www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 1 - 21
AS1324
1.5MHz, 600mA Synchronous DC/DC Converter
1 General Description
The AS1324 is a high-efficiency, constant-frequency synchronous
buck converter available in adjustable- and fixed-voltage versions.
The wide input voltage range (2.7V to 5.5V), automatic powersave
mode and minimal external component requirements make the
AS1324 perfect for any single Li-Ion battery-powered application.
Typical supply current with no load is 30µA and decreases to ≤1µA
in shutdown mode.
The AS1324 is available as the standard versions listed in Table 1.
An internal synchronous switch increases efficiency and eliminates
the need for an external Schottky diode. The internally fixed
switching frequency (1.5MHz) allows for the use of small surface
mount external components.
Very low output voltages can be delivered with the internal 0.6V
feedback reference voltage.
The AS1324 is available in a 5-pin TSOT-23 package.
Figure 1. Typical Application Diagram – High Efficiency Step
Down Converter
2 Key Features
High Efficiency: Up to 96%
Output Current: 600mA
Input Voltage Range: 2.7V to 5.5V
Constant Frequency Operation: 1.5MHz
Variable- and Fixed-Output Voltages
No Schottky Diode Required
Automatic Powersave Operation
Low Quiescent Current: 30µA
Internal Reference: 0.6V
Shutdown Mode Supply Current: ≤1µA
Thermal Protection
5-pin TSOT-23 Package
3 Applications
The device is ideal for mobile communication devices, laptops and
PDAs, ultra-low-power systems, threshold detectors/discriminators,
telemetry and remote systems, medical instruments, or any other
space-limited application with low power-consumption requirements.
Table 1. Standard Versions
Model Output Voltage
AS1324-AD Adjustable via External Resistors
AS1324-12 Fixed: 1.2V
AS1324-15 Fixed: 1.5V
AS1324-18 Fixed: 1.8V
AS1324-18
C
OUT
10µF
C
IN
10µF
GND
2
V
IN
= 2.7V to 5.5V V
OUT
= 1.8V, 600mA
4.7µH
1
EN
5
VOUT
4
VIN
3
SW
5 VOUT
AS1324-18
4 VIN
2
GND
3SW
1
EN
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 2 - 21
AS1324
Datasheet - P i n A s s i g n me n t s
4 Pin Assignments
Figure 2. Pin Assignments (Top View)
4.1 Pin Descriptions
Table 2. Pin Descriptions
Pin Number Pin Name Description
1 EN
Enable Input. Driving this pin above 1.5V enables the device. Driving this pin below 0.3V puts the
device in shutdown mode. In shutdown mode all functions are disabled while SW goes high
impedance, drawing <1µA supply current.
Note: This pin should not be left floating.
2 GND Ground.
3 SW Switch Node Connection to Inductor. This pin connects to the drains of the internal main and
synchronous power MOSFET switches.
4 V
IN
Input Supply Voltage. This pin must be closely decoupled to GND with a ≥ 4.7µF ceramic capacitor.
Connect to any supply voltage between 2.7 to 5.5V.
5
V
FB
Feedback Pin. This pin receives the feedback voltage from the external resistor divider across the
output. (Adjustable voltage variant only.)
V
OUT
Output Voltage Feedback Pin. An internal resistor divider steps the output voltage down for
comparison to the internal reference voltage. (Fixed voltage variants only.)
5VFB
AS1324
4 VIN
2
GND
3
SW
1
EN 5VOUT
AS1324-12/
AS1324-15/
AS1324-18
4 VIN
2GND
3SW
1
EN
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 3 - 21
AS1324
Datasheet - A b s o lu t e M a x i m u m R a t i n gs
5 Absolute Maximum Ratings
Stresses beyond those listed in Table 3 may cause permanent damage to the device. These are stress ratings only, and functional operation of
the device at these or any other conditions beyond those indicated in Section 6 Electrical Characteristics on page 4 is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Table 3. Absolute Maximum Ratings
Parameter Min Max Units Comments
VIN to GND -0.3 6 V
SW, EN, FB to GND -0.3 V
IN
+ 0.3 V
Thermal Resistance Θ
JA
207.4 ºC/W on PCB
ESD 2 kV HBM MIL-Std. 883E 3015.7 methods
Latch-Up -100 +100 mA JEDEC 78
Operating Temperature Range -40 +85 ºC
Storage Temperature Range -65 +125 ºC
Package Body Temperature +260 ºC
The reflow peak soldering temperature (body
temperature) specified is in accordance with IPC/
JEDEC J-STD-020 “Moisture/Reflow Sensitivity
Classification for Non-Hermetic Solid State Surface
Mount Devices”.
