LT8620
1
8620fa
For more information www.linear.com/LT8620
LOAD CURRENT (A)
0
EFFICIENCY (%)
80
90
100
1.75
8620 TA01b
70
60
75
85
95
65
55
50 0.5 0.750.25 1.0 1.25 1.5 2.0
VIN = 12V
VIN = 24V
fSW = 700kHz
TYPICAL APPLICATION
FEATURES DESCRIPTION
65V, 2A Synchronous
Step-Down Regulator
with 2.5µA Quiescent
Current
The LT
®
8620 is a compact, high efficiency, high speed
synchronous monolithic step-down switching regulator
that accepts a wide input voltage range up to 65V, and
consumes only 2.5µA of quiescent current. Top and bottom
power switches are included with all necessary circuitry
to minimize the need for external components. Low ripple
Burst Mode operation enables high efficiency down to
very low output currents while keeping the output ripple
below 10mVP-P. A SYNC pin allows synchronization to an
external clock. Internal compensation with peak current
mode topology allows the use of small inductors and
results in fast transient response and good loop stability.
The EN/UV pin has an accurate 1V threshold and can be
used to program VIN undervoltage lockout or to shut down
the LT8620 reducing the input supply current to 1µA. A
capacitor on the TR/SS pin programs the output voltage
ramp rate during start-up. The PG flag signals when VOUT
is within ±9% of the programmed output voltage as well as
fault conditions. The LT8620 is available in small 16-Lead
MSOP and 3mm × 5mm QFN packages with exposed pads
for low thermal resistance.
5V 2A Step-Down Converter
Efficiency at 5VOUT
APPLICATIONS
n Wide Input Voltage Range: 3.4V to 65V
n Ultralow Quiescent Current Burst Mode
®
Operation:
n 2.5μA IQ Regulating 12VIN to 3.3VOUT
n Output Ripple < 10mVP-P
n High Efficiency Synchronous Operation:
n 94% Efficiency at 1A, 12VIN to 5VOUT
n 92% Efficiency at 1A, 12VIN to 3.3VOUT
n Fast 30ns Minimum Switch-On Time
n Low Dropout Under All Conditions: 250mV at 1A
n Safely Tolerates Inductor Saturation in Overload
n Low EMI
n Adjustable and Synchronizable: 200kHz to 2.2MHz
n Accurate 1V Enable Pin Threshold
n Internal Compensation
n Output Soft-Start and Tracking
n Small Thermally Enhanced 16-Lead MSOP and
24-Lead 3mm × 5mm QFN Packages
n Automotive and Industrial Supplies
n General Purpose Step-Down
n GSM Power Supplies
L, LT , LT C , LT M , Burst Mode, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their respective
owners.
BSTVIN
EN/UV
PG
SYNC
INTVCC
TR/SS
RT
SW
LT8620
GND
BIAS
8620 TA01a
FB
0.1µF
VOUT
5V
2A
4.7µF
VIN
5.5V TO 65V
1µF
10nF
10pF
4.7µH
1M
243k
fSW = 700kHz
60.4k
47µF
LT8620
2
8620fa
For more information www.linear.com/LT8620
PIN CONFIGURATION
ABSOLUTE MAXIMUM RATINGS
VIN, EN/UV ................................................................65V
PG .............................................................................42V
BIAS .......................................................................... 25V
BST Pin Above SW Pin................................................4V
FB, TR/SS, RT, INTVCC . .............................................. 4V
(Note 1)
1
2
3
4
5
6
7
8
SYNC
TR/SS
RT
EN/UV
VIN
VIN
NC
GND
16
15
14
13
12
11
10
9
FB
PG
BIAS
INTVCC
BST
SW
SW
SW
TOP VIEW
17
GND
MSE PACKAGE
16-LEAD PLASTIC MSOP
θJA = 40°C/W, θJC(PAD) = 10°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
24 23 22 21
9 10
TOP VIEW
25
GND
UDD PACKAGE
24-LEAD (3mm × 5mm) PLASTIC QFN
11 12
6
5
4
3
2
1
15
16
17
18
19
20
SYNC
TR/SS
RT
EN/UV
VIN
VIN
NC
GND
FB
PG
BIAS
INTVCC
BST
SW
SW
SW
NC
NC
NC
NC
GND
NC
NC
NC
14
7
13
8
θJA = 46°C/W, θJC(PAD) = 5°C/W
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT8620EMSE#PBF LT8620EMSE#TRPBF 8620 16-Lead Plastic MSOP –40°C to 125°C
LT8620IMSE#PBF LT8620IMSE#TRPBF 8620 16-Lead Plastic MSOP –40°C to 125°C
LT8620HMSE#PBF LT8620HMSE#TRPBF 8620 16-Lead Plastic MSOP –40°C to 150°C
LT8620MPMSE#PBF LT8620MPMSE#TRPBF 8620 16-Lead Plastic MSOP –55°C to 150°C
LT8620EUDD#PBF LT8620EUDD#TRPBF LGGV 24-Lead (3mm × 5mm) Plastic QFN –40°C to 125°C
LT8620IUDD#PBF LT8620IUDD#TRPBF LGGV 24-Lead (3mm × 5mm) Plastic QFN –40°C to 125°C
Consult LT C Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LT C Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
SYNC Voltage . ............................................................6V
Operating Junction Temperature Range (Note 2)
LT8620E ............................................ –40°C to 125°C
LT8620I ............................................. –40°C to 125°C
LT8620H ............................................ –40°C to 150°C
LT8620MP ......................................... –55°C to 150°C
Storage Temperature Range .................. –65°C to 150°C
LT8620
3
8620fa
For more information www.linear.com/LT8620
ELECTRICAL CHARACTERISTICS
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may
cause permanent damage to the device. Exposure to any Absolute Maximum
Rating condition for extended periods may affect device reliability and lifetime.
Note 2: The LT8620E is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C
to 125°C operating junction temperature range are assured by design,
characterization, and correlation with statistical process controls. The
LT8620I is guaranteed over the full –40°C to 125°C operating junction
temperature range. The LT8620H is guaranteed over the full −40°C to
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Input Voltage l2.9 3.4 V
VIN Quiescent Current VEN/UV = 0V, VSYNC = 0V
l
1.0
1.0
3
8
µA
µA
VEN/UV = 2V, Not Switching, VSYNC = 0V
l
1.7
1.7
4
10
µA
µA
VEN/UV = 2V, Not Switching, VSYNC = 2V 0.28 0.5 mA
VIN Current in Regulation VOUT = 0.97V, VIN = 6V, Output Load = 100µA
VOUT = 0.97V, VIN = 6V, Output Load = 1mA
l
l
20
200
50
350
µA
µA
Feedback Reference Voltage VIN = 6V, ILOAD = 0.5A
VIN = 6V, ILOAD = 0.5A
l
0.964
0.958
0.970
0.970
0.976
0.982
V
V
Feedback Voltage Line Regulation VIN = 4.0V to 42V, ILOAD = 0.5A l0.004 0.02 %/V
Feedback Pin Input Current VFB = 1V –20 20 nA
INTVCC Voltage ILOAD = 0mA, VBIAS = 0V
ILOAD = 0mA, VBIAS = 3.3V
3.23
3.25
3.4
3.29
3.57
3.35
V
V
INTVCC Undervoltage Lockout 2.5 2.6 2.7 V
BIAS Pin Current Consumption VBIAS = 3.3V, ILOAD = 1A, 2MHz 7.2 mA
Minimum On-Time ILOAD = 1A, SYNC = 0V
ILOAD = 1A, SYNC = 3.3V
l
l
30
30
45
45
ns
ns
Minimum Off-Time 90 130 ns
Oscillator Frequency RT = 221k, ILOAD = 1A
RT = 60.4k, ILOAD = 1A
RT = 18.2k, ILOAD = 1A
l
l
l
180
665
1.85
210
700
2.00
240
735
2.15
kHz
kHz
MHz
Top Power NMOS On-Resistance ISW = 1A 175 mΩ
Top Power NMOS Current Limit l2.8 4.1 4.9 A
Bottom Power NMOS On-Resistance VINTVCC = 3.4V, ISW = 1A 85 mΩ
Bottom Power NMOS Current Limit VINTVCC = 3.4V 2.9 3.9 4.7 A
SW Leakage Current VIN = 42V, VSW = 0V, 42V –1.5 1.5 µA
EN/UV Pin Threshold EN/UV Rising l0.94 1.0 1.06 V
EN/UV Pin Hysteresis 40 mV
EN/UV Pin Current VEN/UV = 2V –20 20 nA
PG Upper Threshold Offset from VFB VFB Falling l6 9.0 12 %
PG Lower Threshold Offset from VFB VFB Rising l–6 –9.0 –12 %
PG Hysteresis 1.3 %
PG Leakage VPG = 3.3V –40 40 nA
PG Pull-Down Resistance VPG = 0.1V l680 2000 Ω
SYNC Threshold SYNC Falling
SYNC Rising
0.8
1.1
1.0
1.3
1.2
1.5
V
V
SYNC Pin Current VSYNC = 6V –100 100 nA
TR/SS Source Current l1.2 2 2.7 µA
TR/SS Pull-Down Resistance Fault Condition, TR/SS = 0.1V 220 Ω
150°C operating junction temperature range. The LT8620MP is 100%
tested and guaranteed over the full −55°C to 150°C operating junction
temperature range. High junction temperatures degrade operating lifetimes.
