RT8258
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DS8258-02 March 2011 www.richtek.com
Ordering Information
Note :
Richtek products are :
` RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
` Suitable for use in SnPb or Pb-free soldering processes.
1.2A, 24V, 700kHz Step-Down Converter
General Description
The RT8258 is a high voltage buck converter that can support
the input voltage range from 4.5V to 24V and the output
current can be up to 1.2A. Current Mode operation provides
fast transient response and eases loop stabilization.
The chip also provides protection functions such as cycle-
by-cycle current limiting and thermal shutdown protection.
The RT8258 is available in a SOT-23-6 and TSOT-23-6
packages.
Features
zz
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zWide Operating Input Voltage Range : 4.5V to 24V
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zAdjustable Output Voltage Range : 0.8V to 15V
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z1.2A Output Current
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z0.3ΩΩ
ΩΩ
Ω Internal Power MOSFET Switch
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zHigh Efficiency up to 92%
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z700kHz Fixed Switching Frequency
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zSta ble with Low ESR Output Ceramic Capacitors
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zThermal Shutdown
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zCycle-By-Cycle Over Current Protection
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zRoHS Compliant and Halogen Free
Applications
zDistributed Power Systems
zBattery Charger
zPre-Regulator for Linear Regulators
zWLED Drivers
Typical Application Circuit
Pin Configurations
(TOP VIEW)
Marking Information
For marking information, contact our sales representative
directly or through a Richtek distributor located in your
area.
BOOT GND FB
PHASE VIN EN
4
23
56
SOT-23-6/TSOT-23-6
VIN
EN
GND
BOOT
FB
PHASE
4
2
3
5
6
1
L
10µH
CBOOT
10nF
COUT
22µF
R1
100k
R2
32.4k
VOUT
3.3V
CIN
10µF
Chip Enable
VIN
4.5V to 24V
RT8258
D1
B230A
RT8258
Package Type
E : SOT-23-6
J6 : TSOT-23-6
Lead Plating System
G : Green (Halogen Free and Pb Free)
RT8258
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DS8258-02 March 2011www.richtek.com
Function Block Diagram
Table 1. Recommended Component Selection
Functional Pin Description
Pin No. Pin Name Pin Function
1 BOOT
Gate Driver Bootstrap Input Pin. Connect a 10nF or greater capacitor between PHASE
and BOOT pins to supply the MOSFET driver.
2 GND Ground Pin. This pin should be connected to the (-) terminal of the output capacitor and
it should be kept away from the D1 and input capacitor for noise prevention.
3 FB
Output Voltage Feedback Input Pin. An external resistor divider from the output to GND
tapped to the FB pin sets the output voltage. The value of the divider resistors also set
loop bandwidth.
4 EN Chip Enable (Active High). If the EN pin is open, it will be pulled to high by internal
circuit.
5 VIN Power Supply Input Pin. Bypass VIN to GND with a suitable large capacitor to prevent
large voltage spikes from appear ing at the input.
6 PHASE Power Switching Output Pin. Connect this pin to the output inductor.
VOUT 1.2V 1.5V 1.8V 2.5V 3.3V 5V 8V 10V 15V
R1 (kΩ) 100 91 91 100 100 91 91 91 120
R2 (kΩ) 200 100 75 47 32.4 17.4 10 7.87 6.8
L (μH) 3.6 3.6 4.7 6.8 10 15 22 22 33
COUT (μF) 22 22 22 22 22 22 22 22 22
Note : The value of R1 is related to the loop bandwidth of the RT8258. It is strongly recommended to follow the
parameters in above table for the specific output voltage.