The lead finish for Pb-free leaded packages is matte tin
(100% Sn).
Junction Temperature 125 ºC
Junction temperature (T
J
) is calculated from the
ambient temperature (T
AMB
) and power dissipation
(PD) as:
T
J
= T
AMB
+ (PD)(207.4ºC/W) (EQ 1)
Moisture Sensitive Level 1 Represents an unlimited floor life time
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 4 - 21
AS1324
Datasheet - E l e c t r i c a l C h a r a c te r i s t i c s
6 Electrical Characteristics
V
IN
= EN = 3.6V, V
OUT
< V
IN
- 0.5V, T
AMB
= -40 to +85°C,
typ. values @ T
AMB
= +25ºC
(unless otherwise specified).
Note: All limits are guaranteed. The parameters with min and max values are guaranteed with production tests or SQC (Statistical Quality
Control) methods.
Table 4. Electrical Characteristics
Symbol Parameter Conditions Min Typ Max Units
V
IN
Input Voltage Range 2.7 5.5 V
I
Q
Quiescent Current Powersave Mode; V
FB
= 0.62V or V
OUT
= 103%,
I
OUT
= 0mA,
T
AMB
= +25ºC
30 35 µA
I
SHDN
Shutdown Current Shutdown Mode; V
EN
= 0V,
T
AMB
= +25ºC
0.1 1
Regulation
V
FB
Regulated Feedback Voltage
1
1. The device is tested in a proprietary test mode where V
FB
is connected to the output of the error amplifier.
AS1324, I
OUT
= 100mA 0.585 0.6 0.615 V
∆V
FB
Reference Voltage
Line Regulation V
IN
= 2.7V to 5.5V 0.1 1 %/V
I
VFB
Feedback Current
T
AMB
= +25ºC
-30 30 nA
V
OUT
Regulated Output Voltage
AS1324-AD, I
OUT
= 100mA
2
2. Please see Feedback Resistor Selection on page 13 for resistor values.
V
FB
V
AS1324-12, I
OUT
= 100mA 1.164 1.20 1.236
AS1324-15, I
OUT
= 100mA 1.455 1.50 1.545
AS1324-18, I
OUT
= 100mA 1.746 1.80 1.854
∆V
OUT
Output Voltage
Line Regulation V
IN
= 2.7 to 5.5V 0.1 1 %/V
V
LOADREG
Output Voltage
Load Regulation I
OUT
= 0 to 100mA 0.02 %/mA
DC-DC Switches
I
PK
Peak Inductor Current V
IN
= 3V, V
FB
= 0.5V or V
OUT
= 90%, T
AMB
=
25ºC 0.5 0.75 1 A
R
PFET
P-Channel FET R
DS(ON)
I
LSW
= 100mA 0.4 Ω
R
NFET
N-Channel FET R
DS(ON)
I
LSW
= -100mA 0.35 Ω
I
LSW
SW Leakage V
EN
= 0V, V
SW
= 0V or 5V ±0.01 ±1 µA
Control Inputs
V
EN
EN Threshold 0.3 1 1.5 V
I
EN
EN Leakage Current ±0.01 ±1 µA
Oscillator
f
OSC
Oscillator Frequency V
FB
= 0.6V or V
OUT
= 100% 1.2 1.5 1.8 MHz
V
FB
= 0V or V
OUT
= 0V, T
AMB
= 25ºC 115 kHz
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 5 - 21
AS1324
Datasheet - Ty p i c a l O p e r a t i n g Ch a r a c t e r i s t i c s
7 Typical Operating Characteristics
Parts used for measurement: 4.7µH (MOS6020-472) Inductor, 10µF (GRM188R60J106ME47) C
IN
and C
OUT
.
Figure 3. Efficiency vs. Input Voltage; V
OUT
= 1.8V Figure 4. Efficiency vs. Output Current; V
OUT
= 1.2V
50
55
60
65
70
75
80
85
90
95
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
Input Voltage (V)
Efficiency (%) .
IOUT = 600mA
IOUT = 100mA
IOUT = 10mA
IOUT = 1mA
50
55
60
65
70
75
80
85
90
95
100
1 10 100 1000
Output Current (mA)
Efficiency (%) .
VIN = 2.5V
VIN = 2.7V
VIN = 3.7V
VIN = 4.2V
VIN = 5.5V
Figure 5. Efficiency vs. Output Current; V
OUT
= 1.5V Figure 6. Efficiency vs. Output Current; V
OUT
= 1.8V
50
55
60
65
70
75
80
85
90
95
100
1 10 100 1000
Output Current (mA)
Efficiency (%) .