Operating lifetime is derated at junction temperatures greater than 125°C.
Note 3: This IC includes overtemperature protection that is intended to protect
the device during overload conditions. Junction temperature will exceed 150°C
when overtemperature protection is active. Continuous operation above the
specified maximum operating junction temperature will reduce lifetime.
LT8620
4
8620fa
For more information www.linear.com/LT8620
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency at 3.3VOUT Efficiency vs Frequency at 1A Reference Voltage
EN Pin Thresholds Load Regulation Line Regulation
Efficiency at 5VOUT Efficiency at 3.3VOUT Efficiency at 5VOUT
LOAD CURRENT (A)
0
EFFICIENCY (%)
80
90
100
1.751.5
8620 G01
70
60
75
85
95
65
55
50 0.25 0.5 1.0 1.250.75 2.0
VIN = 48V
VIN = 36V
VIN = 24V
VIN = 12V
fSW = 700kHz
L = IHLP2525CZ-01, 4.7µH
LOAD CURRENT (A)
0
EFFICIENCY (%)
80
90
100
1.0 1.25 1.5 1.75
8620 G02
70
60
75
85
95
65
55
50 0.25 0.5 0.75 2.0
VIN = 48V
VIN = 36V
VIN = 24V
VIN = 12V
fSW = 700kHz
L = IHLP2525CZ-01, 4.7µH
LOAD CURRENT (mA)
30
EFFICIENCY (%)
90
100
20
10
80
50
70
60
40
0.01 10010 1000
8620 G03
0
0.1 1.0
VIN = 48V
VIN = 36V
VIN = 24V
VIN = 12V
fSW = 700kHz
L = IHLP2525CZ-01, 4.7µH
LOAD CURRENT (mA)
30
EFFICIENCY (%)
90
100
20
10
80
50
70
60
40
0.01 10 100 1000
8620 G04
0
0.1 1.0
VIN = 48V
VIN = 36V
VIN = 24V
VIN = 12V
fSW = 700kHz
L = IHLP2525CZ-01, 4.7µH
SWITCHING FREQUENCY (MHz)
0.25
88
90
94
8620 G05
86
84
1.751.250.75 2.25
82
80
92
EFFICIENCY (%)
VIN = 12V
VIN = 24V
VOUT = 3.3V
L = IHLP2525CZ-01, 4.7µH
TEMPERATURE (°C)
–50
0.961
REFERENCE VOLTAGE (V)
0.963
0.967
0.969
0.971
0.979
25 75 100 125
8620 G06
0.965
0.975
0.977
0.973
–25 0 50 150
TEMPERATURE (°C)
–50
0.95
EN THRESHOLD (V)
0.96
0.98
0.99
1.00
50 75
1.03
8620 G07
0.97
0
–25 100 125
25 150
1.01
1.02
EN RISING
EN FALLING
LOAD CURRENT (A)
0
–0.15
CHANGE IN VOUT (%)
–0.1
–0.05
0.05
0.5 1 1.5
8620 G08
0.1
0.15
0
2
VOUT = 5V
VIN = 12V
INPUT VOLTAGE (V)
5
CHANGE IN VOUT (%)
0.02
0.00
0.12
0.10
55
8620 G09
–0.04
–0.06
–0.02
0.08
0.06
0.04
–0.08 15 35 45 65
25
VOUT = 5V
ILOAD = 1A
TA = 25°C, unless otherwise noted.
LT8620
5
8620fa
For more information www.linear.com/LT8620
TYPICAL PERFORMANCE CHARACTERISTICS
Top FET Current Limit vs Duty Cycle Top FET Current Limit
Bottom FET Current Limit Switch Drop
Minimum On-Time
Switch Drop
No Load Supply Current
INPUT VOLTAGE (V)
0
0
INPUT CURRENT (µA)
0.5
1.5
2.0
2.5
5.0
3.5
20 30
8620 G10
1.0
4.0
4.5
3.0
10 40 50 60
VOUT = 3.3V
IN REGULATION
DUTY CYCLE
0
CURRENT LIMIT (A)
3.5
0.6 1.0
8620 G11
3.0
2.5
2.0 0.2 0.4 0.8
4.0
TEMPERATURE (°C)
–50
20
MINIMUM ON-TIME (ns)
22
24
34
28
050 75 100
8620 G16
30
32
26
–25 25 125
ILOAD = 1A, VSYNC = 0V
ILOAD = 1A, VSYNC = 3V
ILOAD = 2A, VSYNC = 0V
ILOAD = 2A, VSYNC = 3V
TEMPERATURE (°C)
–50 –25
0
SWITCH DROP (mV)
200
150
350
050 75
8620 G14
50
100
300
250
25 100 125
TOP SWITCH
BOTTOM SWITCH
SWITCH CURRENT = 1A
SWITCH CURRENT (A)
0 0.25
0
SWITCH DROP (mV)
50
150
200
250
1.25
450
8620 G15
100
1
0.5 0.75 1.751.5 2
300
350
400
TOP SWITCH
BOTTOM SWITCH
TA = 25°C, unless otherwise noted.
