RT8259
1.2A, 24V, 1.4MHz Step-Down Converter
General Description
Features
The RT8259 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.
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Wide Operating Input Voltage Range : 4.5V to 24V
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Adjustable Output Voltage Range : 0.8V to 15V
1.2A Output Current
0.3Ω
Ω Internal Power MOSFET Switch
High Efficiency up to 92%
1.4MHz Fixed Switching Frequency
Stable with Low ESR Output Ceramic Capacitors
Thermal Shutdown
Cycle-By-Cycle Over Current Protection
RoHS Compliant and Halogen Free
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The chip also provides protection functions such as cycleby-cycle current limiting and thermal shutdown protection.
The RT8259 is available in a SOT-23-6 and TSOT-23-6
packages.
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Ordering Information
RT8259
Applications
Package Type
E : SOT-23-6
J6 : TSOT-23-6
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Lead Plating System
G : Green (Halogen Free and Pb Free)
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Note :
Distributed Power Systems
Battery Charger
Pre-Regulator for Linear Regulators
WLED Drivers
Pin Configurations
Richtek products are :
`
RoHS compliant and compatible with the current require-
`
Suitable for use in SnPb or Pb-free soldering processes.
(TOP VIEW)
ments of IPC/JEDEC J-STD-020.
PHASE VIN
6
Marking Information
5
4
2
3
BOOT GND
For marking information, contact our sales representative
directly or through a Richtek distributor located in your
area.
EN
FB
SOT-23-6/TSOT-23-6
Typical Application Circuit
VIN
4.5V to 24V
5 VIN
C1
10µF
Chip Enable
Open =
Automatic Startup
DS8259-03 March 2011
BOOT
RT8259
PHASE 6
4 EN
VOUT
3.3V
1
CB
10nF
D1
B230A
L1
4.7µH
R1
62k
FB 3
GND
2
C2
22µF
R2
19.6k
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1
RT8259
Table 1. Recommended Component Selection, C2 = 22μ
μF
V OUT (V)
1.2
1.8
2.5
3.3
5
8
10
15
L1 (μH)
2
2
3.6
4.7
6.8
10
10
15
R1 (kΩ)
62
62
62
62
62
68
68
68
R2 (kΩ)
124
49.9
29.4
19.6
12
7.5
5.9
3.9
Functional Pin Description
Pin No.
Pin Name
Pin Function
Bootstrap. A capacitor is connected between PHASE and BOOT pins to form a floating
supply across the power switch driver. This capacitor is needed to drive the power switch‘s
gate above the supply voltage.
Ground. This pin is the voltage reference for the regulated output voltage. For this reason,
care must be taken in its layout. This node should be placed outside of the D1 to C1 ground
path to prevent switching current spikes from inducing voltage noise into the part.
1
BOOT
2
GND
3
FB
Feedback. 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
Supply Voltage. Bypass VIN to GND with a suitable large capacitor to prevent large voltage
spikes from appearing at the input.
6
PHASE
Switch Output.
Function Block Diagram
VIN
-
X20
1µA
Current Sense Amp
EN
3V
FB
1.1V
Ω
25mΩ
Ramp
Generator
Regulator
10k
+
BOOT
-
Oscillator
1.4MHz
+
Shutdown Reference
Comparator
S
Q
+
EA
-
400k
30pF
+
Driver
R
PWM
Comparator
PHASE
Bootstrap
Control
OC Limit Clamp
GND
1pF
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DS8259-03 March 2011
RT8259
Absolute Maximum Ratings
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(Note 1)
Supply Voltage, VIN -----------------------------------------------------------------------------------------------PHASE Voltage ----------------------------------------------------------------------------------------------------BOOT Voltage ------------------------------------------------------------------------------------------------------All Other Pins -------------------------------------------------------------------------------------------------------Output Voltage -----------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
T/SOT-23-6 ----------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2)
T/SOT-23-6, θJA -----------------------------------------------------------------------------------------------------Junction Temperature ---------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) -----------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------ESD Susceptibility (Note 3)
HBM (Human Body Mode) ---------------------------------------------------------------------------------------MM (Machine Mode) -----------------------------------------------------------------------------------------------
Recommended Operating Conditions
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26V
−0.3V to (VIN + 0.3V)
VPHASE + 6V
0.3V to 6V
−0.3V to 15V
0.4W
250°C/W
150°C
260°C
−65°C to 150°C
2kV
200V
(Note 4)
Supply Voltage, VIN -----------------------------------------------------------------------------------------------Output Voltage, VOUT ---------------------------------------------------------------------------------------------EN Voltage, VEN ----------------------------------------------------------------------------------------------------Junction Temperature Range ------------------------------------------------------------------------------------Ambient Temperature Range -------------------------------------------------------------------------------------
4.5V to 24V
0.8V to 15V
0V to 5.5V
−40°C to 125°C
−40°C to 85°C
Electrical Characteristics
(VIN = 12V, TA = 25° C unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
0.784
0.8
0.816
V
Feedback Reference Voltage
V FB
4.5V ≤ VIN ≤ 24V
Feedback Current
IFB
VFB = 0.8V
--
0.1
0.3
μA
Switch On Resistance
Switch Leakage
RDS(ON)
VEN = 0V, VPHASE = 0V
---
0.3
--
-10
Ω
μA
Current Limit
ILIM
VBOOT − VPHASE = 4.8V
1.6
2.1
--
A
Oscillator Frequency
fSW
1.2
1.4
1.6
MHz
--
80
--
%
--
100
--
ns
3.9
4.2
4.5
V
--
200
--
mV
V IH
1.4
--
--
V IL
--
--
0.4
--
1
--
Maximum Duty Cycle
Minimum On-Time
tON
Under Voltage Lockout
Threshold
Rising
Under Voltage Lockout
Threshold Hysteresis
Logic-High
EN Input Voltage
Logic-Low
EN Pull Up Current
VEN = 0V
V
μA
To be continued
DS8259-03 March 2011
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RT8259
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Shutdown Current
ISHDN
VEN = 0V
--
25
--
μA
Quiescent Current
IQ
VEN = 2V, VFB = 1V (Not Switching)
--
0.55
1
mA
Thermal Shutdown
TSD
--
150
--
°C
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 high effective four layers thermal conductivity test board of
JEDEC 51-7 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.