VIN = 2.5V
VIN = 2.7V
VIN = 3.7V
VIN = 4.2V
VIN = 5.5V
50
55
60
65
70
75
80
85
90
95
100
1 10 100 1000
Output Current (mA)
Efficiency (%) .
VIN = 2.5V
VIN = 2.7V
VIN = 3.7V
VIN = 4.2V
VIN = 5.5V
Figure 7. Efficiency vs. Output Current; V
OUT
= 2.5V Figure 8. Efficiency vs. Output Current; V
OUT
= 3.3V
50
55
60
65
70
75
80
85
90
95
100
1 10 100 1000
Output Current (mA)
Efficiency (%) .
VIN = 3.7V
VIN = 4.2V
VIN = 5.5V
50
55
60
65
70
75
80
85
90
95
100
1 10 100 1000
Output Current (mA)
Efficiency (%) .
VIN = 4.2V
VIN = 5.5V
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 6 - 21
AS1324
Datasheet - Ty p i c a l O p e r a t i n g Ch a r a c t e r i s t i c s
Figure 9. Switching Frequency vs. Supply Voltage Figure 10. Switching Frequency vs. Temperature
1.4
1.45
1.5
1.55
1.6
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
Input Voltage (V)
Switching Frequency (MHz) .
1.4
1.45
1.5
1.55
1.6
-45 -30 -15 0 15 30 45 60 75 90
Temperature (°C)
Switching Frequency (MHz) .
Figure 11. Feedback Voltage vs. Temperature Figure 12. Output Voltage vs. Input Voltage
0.59
0.595
0.6
0.605
0.61
-45 -30 -15 0 15 30 45 60 75 90
Temperature (C°)
Feedback Voltage (V) .
1.6
1.65
1.7
1.75
1.8
1.85
1.9
1.95
2
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
Input Voltage (V)
Output Voltage (V) .
IOUT = 600mA
IOUT = 100mA
IOUT = 10mA
IOUT = 1mA
IOUT = 100µA
Figure 13. V
OUT
vs. I
OUT
; V
OUTNOM
= 1.2V Figure 14. V
OUT
vs. I
OUT
; V
OUTNOM
= 1.5V
1.1
1.15
1.2
1.25
1.3
0 100 200 300 400 500 600
Output Current (mA)
Output Voltage (V) .
Vin=2.5V
Vin=2.7V
Vin=5.5V
1.4
1.45
1.5
1.55
1.6
0 100 200 300 400 500 600
Output Current (mA)
Output Voltage (V) .
Vin=2.5V
Vin=2.7V
Vin=5.5V
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 7 - 21
AS1324
Datasheet - Ty p i c a l O p e r a t i n g Ch a r a c t e r i s t i c s
Figure 15. Quiescent Current vs. Input Voltage Figure 16. Quiescent Current vs. Temperature
0
5
10
15
20
25
30
35
40
45
50
2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
Input Voltage (V)
Quiescent Current (µA) .
0
5
10
15
20
25
30
35
40
45
50
-45 -30 -15 0 15 30 45 60 75 90
Temperature (°C)
Quiescent Current (µA) .
Figure 17. Load Step 0mA to 600mA Figure 18. Load Step 10mA to 200mA
I
SW
V
OUT
I
OUT
500mA/DIV 200mV/DIV
500µs/DIV
600mA/DIV
500mA/DIV 200mV/DIV
I
SW
500µs/DIV
V
OUT
I
OUT
200mA/DIV
Figure 19. Startup Figure 20. Powersave Mode
560mA/DIV 5V/DIV
EN
I
SW
1ms/DIV
V
OUT
1V/DIV
100mV/DIV 5V/DIV
V
SW
I
SW
5µs/DIV
V
OUT
200mA/DIV
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 8 - 21
AS1324
Datasheet - D e t a i l e d D e s c r i p t i o n
8 Detailed Description
The AS1324 is a high-efficiency buck converter that uses a constant-frequency current-mode architecture. The device contains two internal
MOSFET switches and is available in adjustable- and fixed-output voltage versions.