Dropout Voltage Switching Frequency
LOAD CURRENT (A)
0
DROPOUT VOLTAGE (mV)
400
8620 G17
200
01
0.50.25 0.75 1.51.25 1.75
600
300
100
500
2
TEMPERATURE (°C)
–50
SWITCHING FREQUENCY (kHz)
730
50
8620 G18
700
680
–25 250 75
670
660
740 RT = 60.4k
720
710
690
100 125 150
DUTY CYCLE = 5%
TEMPERATURE (°C)
–55
–25
5
35
65
95
125
155
3.0
3.5
4.0
4.5
5.0
CURRENT LIMIT (A)
8620 G12
TEMPERATURE (°C)
–55
–25
5
35
65
95
125
155
3.0
3.5
4.0
4.5
5.0
CURRENT LIMIT (A)
8620 G13
LT8620
6
8620fa
For more information www.linear.com/LT8620
TYPICAL PERFORMANCE CHARACTERISTICS
RT Programmed Switching
Frequency VIN UVLO
Burst Frequency Frequency Foldback
Minimum Load to Full Frequency
(SYNC DC High)
Soft-Start Tracking Soft-Start Current PG High Thresholds
PG Low Thresholds
LOAD CURRENT (mA)
0
SWITCHING FREQUENCY (kHz)
400
500
600
200
8620 G19
300
200
050 100 150
100
800 VIN = 12V
VOUT = 5V
700
FB VOLTAGE (V)
0
SWITCHING FREQUENCY (kHz)
300
400
500
0.6 1
8620 G21
200
100
00.2 0.4 0.8
600
700
800 VOUT = 3.3V
VIN = 12V
VSYNC = 0V
RT = 60.4k
TR/SS VOLTAGE (V)
0
FB VOLTAGE (V)
0.8
1.0
1.2
0.6 1.0
8620 G22
0.6
0.4
0.2 0.4 0.8 1.2 1.4
0.2
0
V
SS
= 0.5V
TEMPERATURE (°C)
–55
–25
5
35
65
95
125
155
1.8
1.9
2.0
2.1
2.2
SS PIN CURRENT (µA)
8620 G23
TEMPERATURE (°C)
–50
7.0
PG THRESHOLD OFFSET FROM VREF (%)
7.5
8.5
9.0
9.5
12.0
10.5
0 25 75 100 125
8620 G24
8.0
11.0
11.5
10.0
–25 50 150
FB RISING
FB FALLING
TEMPERATURE (°C)
–50
–12.0
PG THRESHOLD OFFSET FROM VREF (%)
–11.5
–10.5
–10.0
–9.5
–7.0
–8.5
075 100 125
8620 G25
–11.0
–8.0
–7.5
–9.0
–25 25 50 150
FB RISING
FB FALLING
SWITCHING FREQUENCY (MHz)
0.2
RT PIN RESISTOR (kΩ)
150
200
250
1.8
8620 G26
100
50
125
175
225
75
25
00.6 11.4 2.2
TEMPERATURE (°C)
–55
INPUT VOLTAGE (V)
3.4
35
8620 G27
2.8
2.4
–25 5 65
2.2
2.0
3.6
3.2
3.0
2.6
95 125 155
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
60
80
100
15 25 55 65
8620 G20
40
20
0
5 35 45
VOUT = 5V
fSW = 700kHz
TA = 25°C, unless otherwise noted.
LT8620
7
8620fa
For more information www.linear.com/LT8620
TYPICAL PERFORMANCE CHARACTERISTICS
Bias Pin Current Bias Pin Current
Switching Waveforms, Full
Frequency Continuous Operation
Switching Waveforms,
Burst Mode Operation Switching Waveforms
Transient Response; Load Current
Stepped from 1A to 2A
INPUT VOLTAGE (V)
5
BIAS PIN CURRENT (mA)
4.0
65
8620 G28
3.5
2.5 15 25 45
35 55
4.5
3.0
VBIAS = 5V
VOUT = 5V
ILOAD = 1A
fSW = 700kHz
SWITCHING FREQUENCY (MHz)
0
0
BIAS PIN CURRENT (mA)
2
4
6
8
10
0.5 1 1.5 2
8620 G29
2.5
VBIAS = 5V
VOUT = 5V
VIN = 12V
ILOAD = 1A
IL
1A/DIV
VSW
5V/DIV
500ns/DIV
12VIN TO 5VOUT AT 1A
8620 G30
IL
200mA/DIV
VSW
5V/DIV
2µs/DIV
12VIN TO 5VOUT AT 10mA
VSYNC = 0V
8620 G31
IL
1A/DIV
VSW
20V/DIV
500ns/DIV
48VIN TO 5VOUT AT 1A
8620 G32
ILOAD
1A/DIV
VOUT
100mV/DIV
50µs/DIV
1A TO 2A TRANSIENT
12VIN, 5VOUT
COUT = 47µF
8620 G33
TA = 25°C, unless otherwise noted.
Transient Response; Load Current
Stepped from 50mA
(Burst Mode Operation) to 1A
ILOAD
1A/DIV
VOUT
200mV/DIV
50µs/DIV
50mA (Burst Mode Operation) TO 1A TRANSIENT
12VIN, 5VOUT
COUT = 47µF
8620 G34
Start-Up Dropout Performance
VIN
2V/DIV
VOUT
2V/DIV
100ms/DIV
2.5Ω LOAD
(2A IN REGULATION)
8620 G35
VIN
VOUT
Start-Up Dropout Performance
VIN
2V/DIV
VOUT
2V/DIV
100ms/DIV
20Ω LOAD
(250mA IN REGULATION)
8620 G36
VIN
VOUT
LT8620
8
8620fa
For more information www.linear.com/LT8620
PIN FUNCTIONS
SYNC: External Clock Synchronization Input. Ground this
pin for low ripple Burst Mode operation at low output loads.
Tie to a clock source for synchronization to an external
frequency. Apply a DC voltage of 3V or higher or tie to
INTVCC for pulse-skipping mode. When in pulse-skipping
mode, the IQ will increase to several hundred µA. Do not
float this pin.
TR/SS: Output Tracking and Soft-Start Pin. This pin allows
user control of output voltage ramp rate during start-up. A
TR/SS voltage below 0.97V forces the LT8620 to regulate
the FB pin to equal the TR/SS pin voltage. When TR/SS
is above 0.97V, the tracking function is disabled and the
internal reference resumes control of the error amplifier.
An internal 2μA pull-up current from INTVCC on this pin
allows a capacitor to program output voltage slew rate.
This pin is pulled to ground with an internal 220Ω MOS-
FET during shutdown and fault conditions; use a series
resistor if driving from a low impedance output. This pin
may be left floating if the tracking function is not needed.
RT: A resistor is tied between RT and ground to set the
switching frequency.
EN/UV: The LT8620 is shut down when this pin is low and
active when this pin is high. The hysteretic threshold volt-
age is 1.00V going up and 0.96V going down. Tie to VIN
if the shutdown feature is not used. An external resistor
divider from VIN can be used to program a VIN threshold
below which the LT8620 will shut down.
VIN: The VIN pins supply current to the LT8620 internal
circuitry and to the internal topside power switch. These
pins must be tied together and be locally bypassed. Be
sure to place the positive terminal of the input capacitor
as close as possible to the VIN pins, and the negative
capacitor terminal as close as possible to the PGND pins.
NC: No Connect. This pin is not connected to internal
circuitry.
SW: The SW pins are the outputs of the internal power
switches. Tie these pins together and connect them to the
inductor and boost capacitor. This node should be kept
small on the PCB for good performance.
BST: This pin is used to provide a drive voltage, higher
than the input voltage, to the topside power switch. Place
a 0.1µF boost capacitor as close as possible to the IC.
INTVCC: Internal 3.4V Regulator Bypass Pin. The in-
ternal power drivers and control circuits are powered
from this voltage. INTVCC maximum output current is
20mA. Do not load the INTVCC pin with external cir-
cuitry. INTVCC current will be supplied from BIAS if
VBIAS > 3.1V, otherwise current will be drawn from VIN.
Voltage on INTVCC will vary between 2.8V and 3.4V when
VBIAS is between 3.0V and 3.6V. Decouple this pin to power
ground with at least a 1μF low ESR ceramic capacitor
placed close to the IC.