Driver
R
Q
S
Bootstrap
Control
+
-
Ramp
Generator
Oscillator
700kHz
+
-PWM
Comparator
EA
Reference
Regulator
+
-
1.1V
1µA
+
-
400k 30pF
1pF
Shutdown
Comparator
OC Limit Clamp
Current Sense Amp
X20
BOOT
GND
FB
EN
VIN
PHASE
10k
3V
25mΩ
RT8258
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DS8258-02 March 2011 www.richtek.com
Electrical Characteristics
Parameter Symbol Test Conditions Min Typ Max Unit
Feedback Reference Voltage VFB 4.5V ≤ VIN ≤ 24V 0.784 0.8 0.816 V
Feedback Current IFB V
FB = 0.8V -- 0.1 0.3 μA
Switch On Resistance RDS(ON)
-- 0.3 -- Ω
Switch Leakage VEN = 0V, VPHASE = 0V -- -- 10 μA
Current Limit ILIM V
BOOT − VPHASE = 4.8V 1.6 2.1 -- A
Oscillator Frequency fSW 600 700 800 kHz
Maximum Duty Cycle -- 90 -- %
Minimum On-Time tON -- 100 -- ns
Under Voltage Lock out
Threshold Voltage Rising 3.9 4.2 4.5 V
Under Voltage Lock out
Threshold Hysteresis -- 270 -- mV
Logic High 1.4 -- --
EN Input Voltage Logic Low -- -- 0.4 V
EN Pull Up Current VEN = 0V -- 1 -- μA
Shutdown Current ISHDN V
EN = 0V -- 25 -- μA
Quiescent Current IQ V
EN = 2V, VFB = 1V (No Switching) -- 0.55 1 mA
Thermal Shutdown TSD -- 150 -- °C
(VIN = 12V, TA = 25°C unless otherwise specified)
Absolute Maximum Ratings (Note 1)
zSupply Voltage, VIN ------------------------------------------------------------------------------------------------ 26V
zPHASE Voltage ----------------------------------------------------------------------------------------------------- −0.3V to (VIN + 0.3V)
zBOOT Voltage ------------------------------------------------------------------------------------------------------- VPHASE + 6V
zAll Other Pins-------------------------------------------------------------------------------------------------------- 0.3V to 6V
zPower Dissipation, PD @ TA = 25°C
T/SOT-23-6 ----------------------------------------------------------------------------------------------------------- 0.4W
zPackage Thermal Resistance (Note 2)
T/SOT-23-6, θJA ------------------------------------------------------------------------------------------------------ 250°C/W
zJunction Temperature ---------------------------------------------------------------------------------------------- 150°C
zLead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------ 260°C
zStorage Temperature Range -------------------------------------------------------------------------------------- −65°C to 150°C
zESD Susceptibility (Note 3)
HBM (Human Body Mode) ---------------------------------------------------------------------------------------- 2kV
MM (Machine Mode) ----------------------------------------------------------------------------------------------- 200V
Recommended Operating Conditions (Note 4)
zSupply Voltage, VIN ------------------------------------------------------------------------------------------------ 4.5V to 24V
zOutput Voltage, VOUT ---------------------------------------------------------------------------------------------- 0.8V to 15V
zEN Voltage, VEN ----------------------------------------------------------------------------------------------------- 0V to 5.5V
zJunction Temperature Range ------------------------------------------------------------------------------------- −40°C to 125°C
zAmbient Temperature Range ------------------------------------------------------------------------------------- −40°C to 85°C
RT8258
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DS8258-02 March 2011www.richtek.com
Note 1. Stresses listed as the above "Absolute Maximum Ratings" may cause permanent damage to the device. These are for
stress ratings. Functional operation of the device at these or any other conditions beyond those indicated in the
operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended
periods may remain possibility to affect device reliability.