Figure 21. AS1324 - Block Diagram
AS1324
OSC
Frequency
Shift
0.6V
Reference
+
–
Error
Amp
Shutdown
Ramp
Compensator
R2
0.6V
FB
OSCN
Digital
Logic
0.6V +
∆VOVL
0.6V -
∆VOVL
–
OVDET
+
–
+
–
ICOMP
+
–
IRCMP
+
Anti-
Shoot
Through
Not applicable to AS1324
AS1324-12: R1 + R2 = 600kΩ
AS1324-15: R1 + R2 = 750kΩ
AS1324-18: R1 + R2 = 900kΩ
R1
PMOS
1
EN
5
V
OUT
/V
FB
4
VIN
3
SW
2
GND
NMOS
C
OUT
10µF
V
OUT
4.7µH
C
IN
10µF
V
IN
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 9 - 21
AS1324
Datasheet - D e t a i l e d D e s c r i p t i o n
8.1 Main Control Loop
During PWM operation the converters use a 1.5MHz fixed-frequency, current-mode control scheme. Basis of the current-mode PWM controller is
an open-loop, multiple input comparator that compares the error-amp voltage feedback signal against the sum of the amplified current-sense
signal and the slope-compensation ramp. At the beginning of each clock cycle, the internal high-side PMOS turns on until the PWM comparator
trips. During this time the current in the inductor ramps up, sourcing current to the output and storing energy in the inductor’s magnetic field.
When the PMOS turns off, the internal low-side NMOS turns on. Now the inductor releases the stored energy while the current ramps down, still
providing current to the output. The output capacitor stores charge when the inductor current exceeds the load and discharges when the inductor
current is lower than the load. Under overload conditions, when the inductor current exceeds the current limit, the high-side PMOS is turned off
and the low-side NMOS remains on until the next clock cycle.
When the PMOS is off, the NMOS is turned on until the inductor current starts to reverse (as indicated by the current reversal comparator
(IRCMP)), or the next clock cycle begins. The IRCMP detects the zero crossing.
The peak inductor current (I
PK
) is controlled by the error amplifier. When I
OUT
increases, V
FB
decreases slightly relative to the internal 0.6V
reference, causing the error amplifier’s output voltage to increase until the average inductor current matches the new load current.
The over-voltage detection comparator (OVDET) guards against transient overshoots by turning the main switch off and keeping it off until the
transient is removed.
8.2 Powersave Operation
The AS1324 uses an automatic powersave mode where the peak inductor current (I
PK
) is set to approximately 200mA while independent of the
output load. In powersave mode, load current is supplied solely from the output capacitor. As the output voltage drops, the error amplifier output
rises above the powersave threshold signaling to switch into PWM fixed frequency mode and turn the PMOS on. This process repeats at a rate
determined by the load demand.
Each burst event can last from a few cycles at light loads to almost continuous cycling (with short sleep intervals) at moderate loads. In between
bursts, the power MOSFETs are turned off, as is any unneeded circuitry, reducing quiescent current to 30µA.
8.3 Short-Circuit Protection
In cases where the AS1324 output is shorted to ground, the oscillator frequency (f
OSC
) is reduced to 1/13 the nominal frequency (
≅
115kHz).
This frequency reduction ensures that the inductor current has more time to decay, thus preventing runaway conditions. f
OSC
will progressively
increase to 1.5MHz when V
FB
/V
OUT
> 0V.
8.4 Shutdown
Connecting EN to GND or logic low places the AS1324 in shutdown mode and reduces the supply current to 0.1µA. In shutdown the control
circuitry and the internal NMOS and PMOS turn off and SW becomes high impedance disconnecting the input from the output. The output
capacitance and load current determine the voltage decay rate. For normal operation connect EN to V
IN
or logic high.
Note: Pin EN should not be left floating.
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 10 - 21
AS1324
Datasheet - A p p l ic a t i o n I n f o r m a t i o n
9 Application Information
The AS1324 is perfect for mobile communications equipment like cell phones and smart phones, digital cameras and camcorders, portable MP3
and DVD players, PDA’s and palmtop computers and any other handheld instruments.
Figure 22. Single Li-Ion 1.2V/600mA Regulator for High-Efficiency
Figure 23. 5V Input to 3.3V/600mA Buck Regulator
Figure 24. Single Li-Ion 1.5V/600mA Regulator for High-Efficiency
AS1324
COUT
10µF
CIN
2.2µF
VIN
2.7 to 4.2V
VOUT
1.2V
600mA
4.7µH
301kΩ
22pF
301kΩ
R2
R
1
3
SW
4
VIN
1
EN
5
V
FB
GND
2
COUT
10µF
CIN
4.7µF
VIN
5V
VOUT
3.3V
600mA
4.7µH
66.5kΩ
22pF
301kΩ
R2
R1
AS1324
3
SW
4
VIN
1
EN
5
V
FB
GND
2
COUT
10µF
CIN
4.7µF
VIN
2.7 to 4.2V
VOUT
1.5V
600mA
4.7µH
AS1324-15
3
SW
4
VIN
1
EN
5
V
OUT
GND
2
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 11 - 21
AS1324
Datasheet - A p p l ic a t i o n I n f o r m a t i o n
Figure 25. Single Li-Ion 1.8V/600mA Regulator for Low Output Ripple
9.1 External Component Selection
9.2 Inductor Selection
For most applications the value of the external inductor should be in the range of 2.2 to 6.8µH as the inductor value has a direct effect on the
ripple current. The selected inductor must be rated for its DC resistance and saturation current. The inductor ripple current (∆I
L
) decreases with
higher inductance and increases with higher V
IN
or V
OUT
.