BIAS: The internal regulator will draw current from BIAS
instead of VIN when BIAS is tied to a voltage higher than
3.1V. For output voltages of 3.3V to 25V, this pin should
be tied to VOUT. If this pin is tied to a supply other than
VOUT use a 1µF local bypass capacitor on this pin. If no
supply is available, tie to ground.
PG: The PG pin is the open-drain output of an internal
comparator. PG remains low until the FB pin is within
±9% of the final regulation voltage, and there are no fault
conditions. PG is valid when VIN is above 3.4V, regardless
of EN/UV pin state.
FB: The LT8620 regulates the FB pin to 0.970V.
Connect the feedback resistor divider tap to this pin. Also,
connect a phase lead capacitor between FB and VOUT.
Typically, this capacitor is 4.7pF to 10pF.
GND: Ground. The exposed pad must be connected to the
negative terminal of the input capacitor and soldered to
the PCB in order to lower the thermal resistance.
LT8620
9
8620fa
For more information www.linear.com/LT8620
BLOCK DIAGRAM
+
+
–
+
–
SLOPE COMP
INTERNAL 0.97V REF
OSCILLATOR
200kHz TO 2.2MHz
BURST
DETECT
3.4V
REG
CBST
COUT
VOUT
8620 BD
SW L
BST
SWITCH
LOGIC
AND
ANTI-
SHOOT
THROUGH
ERROR
AMP
SHDN
±9%
VC
SHDN
THERMAL SHDN
INTVCC UVLO
VIN UVLO
SHDN
THERMAL SHDN
VIN UVLO
EN/UV
1V +
–
GND
INTVCC
BIAS
PG
FB
R1C1
R3
OPT
R4
OPT
R2
RT
CSS
OPT
VOUT
TR/SS
2µA
RT
SYNC
VIN
VIN
CIN
CVCC
LT8620
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OPERATION
The LT8620 is a monolithic, constant frequency, current
mode step-down DC/DC converter. An oscillator, with
frequency set using a resistor on the RT pin, turns on
the internal top power switch at the beginning of each
clock cycle. Current in the inductor then increases until
the top switch current comparator trips and turns off the
top power switch. The peak inductor current at which
the top switch turns off is controlled by the voltage on
the internal VC node. The error amplifier servos the VC
node by comparing the voltage on the VFB pin with an
internal 0.97V reference. When the load current increases
it causes a reduction in the feedback voltage relative to
the reference leading the error amplifier to raise the VC
voltage until the average inductor current matches the new
load current. When the top power switch turns off, the
synchronous power switch turns on until the next clock
cycle begins or inductor current falls to zero. If overload
conditions result in more than 3.9A flowing through the
bottom switch, the next clock cycle will be delayed until
switch current returns to a safe level.
If the EN/UV pin is low, the LT8620 is shut down and
draws 1µA from the input. When the EN/UV pin is above
1V, the switching regulator will become active.
To optimize efficiency at light loads, the LT8620 operates
in Burst Mode operation in light load situations. Between
bursts, all circuitry associated with controlling the output
switch is shut down, reducing the input supply current to
1.7μA. In a typical application, 2.5μA will be consumed
from the input supply when regulating with no load. The
SYNC pin is tied low to use Burst Mode operation and can
be tied to a logic high to use pulse-skipping mode. If a
clock is applied to the SYNC pin the part will synchronize to
an external clock frequency and operate in pulse-skipping
mode. While in pulse-skipping mode the oscillator operates
continuously and positive SW transitions are aligned to
the clock. During light loads, switch pulses are skipped
to regulate the output and the quiescent current will be
several hundred µA.
To improve efficiency across all loads, supply current to
internal circuitry can be sourced from the BIAS pin when
biased at 3.3V or above. Else, the internal circuitry will draw
current from VIN. The BIAS pin should be connected to
VOUT if the LT8620 output is programmed at 3.3V or above.
Comparators monitoring the FB pin voltage will pull the
PG pin low if the output voltage varies more than ±9%
(typical) from the set point, or if a fault condition is present.
The oscillator reduces the LT8620’s operating frequency
when the voltage at the FB pin is low. This frequency
foldback helps to control the inductor current when the
output voltage is lower than the programmed value which
occurs during start-up or overcurrent conditions. When
a clock is applied to the SYNC pin or the SYNC pin is
held DC high, the frequency foldback is disabled and the
switching frequency will slow down only during overcur-
rent conditions.
LT8620
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APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT8620 operates
in low ripple Burst Mode operation, which keeps the out-
put capacitor charged to the desired output voltage while
minimizing the input quiescent current and minimizing
output voltage ripple. In Burst Mode operation the LT8620
delivers single small pulses of current to the output capaci-
tor followed by sleep periods where the output power is
supplied by the output capacitor. While in sleep mode the
LT8620 consumes 1.7μA.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 1a) and the percentage
of time the LT8620 is in sleep mode increases, resulting in
much higher light load efficiency than for typical convert-
ers. By maximizing the time between pulses, the converter
quiescent
current approaches 2.5µA for a typical application
when there is no output load. Therefore, to optimize the
quiescent current performance at light loads, the current
in the feedback resistor divider must be minimized as it
appears to the output as load current.
In order to achieve higher light load efficiency, more energy
must be delivered to the output during the single small
pulses in Burst Mode operation such that the LT8620 can
stay in sleep mode longer between each pulse. This can be
achieved by using a larger value inductor (i.e. 4.7µH), and
should be considered independent of switching frequency
when choosing an inductor. For example, while a lower
inductor value would typically be used for a high switch-
ing frequency application, if high light load efficiency is
desired, a higher inductor value should be chosen.
While in Burst Mode operation the current limit of the top
switch is approximately 400mA resulting in output voltage
ripple shown in Figure 2. Increasing the output capacitance
will decrease the output ripple proportionally. As load ramps
upward from zero the switching frequency will increase
but only up to the switching frequency programmed by
the resistor at the RT pin as shown in Figure 1a. The out-
put load at which the LT8620 reaches the programmed
frequency varies based on input voltage, output voltage,
and inductor choice.
Figure 1. SW Frequency vs Load Information in Burst Mode Operation (1a) and Pulse-Skipping Mode (1b)
Minimum Load to Full Frequency (SYNC DC High)
Burst Frequency
(1a) (1b)
LOAD CURRENT (mA)
0
SWITCHING FREQUENCY (kHz)
400
500
600
200
8620 F01a
300
200
050 100 150
100
800 VIN = 12V
VOUT = 5V
700
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
60
80
100
15 25 55 65
8620 F01b
40
20
0
5 35 45
VOUT = 5V
fSW = 700kHz
LT8620
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Figure 2. Burst Mode Operation
VSW
5V/DIV
IL
500mA/DIV
VOUT
10mV/DIV
2µs/DIV
12VIN TO 5VOUT AT 10mA
VSYNC = 0V
8620 F02
For some applications it is desirable for the LT8620 to
operate in pulse-skipping mode, offering two major differ-
ences from Burst Mode operation. First is the clock stays
awake at all times and all switching cycles are aligned to
the clock. In this mode much of the internal circuitry is
awake at all times, increasing quiescent current to several
hundred µA. Second is that full switching frequency is
reached at lower output load than in Burst Mode operation
(see Figure 1b). To enable pulse-skipping mode, the SYNC
pin is tied high either to a logic output or to the INTVCC
pin. When a clock is applied to the SYNC pin the LT8620
will also operate in pulse-skipping mode.