Note 2. θJA is measured in the natural convection at TA = 25°C on a low effective single layer thermal conductivity test board of
JEDEC 51-3 thermal measurement standard.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
RT8258
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Typical Operating Characteristics
Efficiency vs. Output Current
0
10
20
30
40
50
60
70
80
90
100
0 0.2 0.4 0.6 0.8 1 1.2
Load Current (A)
Efficiency (%)
Output Voltage vs. Output Current
3.234
3.256
3.278
3.300
3.322
3.344
3.366
0 0.2 0.4 0.6 0.8 1 1.2
Output Current (A)
Output Voltage (V)
Output Voltage v s. Temperature
3.234
3.256
3.278
3.300
3.322
3.344
3.366
-50 -25 0 25 50 75 100 125
Temperature (°C)
Output Voltage (V)
Frequency vs. Temperature
550
600
650
700
750
-50-25 0 25 50 75100125
Temperature (°C)
Frequency (kHz) 1
VOUT = 3.3V
VIN = 12V
VIN = 24V
VIN = 12V
VIN = 24V
IOUT = 0A
VIN = 12V
VIN = 24V
VIN = 12V, VOUT = 3.3V, IOUT = 0A
Quiescent Current vs. Temperature
400
425
450
475
500
525
550
575
600
-50 -25 0 25 50 75 100 125
Temperature (°C)
Quiescent Current (μA
)
VIN = 12V
VIN = 24V
VEN = 2V, VFB = 1V
Efficiency vs. Output Current
0
10
20
30
40
50
60
70
80
90
100
0 0.2 0.4 0.6 0.8 1 1.2
Output Current (A)
Efficiency (%)
VIN = 12V
VIN = 24V
VOUT = 5V
RT8258
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Load Transient Response
Time (250μs/Div)
VOUT
(0.1V/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 0A to 1.2A
IOUT
(0.5A/Div)
Load Transient Response
Time (50μs/Div)
IOUT
(0.5A/Div)
VOUT
(0.1V/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 0.6A to 1.2A
Output Ripple Voltage
Time (500ns/Div)
IL
(1A/Div)
VPHASE
(10V/Div)
VOUT
(10mV/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 1.2A
Output Ripple Voltage
Time (500ns/Div)
IL
(1A/Div)
VPHASE
(10V/Div)
VOUT
(10mV/Div)
VIN = 24V, VOUT = 3.3V, IOUT = 1.2A
Power Off from EN
Time (50μs/Div)
VEN
(2V/Div)
VOUT
(1V/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 2A
Power On from EN
Time (100μs/Div)
VEN
(2V/Div)
VOUT
(1V/Div) VIN = 12V, VOUT = 3.3V, IOUT = 1.2A
RT8258
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DS8258-02 March 2011 www.richtek.com
Application Information
The RT8258 is an asynchronous high voltage buck converter
that can support the input voltage range from 4.5V to 24V
and the output current can be up to 1.2A.
Output Voltage Setting
The resistive voltage divider allows the FB pin to sense a
fraction of the output voltage as shown in Figure 1.
Figure 1. Output Voltage Setting
For adjustable voltage mode, the output voltage is set by
an external resistive voltage divider according to the
following equation :
⎛⎞
+
⎜⎟
⎝⎠
OUT FB R1
V = V1
R2
OUT OUT
LIN
VV
I = 1
fL V
⎡⎤⎡ ⎤
Δ×−
⎢⎥⎢ ⎥
×
⎣⎦⎣ ⎦
Having a lower ripple current reduces not only the ESR
losses in the output capacitors but also the output voltage
ripple. High frequency with small ripple current can achieve
highest efficiency operation. However, it requires a large
inductor to achieve this goal.
For the ripple current selection, the value of ΔIL = 0.34(IMAX)
will be a reasonable starting point. The largest ripple current
occurs at the highest VIN. To guarantee that the ripple
current stays below the specified maximum, the inductor
value should be chosen according to the following
equation :
OUT OUT
L(MAX) IN(MAX)
VV
L = 1
fI V
⎡⎤⎡ ⎤
×−
⎢⎥⎢ ⎥
×Δ
⎣⎦⎣ ⎦
Inductor Core Selection
The inductor type must be selected once the value for L is
known. Generally speaking, high efficiency converters can
not afford the core loss found in low cost powdered iron
cores. So, the more expensive ferrite or mollypermalloy
cores will be a better choice.
The selected inductance rather than the core size for a
fixed inductor value is the key for actual core loss. As the
inductance increases, core losses decrease. Unfortunately,
increase of the inductance requires more turns of wire and
therefore the copper losses will increase.
Ferrite designs are preferred at high switching frequency
due to the characteristics of very low core losses. So,
design goals can focus on the reduction of copper loss
and the saturation prevention.
Ferrite core material saturates “hard”, which means that
inductance collapses abruptly when the peak design current
is exceeded. The previous situation results in an abrupt
increase in inductor ripple current and consequent output
voltage ripple.
Inductor Selection
The inductor value and operating frequency determine the
ripple current according to a specific input and output
voltage. The ripple current ΔIL increases with higher VIN
and decreases with higher inductance.
RT8258
GND
FB
R1
R2
VOUT
External Bootstrap Diode
Connect a 10nF low ESR ceramic capacitor between the
BOOT pin and PHASE pin. This capacitor provides the
gate driver voltage for the high side MOSFET.
It is recommended to add an external bootstrap diode
between an external 5V and the BOOT pin for efficiency
improvement when input voltage is lower than 5.5V or duty
ratio is higher than 65%. The bootstrap diode can be a low
cost one such as 1N4148 or BAT54.