In Equation (EQ 2) the maximum inductor current in PWM mode under static load conditions is calculated. The saturation current of the inductor
should be rated higher than the maximum inductor current as calculated with Equation (EQ 3). This is recommended because the inductor
current will rise above the calculated value during heavy load transients.
Where:
f = Switching Frequency (1.5 MHz typical)
L = Inductor Value
I
Lmax
= Maximum Inductor current
∆
I
L =
Peak to Peak inductor ripple current
The recommended starting point for setting ripple current is ∆
I
L
= 240mA (40% of 600mA).
The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation.
Thus, a 720mA rated inductor should be sufficient for most applications (600mA + 120mA). A easy and fast approach is to select the inductor
current rating fitting to the maximum switch current limit of the converter.
Note: For highest efficiency, a low DC-resistance inductor is recommended.
Accepting larger values of ripple current allows the use of low inductance values, but results in higher output voltage ripple, greater core losses,
and lower output current capability.
The total losses of the coil have a strong impact on the efficiency of the dc/dc conversion and consist of both the losses in the dc resistance and
the following frequency-dependent components:
1. The losses in the core material (magnetic hysteresis loss, especially at high switching frequencies)
2. Additional losses in the conductor from the skin effect (current displacement at high frequencies)
3. Magnetic field losses of the neighboring windings (proximity effect)
4. Radiation losses
COUT
22µF
CIN
10µF
VOUT
1.8V
600mA
4.7µH
VIN
2.7 to 4.2V
AS1324-18
3
SW
4
VIN
1
EN
5
V
OUT
GND
2
(EQ 2)
∆I
L
V
OUT
1V
OUT
V
IN
--------------
–
L f×
------------------------
×=
(EQ 3)
I
LMAX
I
OUTMAX
∆I
L
2
--------
+=
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 12 - 21
AS1324
Datasheet - A p p l ic a t i o n I n f o r m a t i o n
Figure 26. Efficiency Comparison of Different Inductors, V
IN
= 2.7V, V
OUT
= 1.8V and 1.2V
9.3 Output Capacitor Selection
The advanced fast-response voltage mode control scheme of the AS1324 allows the use of tiny ceramic capacitors. Because of their lowest
output voltage ripple low ESR ceramic capacitors are recommended. X7R or X5R dielectric output capacitor are recommended.
At high load currents, the device operates in PWM mode and the RMS ripple current is calculated as:
While operating in PWM mode the overall output voltage ripple is the sum of the voltage spike caused by the output capacitor ESR plus the
voltage ripple caused by charging and discharging the output capacitor:
Higher value, low cost ceramic capacitors are available in very small case sizes, and their high ripple current, high voltage rating, and low ESR
make them ideal for switching regulator applications. Because the AS1324 control loop is not dependant on the output capacitor ESR for stable
operation, ceramic capacitors can be used to achieve very low output ripple and accommodate small circuit size.
At light loads, the converter operates in powersave mode and the output voltage ripple is in direct relation to the output capacitor and inductor
value used. Larger output capacitor and inductor values minimize the voltage ripple in powersave mode and tighten DC output accuracy in
powersave mode.
Table 5. Recommended Inductors
Part Number L DCR Current Rating Dimensions (L/W/T) Manufacturer
LQH32CN2R2M33 2.2µH 97mΩ790mA 3.2x2.5x2.0mm Murata
www.murata.com
LQH32CN4R7M33 4.7µH 150mΩ650mA 3.2x2.5x2.0mm
LPS3008-222MLC 2.2µH 175mΩ1100mA 3.1x3.1x0.8mm Coilcraft
www.coilcraft.com
LPS3015-222MLC 2.2µH 110mΩ2000mA 3.1x3.1x1.5mm
MOS6020-222MLC 2.2µH 35mΩ3260mA 6.0x6.8x2.4mm
MOS6020-472MLC 4.7µH 50mΩ1820mA 6.0x6.8x2.4mm
CDRH3D16NP-2R2N 2.2µH 72mΩ1200mA 4.0x4.0x1.8mm Sumida
www.sumida.com
CDRH3D16ND-4R7N 4.7µH 105mΩ900mA 4.0x4.0x1.8mm
70
75
80
85
90
95
1 10 100 1000
Output Current (mA)
Efficiency (%) .