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
R1=R2
V
OUT
0.970V – 1
(1)
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
If low input quiescent current and good light-load efficiency
are desired, use large resistor values for the FB resistor
divider. The current flowing in the divider acts as a load
current, and will increase the no-load input current to the
converter, which is approximately:
IQ=1.7µA +VOUT
R1+R2
VOUT
V
IN
1
n
(2)
where 1.7µA is the quiescent current of the LT8620 and
the second term is the current in the feedback divider
reflected to the input of the buck operating at its light
load efficiency n. For a 3.3V application with R1 = 1M and
R2 = 412k, the feedback divider draws 2.3µA. With VIN =
12V and n = 80%, this adds 0.8µA to the 1.7µA quiescent
current resulting in 2.5µA no-load current from the 12V
supply. Note that this equation implies that the no-load
current is a function of VIN; this is plotted in the Typical
Performance Characteristics section.
When using large FB resistors, a 4.7pF to 10pF phase-lead
capacitor should be connected from VOUT to FB.
Setting the Switching Frequency
The LT8620 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2.2MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary RT value for a desired switching
frequency is in Table 1.
The RT resistor required for a desired switching frequency
can be calculated using:
RT=
46.5
fSW
– 5.2
(3)
where RT is in kΩ and fSW is the desired switching fre-
quency in MHz.
Table 1. SW Frequency vs RT Value
fSW (MHz) RT (kΩ)
0.2 232
0.3 150
0.4 110
0.5 88.7
0.6 71.5
0.7 60.4
0.8 52.3
1.0 41.2
1.2 33.2
14 28.0
1.6 23.7
1.8 20.5
2.0 18.2
2.2 15.8
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Operating Frequency Selection and Trade-Offs
Selection of the operating frequency is a trade-off between
efficiency, component size, and input voltage range. The
advantage of high frequency operation is that smaller induc-
tor and capacitor values may be used. The disadvantages
are lower efficiency and a smaller input voltage range.
The highest switching frequency (fSW(MAX)) for a given
application can be calculated as follows:
fSW(MAX) =
V
OUT
+V
SW(BOT)
tON(MIN) V
IN – VSW(TOP) +VSW(BOT)
( )
(4)
where VIN is the typical input voltage, VOUT is the output
voltage, VSW(TOP) and VSW(BOT) are the internal switch
drops (~0.3V, ~0.15V, respectively at maximum load)
and tON(MIN) is the minimum top switch on-time (see the
Electrical Characteristics). This equation shows that a
slower switching frequency is necessary to accommodate
a high VIN/VOUT ratio.
For transient operation, VIN may go as high as the abso-
lute maximum rating of 65V regardless of the RT value,
however the LT8620 will reduce switching frequency as
necessary to maintain control of inductor current to as-
sure safe operation.
The LT8620 is capable of a maximum duty cycle of ap-
proximately 99%, and the VIN-to-VOUT dropout is limited
by the RDS(ON) of the top switch. In this mode the LT8620
skips switch cycles, resulting in a lower switching frequency
than programmed by RT.
For applications that cannot allow deviation from the pro-
grammed switching frequency at low VIN/VOUT ratios use
the following formula to set switching frequency:
V
IN(MIN) =
V
OUT
+V
SW(BOT)
1– fSW • tOFF(MIN)
– VSW(BOT) +VSW(TOP)
(5)
where VIN(MIN) is the minimum input voltage without
skipped cycles, VOUT is the output voltage, VSW(TOP) and
VSW(BOT) are the internal switch drops (~0.3V, ~0.15V,
respectively at maximum load), fSW is the switching fre-
quency (set by RT), and tOFF(MIN) is the minimum switch
off-time. Note that higher switching frequency will increase
the minimum input voltage below which cycles will be
dropped to achieve higher duty cycle.
Inductor Selection and Maximum Output Current
The LT8620 is designed to minimize solution size by
allowing the inductor to be chosen based on the output
load requirements of the application. During overload or
short-circuit conditions the LT8620 safely tolerates opera-
tion with a saturated inductor through the use of a high
speed peak-current mode architecture.
A good first choice for the inductor value is:
L=
V
OUT
+V
SW(BOT)
fSW
(6)
where fSW is the switching frequency in MHz, VOUT is
the output voltage, VSW(BOT) is the bottom switch drop
(~0.15V) and L is the inductor value in μH.
To avoid overheating and poor efficiency, an inductor must
be chosen with an RMS current rating that is greater than
the maximum expected output load of the application. In
addition, the saturation current (typically labeled ISAT)
rating of the inductor must be higher than the load current
plus 1/2 of in inductor ripple current:
I
L(PEAK) =ILOAD(MAX) +
1
2
∆IL
(7)
where ∆IL is the inductor ripple current as calculated in
Equation 9 and ILOAD(MAX) is the maximum output load
for a given application.
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APPLICATIONS INFORMATION
As a quick example, an application requiring 1A output
should use an inductor with an RMS rating of greater than
1A and an ISAT of greater than 1.3A. During long duration
overload or short-circuit conditions, the inductor RMS
routing requirement is greater to avoid overheating of the
inductor. To keep the efficiency high, the series resistance
(DCR) should be less than 0.04Ω, and the core material
should be intended for high frequency applications.
The LT8620 limits the peak switch current in order to
protect the switches and the system from overload faults.
The top switch current limit (ILIM) is at least 3.8A at low
duty cycles and decreases linearly to 2.8A at DC = 0.8. The
inductor value must then be sufficient to supply the desired
maximum output current (IOUT(MAX)), which is a function
of the switch current limit (ILIM) and the ripple current.
IOUT(MAX) =ILIM –
∆I
L
2
(8)
The peak-to-peak ripple current in the inductor can be
calculated as follows:
∆IL=VOUT
L•fSW
• 1– VOUT
V
IN(MAX)
(9)
where fSW is the switching frequency of the LT8620, and
L is the value of the inductor. Therefore, the maximum
output current that the LT8620 will deliver depends on
the switch current limit, the inductor value, and the input
and output voltages. The inductor value may have to be
increased if the inductor ripple current does not allow
sufficient maximum output current (IOUT(MAX)) given the
switching frequency, and maximum input voltage used in
the desired application.
In order to achieve higher light load efficiency, more energy
must be delivered to the output during the single small
pulses in Burst Mode operation such that the LT8620 can
stay in sleep mode longer between each pulse. This can be
achieved by using a larger value inductor (i.e. 4.7µH), and
should be considered independent of switching frequency
when choosing an inductor. For example, while a lower
inductor value would typically be used for a high switch-
ing frequency application, if high light load efficiency is
desired, a higher inductor value should be chosen.
The optimum inductor for a given application may differ
from the one indicated by this design guide. A larger value
inductor provides a higher maximum load current and
reduces the output voltage ripple. For applications requir-
ing smaller load currents, the value of the inductor may
be lower and the LT8620 may operate with higher ripple
current. This allows use of a physically smaller inductor,
or one with a lower DCR resulting in higher efficiency. Be
aware that low inductance may result in discontinuous
mode operation, which further reduces maximum load
current.
For more information about maximum output current
and discontinuous operation, see Linear Technology’s
Application Note 44.
Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5),
a minimum inductance is required to avoid sub-harmonic
oscillation. See Application Note 19.
Input Capacitor
Bypass the input of the LT8620 circuit with a ceramic ca-
pacitor of X7R or X5R type placed as close as possible to
the VIN and PGND pins. Y5V types have poor performance
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypass the LT8620 and will easily handle the ripple current.
Note that larger input capacitance is required when a lower
switching frequency is used. If the input power source has
high impedance, or there is significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT8620 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT8620 (see the PCB Layout section).
A second precaution regarding the ceramic input capaci-
tor concerns the maximum input voltage rating of the
LT8620. A ceramic input capacitor combined with trace
or cable inductance forms a high quality (under damped)
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APPLICATIONS INFORMATION
tank circuit. If the LT8620 circuit is plugged into a live
supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT8620’s voltage rating.