The external 5V can be a 5V fixed input from system or a
5V output of the RT8268.
Where VFB is the feedback reference voltage (0.8V typ.).
Figure 2. External Bootstrap Diode
PHASE
BOOT
RT8258 10nF
5V
RT8258
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DS8258-02 March 2011www.richtek.com
CIN and COUT Selection
The input capacitance, CIN, is needed to filter the trapezoidal
current at the source of the top MOSFET. To prevent large
ripple current, a low ESR input capacitor sized for the
maximum RMS current should be used. The RMS current
is given by :
OUT IN
RMS OUT(MAX) IN OUT
VV
I = I 1
VV
−
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief.
Choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
size or height requirements in the design.
The selection of COUT is determined by the required Effective
Series Resistance (ESR) to minimize voltage ripple.
Moreover, the amount of bulk capacitance is also a key for
COUT selection to ensure that the control loop is stable.
Loop stability can be checked by viewing the load transient
response as described in a later section.
The output ripple, ΔVOUT , is determined by :
OUT L OUT
1
VIESR
8fC
⎡⎤
Δ≤Δ +
⎢⎥
⎣⎦
The output ripple will be highest at the maximum input
voltage since ΔIL increases with input voltage. Multiple
capacitors placed in parallel may be needed to meet the
ESR and RMS current handling requirement. Dry tantalum,
special polymer, aluminum electrolytic and ceramic
capacitors are all available in surface mount packages.
Special polymer capacitors offer very low ESR value.
However, it provides lower capacitance density than other
types. Although Tantalum capacitors have the highest
capacitance density, it is important to only use types that
pass the surge test for use in switching power supplies.
Aluminum electrolytic capacitors have significantly higher
ESR. However, it can be used in cost-sensitive applications
for ripple current rating and long term reliability
considerations. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient and
audible piezoelectric effects. The high Q of ceramic
capacitors with trace inductance can also lead to significant
ringing.
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at input and
output. When a ceramic capacitor is used at the input and
the power is supplied by a wall adapter through long wires,
a load step at the output can induce ringing at the input,
VIN. At best, this ringing can couple to the output and be
mistaken as loop instability. At worst, a sudden inrush of
current through the long wires can potentially cause a
voltage spike at VIN large enough to damage the part.
Do not allow the core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy materials
are small and do not radiate energy. However, they are
usually more expensive than the similar powdered iron
inductors. The rule for inductor choice mainly depends on
the price vs. size requirement and any radiated field/EMI
requirements.
Diode Selection
When the power switch turns off, the path for the current
is through the diode connected between the switch output
and ground. This forward biased diode must have a
minimum voltage drop and recovery times. Schottky diode
is recommended and it should be able to handle those
current. The reverse voltage rating of the diode should be
greater than the maximum input voltage, and current rating
should be greater than the maximum load current. For
more detail, please refer to Table 3.
Checking T ransient Re sponse
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, VOUT immediately shifts by an amount
equal to ΔILOAD (ESR) and also begins to charge or
discharge COUT generating a feedback error signal for the
regulator to return VOUT to its steady-state value. During
this recovery time, VOUT can be monitored for overshoot or
ringing that would indicate a stability problem.
RT8258
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DS8258-02 March 2011 www.richtek.com
Layout Consideration
Follow the PCB layout guidelines for optimal performance
of RT8258.
`Keep the traces of the main current paths as short and
wide as possible.
`Put the input capacitor as close as possible to the device
pins (VIN and GND).
`PHASE node is with high frequency voltage swing and
should be kept at small area. Keep sensitive components
away from the PHASE node to prevent stray capacitive
noise pick-up.
`Place the feedback components to the FB pin as close
as possible.
`Connect the GND to a ground plane for noise reduction
and thermal dissipation.
Thermal Considerations
For continuous operation, do not exceed the maximum
operation junction temperature 125°C. The maximum power
dissipation depends on the thermal resistance of IC
package, PCB layout, the rate of surroundings airflow and
temperature difference between junction to ambient. The
maximum power dissipation can be calculated by following
formula :
PD(MAX) = (TJ(MAX) − TA ) / θJA
where TJ(MAX) is the maximum operation junction
temperature, TA is the ambient temperature and the θJA is
the junction to ambient thermal resistance.