LQH32CN2R2
LPS3008-222
LPS3015-222 MOS6020-222
LQH32CN4R7 MOS6020-472
70
75
80
85
90
95
1 10 100 1000
Output Current (mA)
Efficiency (%) .
LQH32CN2R2
LPS3008-222
LPS3015-222
MOS6020-222
LQH32CN4R7 MOS6020-472
V
OUT
= 1.8V V
OUT
= 1.2V
(EQ 4)
I
RMSC
OUT
V
OUT
1V
OUT
V
IN
--------------
–
L f×
------------------------ 1
2 3×
-----------------
××=
(EQ 5)
∆V
OUT
V
OUT
1V
OUT
V
IN
--------------
–
L f×
------------------------ 1
8 C
OUT
×f×
-------------------------------- ESR+
××=
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 13 - 21
AS1324
Datasheet - A p p l ic a t i o n I n f o r m a t i o n
9.4 Input Capacitor Selection
In continuous mode, the source current of the PMOS is a square wave of the duty cycle V
OUT
/V
IN
. To prevent large voltage transients while
minimizing the interference with other circuits caused by high input voltage spikes, a low ESR input capacitor sized for the maximum RMS
current must be used. The maximum RMS capacitor current is given as:
where the maximum average output current I
MAX
equals the peak current minus half the peak-to-peak ripple current, I
MAX
= I
LIM
- ∆I
L
/2
This formula has a maximum at V
IN
= 2V
OUT
where I
RMS
= I
OUT
/2. This simple worst-case condition is commonly used for design because
even significant deviations only provide negligible affects.
The input capacitor can be increased without any limit for better input voltage filtering. Take care when using small ceramic input capacitors.
When a small ceramic capacitor is used at the input, and the power is being supplied through long wires, such as from a wall adapter, a load step
at the output, or V
IN
step on the input, can induce ringing at the VIN pin. This ringing can then couple to the output and be mistaken as loop
instability, or could even damage the part by exceeding the maximum ratings.
9.4.1 Ceramic Input and Output Capacitors
When choosing ceramic capacitors for C
IN
and C
OUT
, the X5R or X7R dielectric formulations are recommended. These dielectrics have the
best temperature and voltage characteristics for a given value and size. Y5V and Z5U dielectric capacitors, aside from their wide variation in
capacitance over temperature, become resistive at high frequencies and therefore should not be used.
Because ceramic capacitors lose a lot of their initial capacitance at their maximum rated voltage, it is recommended that either a higher input
capacity or a capacitance with a higher rated voltage is used.
9.5 Feedback Resistor Selection
In the AS1324-AD, the output voltage is set by an external resistor divider connected to V
FB
(see Figure 27). This circuitry allows for remote
voltage sensing and adjustment.
Figure 27. Setting the AS1324 Output Voltage
Resistor values for the circuit shown in Figure 27 can be calculated as:
The output voltage can be adjusted by selecting different values for R
1
and R
2
. For R
1
a value between 10kΩ and 500kΩ is recommended. A
higher resistance of R
1
and R
2
will result in a lower leakage current at the output. It is recommended to keep V
IN
500mV higher than V
OUT
.
Table 6. Recommended Input and Output Capacitor
Part Number C TC Code Rated Voltage Dimensions (L/W/T) Manufacturer
JMK212BJ226MG-T 22µF X5R 6.3V 0805 Taiyo Yuden
www.t-yuden.com
GRM188R60J106ME47 10µF X5R 6.3V 0603 Murata
www.murata.com
GRM21BR71A475KA73 4.7µF X7R 10V 0805
(EQ 6)
I
RMS
I
MAX
V
OUT
V
IN
V
OUT
–( )× V
IN
------------------------------------------------------------
×=
R2
0.6V ≤ VOUT ≤ 5.5V
R1
AS1324
5
VFB
2
GND
R1<<R2
(EQ 7)
V
OUT
0,6 1 R
2
R
1
-------
+×=
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 14 - 21
AS1324
Datasheet - A p p l ic a t i o n I n f o r m a t i o n
9.6 Efficiency
The efficiency of a switching regulator is equivalent to:
Efficiency = (P
OUT
/P
IN
)100% (EQ 8)
For optimum design, an analysis of the AS1324 is needed to determine efficiency limitations and to determine design changes for improved
efficiency. Efficiency can be expressed as:
Efficiency = 100% – (L
1
+ L
2
+ L
3
+ ...) (EQ 9)
Where:
L
1
, L
2
, L
3
, etc. are the individual losses as a percentage of input power.