This situation is easily avoided (see Linear Technology
Application Note 88).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated
by the LT8620 to produce the DC output. In this role it
determines the output ripple, thus low impedance at the
switching frequency is important. The second function
is to store energy in order to satisfy transient loads and
stabilize the LT8620’s control loop. Ceramic capacitors
have very low equivalent series resistance (ESR) and
provide the best ripple performance. For good starting
values, see the Typical Applications section.
Use X5R or X7R types. This choice will provide low output
ripple and good transient response. Transient performance
can be improved with a higher value output capacitor and
the addition of a feedforward capacitor placed between
VOUT and FB. Increasing the output capacitance will also
decrease the output voltage ripple. A lower value of output
capacitor can be used to save space and cost but transient
performance will suffer and may cause loop instability. See
the Typical Applications in this data sheet for suggested
capacitor values.
When choosing a capacitor, special attention should be
given to the data sheet to calculate the effective capacitance
under the relevant operating conditions of voltage bias and
temperature. A physically larger capacitor or one with a
higher voltage rating may be required.
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT8620 due to their piezoelectric
nature. When in Burst Mode operation, the LT8620’s
switching frequency depends on the load current, and
at very light loads the LT8620 can excite the ceramic
capacitor at audio frequencies, generating audible noise.
Since the LT8620 operates at a lower current limit during
Burst Mode operation, the noise is typically very quiet to a
casual ear. If this is unacceptable, use a high performance
tantalum or electrolytic capacitor at the output. Low noise
ceramic capacitors are also available.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT8620. As
previously mentioned, a ceramic input capacitor combined
with trace or cable inductance forms a high quality (un-
derdamped) tank circuit. If the LT8620 circuit is plugged
into a live supply, the input voltage can ring to twice its
nominal value, possibly exceeding the LT8620’s rating.
This situation is easily avoided (see Linear Technology
Application Note 88).
Enable Pin
The LT8620 is in shutdown when the EN pin is low and
active when the pin is high. The rising threshold of the EN
comparator is 1.0V, with 40mV of hysteresis. The EN pin
can be tied to VIN if the shutdown feature is not used, or
tied to a logic level if shutdown control is required.
Adding a resistor divider from VIN to EN programs the
LT8620 to regulate the output only when VIN is above a
desired voltage (see the Block Diagram). Typically, this
threshold, VIN(EN), is used in situations where the input
supply is current limited, or has a relatively high source
resistance. A switching regulator draws constant power
from the source, so source current increases as source
voltage drops. This looks like a negative resistance load
to the source and can cause the source to current limit or
latch low under low source voltage conditions. The VIN(EN)
threshold prevents the regulator from operating at source
voltages where the problems might occur. This threshold
can be adjusted by setting the values R3 and R4 such that
they satisfy the following equation:
V
IN(EN) =
R3
R4 +1
• 1.0V
(10)
where the LT8620 will remain off until VIN is above VIN(EN).
Due to the comparator’s hysteresis, switching will not stop
until the input falls slightly below VIN(EN).
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When operating in Burst Mode operation for light load
currents, the current through the VIN(EN) resistor network
can easily be greater than the supply current consumed
by the LT8620. Therefore, the VIN(EN) resistors should be
large to minimize their effect on efficiency at low loads.
INTVCC Regulator
An internal low dropout (LDO) regulator produces the 3.4V
supply from VIN that powers the drivers and the internal
bias circuitry. The INTVCC can supply enough current for
the LT8620’s circuitry and must be bypassed to ground
with a minimum of 1μF ceramic capacitor. Good bypassing
is necessary to supply the high transient currents required
by the power MOSFET gate drivers. To improve efficiency
the internal LDO can also draw current from the BIAS
pin when the BIAS pin is at 3.1V or higher. Typically the
BIAS pin can be tied to the output of the LT8620, or can
be tied to an external supply of 3.3V or above. If BIAS is
connected to a supply other than VOUT, be sure to bypass
with a local ceramic capacitor. If the BIAS pin is below
3.0V, the internal LDO will consume current from VIN.
Applications with high input voltage and high switching
frequency where the internal LDO pulls current from VIN
will increase die temperature because of the higher power
dissipation across the LDO. Do not connect an external
load to the INTVCC pin.
Output Voltage Tracking and Soft-Start
T
he LT8620 allows the user to program its output voltage
ramp rate by means of the TR/SS pin. An internal 2μA
pulls up the TR/SS pin to INTVCC. Putting an external
capacitor on TR/SS enables soft starting the output to pre-
vent current surge on the input supply. During the soft-start
ramp the output voltage will proportionally track the TR/SS
pin voltage. For output tracking applications, TR/SS can
be externally driven by another voltage source. From 0V to
0.97V, the TR/SS voltage will override the internal 0.97V
reference input to the error amplifier, thus regulating the
FB pin voltage to that of TR/SS pin. When TR/SS is above
0.97V, tracking is disabled and the feedback voltage will
regulate to the internal reference voltage. The TR/SS pin
may be left floating if the function is not needed.
An active pull-down circuit is connected to the TR/SS pin
which will discharge the external soft-start capacitor in
the case of fault conditions and restart the ramp when the
faults are cleared. Fault conditions that clear the soft-start
capacitor are the EN/UV pin transitioning low, VIN voltage
falling too low, or thermal shutdown.
Output Power Good
When the LT8620’s output voltage is within the ±9%
window of the regulation point, which is a VFB voltage in
the range of 0.883V to 1.057V (typical), the output voltage
is considered good and the open-drain PG pin goes high
impedance and is typically pulled high with an external
resistor. Otherwise, the internal pull-down device will pull
the PG pin low. To prevent glitching both the upper and
lower thresholds include 1.3% of hysteresis.
The PG pin is also actively pulled low during several fault
conditions: EN/UV pin is below 1V, INTVCC has fallen too
low, VIN is too low, or thermal shutdown.
Synchronization
To select low ripple Burst Mode operation, tie the SYNC pin
below 0.4V (this can be ground or a logic low output). To
synchronize the LT8620 oscillator to an external frequency
connect a square wave (with 20% to 80% duty cycle) to
the SYNC pin. The square wave amplitude should have val-
leys that are below 0.4V and peaks above 1.5V (up to 6V).
The LT8620 will not enter Burst Mode operation at low
output loads while synchronized to an external clock, but
instead will pulse skip to maintain regulation. The LT8620
may be synchronized over a 200kHz to 2.2MHz range. The
RT resistor should be chosen to set the LT8620 switching
frequency equal to or below the lowest synchronization
input. For example, if the synchronization signal will be
500kHz and higher, the RT should be selected for 500kHz.
The slope compensation is set by the RT value, while the
minimum slope compensation required to avoid subhar-
monic oscillations is established by the inductor size,
input voltage, and output voltage. Since the synchroniza-
tion frequency will not change the slopes of the inductor
current waveform, if the inductor is large enough to avoid
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APPLICATIONS INFORMATION
subharmonic oscillations at the frequency set by RT, then
the slope compensation will be sufficient for all synchro-
nization frequencies.
For some applications it is desirable for the LT8620 to
operate in pulse-skipping mode, offering two major differ-
ences from Burst Mode operation. First is the clock stays
awake at all times and all switching cycles are aligned to
the clock. Second is that full switching frequency is reached
at lower output load than in Burst Mode operation. These
two differences come at the expense of increased quiescent
current. To enable pulse-skipping mode, the SYNC pin is
tied high either to a logic output or to the INTVCC pin.