For recommended operating conditions specification of the
RT8258, the maximum junction temperature of the die is
125°C. The junction to ambient thermal resistance θJA is
layout dependent. For T/SOT-23-6 package, the thermal
resistance θJA is 250°C/W on standard JEDEC 51-3 single
layer thermal test board. The maximum power dissipation
at TA = 25°C can be calculated by following formula :
PD(MAX) = (125°C − 25°C) / (250°C/W) = 0.4W for
T/SOT-23-6 package
The maximum power dissipation depends on operating
ambient temperature for fixed TJ(MAX) and thermal resistance
θJA . For RT8258 package, the Figure 3 of derating curve
allows the designer to see the effect of rising ambient
temperature on the maximum power dissipation allowed.
Figure 3. Derating Curve for RT8258 Package
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0255075100125
Ambient Temperature (°C)
Maximum Power Dissipation (W)
T/SOT-23-6
Single Layer PCB
Figure 4. PCB Layout Guide
BOOT
GND
FB EN
VIN
PHASE
4
2
3
5
61
VOUT
CB
R2
R1
CIN
VOUT
L
COUT
GND
D1
RT8258
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DS8258-02 March 2011www.richtek.com
Table 3. Suggested Capacitors for CIN a nd COUT
Component Sup plier Series Di mens io n s (mm)
TDK SLF12555T 12.5x12.5x5.5
TAI Y O YUDEN NR8040 8x8x4
TDK SLF12565T 12.5x12.5x6.5
Ta ble 2. Sugge sted Inductors for L
Component Supplier Series VRRM
(V) IOUT
(A) Package
DIODES B230A 30 2 DO-214AC
DIODES B330A 30 3 DO-214AC
PANJIT SK23 30 2 DO-214AC
PANJIT SK33 30 3 DO-214AB
Table 4. Suggested Diode for D1
Location Component Supplier Part No. Capacitance
(μF) Case Size
CIN MURATA GRM31CR61E106K 10 1206
CIN TDK C3225X5R1E106K 10 1206
CIN TAIYO YUDEN TMK316BJ106ML 10 1206
COUT MURATA GRM31CR61C226M 22 1206
COUT TDK C3225X 5R1C226M 22 1206
COUT TAIYO YUDEN EMK316BJ226ML 22 1206
RT8258
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DS8258-02 March 2011 www.richtek.com
AA1
e
b
B
D
C
H
L
SOT-23-6 Surfa ce Mount Package
Dimensions In Millimeters Dimensions In Inches
Symbol Min Max Min Max
A 0.889 1.295 0.031 0.051
A1 0.000 0.152 0.000 0.006
B 1.397 1.803 0.055 0.071
b 0.250 0.560 0.010 0.022
C 2.591 2.997 0.102 0.118
D 2.692 3.099 0.106 0.122
e 0.838 1.041 0.033 0.041
H 0.080 0.254 0.003 0.010
L 0.300 0.610 0.012 0.024
Outline Dimension
RT8258
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DS8258-02 March 2011www.richtek.com
Information that is provided by Richtek Technology Corporation is believed to be accurate and reliable. Richtek reserves the right to make any change in circuit design,
specification or other related things if necessary without notice at any time. No third party intellectual property infringement of the applications should be guaranteed
by users when integrating Richtek products into any application. No legal responsibility for any said applications is assumed by Richtek.
Richtek Technology Corporation
Headquarter
5F, No. 20, Taiyuen Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789 Fax: (8863)5526611
Richtek Technology Corporation
Taipei Office (Marketing)
5F, No. 95, Minchiuan Road, Hsintien City
Taipei County, Taiwan, R.O.C.
Tel: (8862)86672399 Fax: (8862)86672377
Email: marketing@richtek.com
TSOT-23-6 Surfa ce Mount Package
Dimensions In Millimeters Dimensions In Inches
Symbol Min Max Min Max
A 0.700 1.000 0.028 0.039
A1 0.000 0.100 0.000 0.004
B 1.397 1.803 0.055 0.071
b 0.300 0.559 0.012 0.022
C 2.591 3.000 0.102 0.118
D 2.692 3.099 0.106 0.122
e 0.838 1.041 0.033 0.041
H 0.080 0.254 0.003 0.010
L 0.300 0.610 0.012 0.024
AA1
e
b
B
D
C
H
L