Although all dissipative elements in the circuit produce losses, those four main sources should be considered for efficiency calculation:
9.6.1 Input Voltage Quiescent Current Losses
The V
IN
current is the DC supply current given in the electrical characteristics which excludes MOSFET driver and control currents. V
IN
current
results in a small (<0.1%) loss that increases with V
IN
, even at no load. The V
IN
quiescent current loss dominates the efficiency loss at very low
load currents.
9.6.2 I²R Losses
Most of the efficiency loss at medium to high load currents are attributed to I²R loss, and are calculated from the resistances of the internal
switches (R
SW)
and the external inductor (R
L
). In continuous mode, the average output current flowing through inductor L is split between the
internal switches. Therefore, the series resistance looking into the SW pin is a function of both NMOS & PMOS R
DS(ON)
as well as the duty
cycle (DC) and can be calculated as follows:
R
SW
= (R
DS(ON)PMOS
)(DC) + (R
DS(ON)NMOS
)(1 – DC) (EQ 10)
The R
DS(ON)
for both MOSFETs can be obtained from the Electrical Characteristics on page 4. Thus, to obtain I²R losses calculate as follows:
I²R losses = I
OUT
²(R
SW
+ R
L
) (EQ 11)
9.6.3 Switching Losses
The switching current is the sum of the control currents and the MOSFET driver. The MOSFET driver current results from switching the gate
capacitance of the power MOSFETs. If a MOSFET gate is switched from low to high to low again, a packet of charge dQ moves from V
IN
to
ground. The resulting dQ/dt is a current out of V
IN
that is typically much larger than the DC bias current. In continuous mode:
I
GC
= f(Q
PMOS
+ Q
NMOS
)(EQ 12)
Where: Q
PMOS
and Q
NMOS
are the gate charges of the internal MOSFET switches.
The losses of the gate charges are proportional to V
IN
and thus their effects will be more visible at higher supply voltages.
9.6.4 Other Losses
Basic losses in the design of a system should also be considered. Internal battery resistances and copper trace can account for additional
efficiency degradations in battery operated systems. By making sure that C
IN
has adequate charge storage and very low ESR at the given
switching frequency, the internal battery and fuse resistance losses can be minimized. C
IN
and C
OUT
ESR dissipative losses and inductor core
losses generally account for less than 2% total additional loss.
9.7 Thermal Shutdown
Due to its high-efficiency design, the AS1324 will not dissipate much heat in most applications. However, in applications where the AS1324 is
running at high ambient temperature, uses a low supply voltage, and runs with high duty cycles (such as in dropout) the heat dissipated may
exceed the maximum junction temperature of the device.
As soon as the junction temperature reaches approximately 150ºC the AS1324 goes in thermal shutdown. In this mode the internal PMOS &
NMOS switch are turned off. The device will power up again, as soon as the temperature falls below +145°C again.
9.8 Checking Transient Response
The main loop response can be evaluated by examining the load transient response. Switching regulators normally take several cycles to
respond to a step in load current. When a load step occurs, V
OUT
immediately shifts by an amount equivalent to:
V
DROP
= ∆I
OUT
x ESR (EQ 13)
Where:
ESR is the effective series resistance of C
OUT
.
∆I
OUT
also begins to charge or discharge C
OUT
, which generates a feedback error signal. The regulator loop then acts to return V
OUT
to its
steady-state value. During this recovery time V
OUT
can be monitored for overshoot or ringing that would indicate a stability problem.
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 15 - 21
AS1324
Datasheet - A p p l ic a t i o n I n f o r m a t i o n
9.9 Design Example
Figure 28 shows the AS1324 used in a single lithium-ion (3.7V typ) battery-powered mobile phone application. The load current requirement is
600mA (max) but most of the time the device will require only 2mA (standby mode current).
Figure 28. Design Example
For the circuit shown in Figure 28, efficiency at low- and high-load currents is an important consideration when selecting the value for the
external inductor, which is calculated as:
From (EQ 14), substituting V
OUT
= 2.2V, V
IN
= 3.7V, ∆I
L
= 240mA and f = 1.5MHz gives:
Therefore, a standard 2.2µH inductor should be used for this design.