The LT8620 does not operate in forced continuous mode
regardless of SYNC signal. Never leave the SYNC pin
floating.
Shorted and Reversed Input Protection
The LT8620 will tolerate a shorted output. Several features
are used for protection during output short-circuit and
brownout conditions. The first is the switching frequency
will be folded back while the output is lower than the set
point to maintain inductor current control. Second, the
bottom switch current is monitored such that if inductor
current is beyond safe levels switching of the top switch
will be delayed until such time as the inductor current
falls to safe levels.
Frequency foldback behavior depends on the state of the
SYNC pin: If the SYNC pin is low the switching frequency
will slow while the output voltage is lower than the pro-
grammed level. If the SYNC pin is connected to a clock
source or tied high, the LT8620 will stay at the programmed
frequency without foldback and only slow switching if the
inductor current exceeds safe levels.
There is another situation to consider in systems where
the output will be held high when the input to the LT8620
is absent. This may occur in battery charging applications
or in battery-backup systems where a battery or some
other supply is diode ORed with the LT8620’s output. If
the VIN pin is allowed to float and the EN pin is held high
(either by a logic signal or because it is tied to VIN), then
the LT8620’s internal circuitry will pull its quiescent current
through its SW pin. This is acceptable if the system can
tolerate several μA in this state. If the EN pin is grounded
the SW pin current will drop to near 1µA. However, if the
VIN pin is grounded while the output is held high, regard-
less of EN, parasitic body diodes inside the LT8620 can
pull current from the output through the SW pin and
the VIN pin. Figure 3 shows a connection of the VIN and
EN/UV pins that will allow the LT8620 to run only when
the input voltage is present and that protects against a
shorted or reversed input.
Figure 3. Reverse VIN Protection
VIN
VIN
D1
LT8620
EN/UV
8620 F03
GND
PCB Layout
For proper operation and minimum EMI, care must be taken
during printed circuit board layout. Figure 4 shows the
recommended component placement with trace, ground
plane and via locations. Note that large, switched currents
flow in the LT8620’s VIN pins, GND pins, and the input
capacitor. The loop formed by the input capacitor should
be as small as possible by placing the capacitor adjacent
to the VIN and GND pins. When using a physically large
input capacitor the resulting loop may become too large
in which case using a small case/value capacitor placed
close to the VIN and GND pins plus a larger capacitor
further away is preferred. These components, along with
the inductor and output capacitor, should be placed on the
same side of the circuit board, and their connections should
be made on that layer. Place a local, unbroken ground
plane under the application circuit on the layer closest to
the surface layer. The SW and BOOST nodes should be
as small as possible. Finally, keep the FB and RT nodes
LT8620
18
8620fa
For more information www.linear.com/LT8620
APPLICATIONS INFORMATION
Figure 4b. Recommended PCB Layout for the
LT8620 QFN Package
Figure 4a. Recommended PCB Layout for the
LT8620 MSOP Package
small so that the ground traces will shield them from the
SW and BOOST nodes. The exposed pad on the bottom of
the package must be soldered to ground so that the pad
is connected to ground electrically and also acts as a heat
sink thermally. To keep thermal resistance low, extend the
ground plane as much as possible, and add thermal vias
under and near the LT8620 to additional ground planes
within the circuit board and on the bottom side.
High Temperature Considerations
For higher ambient temperatures, care should be taken in
the layout of the PCB to ensure good heat sinking of the
LT8620. The exposed pad on the bottom of the package
must be soldered to a ground plane. This ground should
be tied to large copper layers below with thermal vias;
these layers will spread heat dissipated by the LT8620.
Placing additional vias can reduce thermal resistance
further. The maximum load current should be derated
as the ambient temperature approaches the maximum
junction rating. Power dissipation within the LT8620 can
be estimated by calculating the total power loss from an
efficiency measurement and subtracting the inductor loss.
The die temperature is calculated by multiplying the LT8620
power dissipation by the thermal resistance from junction
to ambient. The LT8620 will stop switching and indicate
a fault condition if safe junction temperature is exceeded.
VOUT
8620 F04
OUTLINE OF LOCAL
GROUND PLANE
SW
BST
BIAS
INTVCC
GND
9
10
11
12
13
14
15 PG
FB
GND
VOUT
16
SYNC
TR/SS
RT
EN/UV
VIN
1
2
3
4
5
6
7
8
VOUT LINE TO BIAS VIAS TO GROUND PLANE
VOUT
8620 F04
OUTLINE OF LOCAL
GROUND PLANE
SW
GND NC NC NC
NC NC NC NC
BST
BIAS
INTVCC
GND
13
14
15
16
17
18
19 PG
FB
GND
VOUT
20
SYNC
TR/SS
RT
EN/UV
VIN
1
2
3
4
5
6
7
8
VOUT LINE TO BIAS VIAS TO GROUND PLANE
9 10 11 12
24 23 22 21
LT8620
19
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For more information www.linear.com/LT8620
TYPICAL APPLICATIONS
BSTVIN
EN/UV
SYNC
INTVCC
TR/SS
RT
SW
LT8620
GND
BIAS
8620 TA02
PG
FB
0.1µF
VOUT
5V
2A
4.7µF
VIN
5.5V TO 65V
1µF
10nF
10pF
2.2µH
1M
243k
fSW = 2MHz
18.2k
47µF
POWER GOOD
100k
L: XFL4020
5V 2MHz Step-Down Converter
5V Step-Down Converter
BSTVIN
EN/UV
SYNC
INTVCC
TR/SS
RT
SW
LT8620
GND
BIAS
8620 TA03
PG
FB
0.1µF
VOUT
5V
2A
4.7µF
VIN
5.5V TO 65V
1µF
10nF
10pF
4.7µH
1M
243k
fSW = 700kHz
L: IHLP2020CZ-01
60.4k
47µF
POWER GOOD
100k
LT8620
20
8620fa
For more information www.linear.com/LT8620
3.3V Step-Down Converter
BSTVIN
EN/UV
SYNC
PG
INTVCC
TR/SS
RT
SW
LT8620
GND
BIAS
8620 TA05
FB
0.1µF
VOUT
3.3V
2A
4.7µF
VIN
3.8V TO 65V
1µF
10nF
10pF
4.7µH
1M
412k
fSW = 700kHz
L: IHLP2020CZ-01
60.4k
47µF
3.3V 2MHz Step-Down Converter
BSTVIN
EN/UV
SYNC
PG
INTVCC
TR/SS
RT
SW
LT8620
GND
BIAS
8620 TA04
FB
0.1µF
VOUT
3.3V
2A
4.7µF
VIN
3.8V TO 65V
1µF
10nF
10pF
1.8µH
1M
412k
fSW = 2MHz
L: XFL4020
18.2k
47µF
TYPICAL APPLICATIONS
LT8620
21
8620fa
For more information www.linear.com/LT8620
1.8V 2MHz Step-Down Converter
12V Step-Down Converter
BSTVIN
EN/UV
SYNC
PG
INTVCC
TR/SS
RT
SW
LT8620
GND
BIAS
8620 TA06
FB
0.1µF
VOUT
1.8V
2A
4.7µF
VIN
3.4V TO 22V
(65V TRANSIENT)
1µF
10nF
10pF
1µH
866k
EXTERNAL SOURCE
>3.1V OR GND
1M
fSW = 2MHz
L: XFL4020
18.2k
100µF
1µF
BSTVIN
EN/UV
SYNC
INTVCC
TR/SS
RT
SW
LT8620
GND
BIAS
8620 TA09
PG
FB
0.1µF
VOUT
12V
2A
4.7µF
VIN
12.5V TO 65V
1µF
10nF
10pF
10µH
1M
88.7k
fSW = 1MHz
L: IHLP2525CZ-01
41.2k
47µF
POWER GOOD
100k
TYPICAL APPLICATIONS
LT8620
22
8620fa
For more information www.linear.com/LT8620
TYPICAL APPLICATIONS
Ultralow EMI 5V 2A Step-Down Converter
1.8V Step-Down Converter
EXTERNAL SOURCE
>3.1V OR GND
BSTVIN
EN/UV
SYNC
PG
INTVCC
TR/SS
RT
SW
LT8620
GND
PGND
BIAS
8620 TA07
FB
0.1µF
VOUT
1.8V
2A
4.7µF
VIN
3.4V TO 65V
1µF
10nF
10pF
4.7µH
866k
1M
fSW = 400kHz
L: IHLP2020CZ-01
110k
120µF
1µF
BSTVIN
EN/UV
PG
SYNC
INTVCC
TR/SS
RT
SW
LT8620
GND
BIAS
8620 TA11
FB
0.1µF
VOUT
5V
2A
4.7µF
VIN
5.5V TO 65V
1µF
10nF
10pF
2.2µH
4.7µH
1M
FB1
BEAD
FB1: TDK MPZ2012S221A
L: XFL4020
243k
fSW = 2MHz
18.2k
4.7µF4.7µF
47µF
LT8620
23
8620fa
For more information www.linear.com/LT8620
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
MSOP (MSE16) 0213 REV F
0.53 ±0.152
(.021 ±.006)
SEATING
PLANE
0.18
(.007)
1.10
(.043)
MAX
0.17 –0.27
(.007 – .011)
TYP
0.86
(.034)
REF
0.50
(.0197)
BSC
16
161514 13121110
12345678
9
9
18
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL
NOT EXCEED 0.254mm (.010") PER SIDE.
0.254
(.010) 0° – 6° TYP
DETAIL “A”
DETAIL “A”
GAUGE PLANE
5.10
(.201)
MIN
3.20 – 3.45
(.126 – .136)
0.889 ±0.127
(.035 ±.005)
RECOMMENDED SOLDER PAD LAYOUT
0.305 ±0.038
(.0120 ±.0015)
TYP
0.50
(.0197)
BSC
BOTTOM VIEW OF
EXPOSED PAD OPTION
2.845 ±0.102
(.112 ±.004)
2.845 ±0.102
(.112 ±.004)
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
1.651 ±0.102
(.065 ±.004)
1.651 ±0.102
(.065 ±.004)
0.1016 ±0.0508
(.004 ±.002)
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
0.280 ±0.076
(.011 ±.003)
REF
4.90 ±0.152
(.193 ±.006)
DETAIL “B”
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
NO MEASUREMENT PURPOSE
0.12 REF
0.35
REF
MSE Package
16-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1667 Rev F)
LT8620
24
8620fa
For more information www.linear.com/LT8620
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
3.00 ±0.10 1.50 REF
5.00 ±0.10
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
PIN 1
TOP MARK
(NOTE 6)
0.40 ±0.10
23 24
1
2
BOTTOM VIEW—EXPOSED PAD
3.50 REF
0.75 ±0.05
R = 0.115
TYP
PIN 1 NOTCH
R = 0.20 OR 0.25
× 45° CHAMFER
0.25 ±0.05
0.50 BSC
0.200 REF
0.00 – 0.05
(UDD24) QFN 0808 REV Ø
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
0.70 ±0.05
0.25 ±0.05
3.50 REF
4.10 ±0.05
5.50 ±0.05
1.50 REF
2.10 ±0.05
3.50 ±0.05
PACKAGE OUTLINE
R = 0.05 TYP
1.65 ±0.10
3.65 ±0.10
1.65 ±0.05
UDD Package
24-Lead Plastic QFN (3mm × 5mm)
(Reference LTC DWG # 05-08-1833 Rev Ø)
3.65 ±0.05
0.50 BSC
LT8620
25
8620fa
For more information www.linear.com/LT8620
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
A 05/15 Added H- and MP-Grade Versions ABS Max Table, Order Information
Clarified Specifications to 150°C and Note 2
Clarified Current Limit Graphs
Clarified RT Programmed Switching Frequency, Soft-Start Current
Clarified TR/SS and BIAS Pin Function Description
Clarified Overload Conditions from 3.8A to 3.9A
2
3
5
6
8
10
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
LT8620
26
8620fa
For more information www.linear.com/LT8620
LINEAR TECHNOLOGY CORPORATION 2014
LT 0515 REV A • PRINTED IN USA
RELATED PARTS
TYPICAL APPLICATION
Ultralow IQ 2.5V, 3.3V Step-Down with LDO
BSTVIN
EN/UV
SYNC
PG
INTVCC
TR/SS
RT
SW
LT8620
GND
BIAS
8620 TA10
FB
0.1µF
VOUT1
3.3V
2A
4.7µF
VIN
3.8V TO 65V
1µF
10nF
10pF
1.8µH
1M
412k
2.2µF
VOUT2
2.5V
20mA
fSW = 2MHz
L: XFL4020
18.2k
47µF
IN
LT3008-2.5
SHDN
OUT
SENSE
PART NUMBER DESCRIPTION COMMENTS
LT8610 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down
DC/DC Converter with IQ = 2.5µA
VIN: 3.4V to 42V, VOUT(MIN) = 0.985V, IQ = 2.5µA,
ISD < 1µA, MSOP-16E Package
LT8610A/
LT8610AB
42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down
DC/DC Converter with IQ = 2.5µA
VIN: 3.4V to 42V, VOUT(MIN) = 0.985V, IQ = 2.5µA,
ISD < 1µA, MSOP-16E Package
LT8611 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down
DC/DC Converter with IQ = 2.5µA and Input/Output Current Limit/Monitor
VIN: 3.4V to 42V, VOUT(MIN) = 0.985V, IQ = 2.5µA,
ISD < 1µA, 3mm × 5mm QFN-24 Package
LT8612 42V, 6A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down
DC/DC Converter with IQ = 2.5µA
VIN: 3.4V to 42V, VOUT(MIN) = 0.985V, IQ = 3µA,
ISD < 1µA, 3mm × 6mm QFN-28 Package
LT8614 42V, 4A, 96% Efficiency, 2.2MHz Silent Switcher Synchronous
Micropower Step-Down DC/DC Converter with IQ = 2.5µA
VIN: 3.4V to 42V, VOUT(MIN) = 0.985V, IQ = 2.5µA,
ISD < 1µA, 3mm × 4mm QFN-20 Package
LT3690 36V with 60V Transient Protection, 4A, 92% Efficiency, 1.5MHz
Synchronous Micropower Step-Down DC/DC Converter with IQ = 70µA
VIN: 3.9V to 36V, VOUT(MIN) = 0.985V, IQ = 70µA,
ISD < 1µA, 4mm × 6mm QFN-26 Package
LT3991 55V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC
Converter with IQ = 2.8µA
VIN: 4.2V to 62V, VOUT(MIN) = 1.21V, IQ = 2.8µA,
ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages
LT3990 62V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC
Converter with IQ = 2.5µA
VIN: 4.2V to 65V, VOUT(MIN) = 1.21V, IQ = 2.5µA,
ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-6E Packages
LT3980 58V with Transient Protection to 80V, 2A (IOUT), 2.4MHz, High Efficiency
Step-Down DC/DC Converter with Burst Mode Operation
VIN: 3.6V to 58V, Transient to 80V, VOUT(MIN) = 0.78V,
IQ = 85µA, ISD < 1µA, 3mm × 4mm DFN-16 and
MSOP-16E Packages
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LT8620