For best overall efficiency use an inductor with a rating of 720mA or greater and less than 0.2Ω series resistance. C
IN
will require an RMS
current rating of at least 0.3A
≅
I
LOAD(MAX)
/2, whereas C
OUT
will require an ESR of less than 0.25Ω. In most cases, a ceramic capacitor will
satisfy this requirement.
For the feedback resistors, select the value for R
1
= 375kΩ. R
2
can then be calculated from (EQ 7) to be:
R
2
= (V
OUT
/0.6 - 1)375k = 1000kΩ
COUT
10µF
CER
CIN
4.7µF
CER
VIN
3.7V
VOUT
2.2V
2.2µH
375kΩ
22pF
1MΩ
R2
R1
AS1324
3
SW
4
VIN
1
EN
5
V
FB
GND
2
(EQ 14)
LV
OUT
f∆I
L
-------------- 1V
OUT
V
IN
--------------
–
×=
(EQ 15)
L2,2V
1,5MHz 240mA×( )
---------------------------------------------------- 12,2V
3 7V,
------------
–
×2,48µH= =
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 16 - 21
AS1324
Datasheet - A p p l ic a t i o n I n f o r m a t i o n
9.10 Layout Considerations
The AS1324 requires proper layout and design techniques for optimum performance.
The power traces (GND, SW, and V
IN
) should be kept as short, direct, and wide as is practical.
Pin V
FB
(AS1324 only) should be connected directly to the feedback resistors (R
1
and R
2
). A potentiometer as replacement for R
1
and R
2
should be avoided to minimize the output voltage ripple and to maintain the stability of the regulator.
The resistive divider (R
1
/R
2
) must be connected between the positive plate of C
OUT
and ground.
The positive plate of C
IN
should be connected as close to V
IN
as is practical since C
IN
provides the AC current to the internal power MOS-
FETs.
Switching node SW should be kept far away from the sensitive V
FB
node.
The negative plates of C
IN
and C
OUT
should be kept as close to each other as is practical. A starpoint to Ground is recommended.
Figure 29. AS1324 Basic PCB Layout
Figure 30. AS1324 Basic Diagram
5
AS1324
43
2
1
Via to GND
Via to V
OUT
V
OUT
SW
GND
V
IN
Via to V
IN
COUT CIN
L1
R1
R2
CFWD
AS1324
1
EN
4
VIN
3
SW
5
VFB
2
GND
C
IN
L
1
R
1
R
2
C
FWD
High Current Path
V
IN
C
OUT
V
OUT
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 17 - 21
AS1324
Datasheet - A p p l ic a t i o n I n f o r m a t i o n
Figure 31. AS1324-18 Basic PCB Layout
Figure 32. AS1324-18 Basic Diagram
5
AS1324-18
43
2
1
V
OUT
SW
GND
V
IN
COUT CIN
L1
Via to V
IN
Via to V
OUT
AS1324-18
1
EN
4
VIN
3
SW
5
VOUT
2
GND
V
IN
C
IN
L
1
High Current Path
C
OUT
V
OUT
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 20 - 21
AS1324
Datasheet
11 Ordering Information
The device is available as the following standard versions.
Note: All products are RoHS compliant.
Buy our products or get free samples online at ICdirect: http://www.ams.com/ICdirect
Technical Support is found at http://www.ams.com/Technical-Support
For further information and requests, please contact us mailto:sales@ams.com
or find your local distributor at http://www.ams.com/distributor
ams
Table 7. Ordering Information
Ordering Code Marking Output Description Delivery Form Package
AS1324-BTTT-AD ASKR adjustable 1.5MHz, 600mA Synchronous DC/DC
Converter Tape and Reel 5-pin TSOT-23
AS1324-BTTT-12 ASKT 1.2V 1.5MHz, 600mA Synchronous DC/DC
Converter Tape and Reel 5-pin TSOT-23
AS1324-BTTT-15 ASKU 1.5V 1.5MHz, 600mA Synchronous DC/DC
Converter Tape and Reel 5-pin TSOT-23
AS1324-BTTT-18 ASKS 1.8V 1.5MHz, 600mA Synchronous DC/DC
Converter Tape and Reel 5-pin TSOT-23
www.ams.com/DC-DC_Step-Up/AS1324 Revision 1.06 21 - 21
AS1324
Datasheet - O r d e r i n g I n f o r m a ti o n
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devices from patent infringement. ams AG reserves the right to change specifications and prices at any time and without notice. Therefore, prior
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