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DS8259-03 March 2011
RT8259
Typical Operating Characteristics
Efficiency vs. Load Current
Efficiency vs. Load Current
100
100
VIN = 12V
VIN = 24V
80
80
70
Efficiency (%)
Efficiency (%)
VIN = 12V
90
90
60
50
40
30
VIN = 24V
70
60
50
40
30
20
20
10
10
VOUT = 3.3V
VOUT = 5V
0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1.1 1.2
3.328
3.36
3.323
VIN = 24V
3.313
1.1 1.2
Output Voltage vs. Temperature
3.38
Output Voltage (V)
Output Voltage (V)
Output Voltage vs. Load Current
3.333
3.318
1
Load Current (A)
Load Current (A)
VIN = 12V
3.308
3.34
VIN = 24V
3.32
3.30
VIN = 12V
3.28
3.26
3.303
3.24
3.298
3.22
IOUT = 0A
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
-50
1.1 1.2
-25
0
Load Current (A)
50
75
100
125
Frequency vs. Temperature
Frequency vs. Input Voltage
1.55
1.55
1.50
1.50
1.45
1.45
Frequency (MHz)1
Frequency (MHz)
25
Temperature (°C)
1.40
1.35
1.30
1.25
1.20
1.40
1.35
1.30
1.25
1.20
VOUT = 3.3V, IOUT = 0.3A
1.15
VIN = 12V, VOUT = 3.3V, IOUT = 0.3A
1.15
4
6.5
9
11.5
14
16.5
Input Voltage (V)
DS8259-03 March 2011
19
21.5
24
-50
-25
0
25
50
75
100
125
Temperature (°C)
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RT8259
Quiescent Current vs. Temperature
0.9
0.8
0.8
Quiescent Current (mA)
Quiescent Current (mA)
Quiescent Current vs. Input Voltage
0.9
0.7
0.6
0.5
0.4
0.3
0.2
0.7
VIN = 24V
0.6
VIN = 12V
0.5
0.4
0.3
0.2
VEN = 2V, VFB = 1V
VEN = 2V, VFB = 1V
0.1
0.1
4
6.5
9
11.5
14
16.5
19
21.5
24
-50
0
25
50
75
100
Temperature (°C)
Load Transient Response
Load Transient Response
VIN = 12V, VOUT = 3.3V, IOUT = 0.6A to 1.2A
VOUT
(50mV/Div)
IOUT
(500mA/Div)
IOUT
(500mA/Div)
Time (50μs/Div)
Time (50μs/Div)
Switching
Switching
VOUT
(5mV/Div)
VOUT
(5mV/Div)
VPHASE
(10V/Div)
VPHASE
(10V/Div)
IL
(1A/Div)
IL
(1A/Div)
VIN = 12V, VOUT = 3.3V, IOUT = 1.2A
Time (250ns/Div)
125
VIN = 12V, VOUT = 3.3V, IOUT = 0A to 1.2A
VOUT
(50mV/Div)
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-25
Input Voltage (V)
VIN = 20V, VOUT = 3.3V, IOUT = 1.2A
Time (250ns/Div)
DS8259-03 March 2011
RT8259
Power Off from EN
Power On from EN
VIN = 12V, VOUT = 3.3V, IOUT = 1.2A
VIN = 12V, VOUT = 3.3V, IOUT = 1.2A
VEN
(5V/Div)
VEN
(5V/Div)
VOUT
(1V/Div)
I IN
(500mA/Div)
VOUT
(1V/Div)
I IN
(500mA/Div)
Time (250μs/Div)
DS8259-03 March 2011
Time (100μs/Div)
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RT8259
Application Information
The RT8259 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.
Output Voltage Setting
The resistive voltage divider allows the FB pin to sense a
fraction of the output voltage as shown in Figure 1.
VOUT
R1
FB
RT8259
R2
GND
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 :
VOUT = VFB ⎛⎜ 1 + R1 ⎞⎟
⎝ R2 ⎠
Where VFB is the feedback reference voltage (0.8V typ.).
External Bootstrap Diode
Connect a 10nF low ESR ceramic capacitor between the
BOOT pin and SW 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 RT8259.
5V
BOOT
RT8259
10nF
PHASE
Figure 2. External Bootstrap Diode
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.
V
V
ΔIL = ⎡⎢ OUT ⎤⎥ × ⎡⎢1− OUT ⎤⎥
f
×
L
VIN ⎦
⎣
⎦ ⎣
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 = 0.4(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 :
⎡ VOUT ⎤ ⎡
VOUT ⎤
L =⎢
× ⎢1−
⎥
⎥
f
×
Δ
I
V
L(MAX) ⎦ ⎣
IN(MAX) ⎦
⎣
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.
Do not allow the core to saturate!
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DS8259-03 March 2011
RT8259
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 4.
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 :
V
VIN
IRMS = IOUT(MAX) OUT
−1
VIN
VOUT
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 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.
Checking Transient Response
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) 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.
The output ripple, ΔVOUT , is determined by :
1
⎤
ΔVOUT ≤ ΔIL ⎡⎢ESR +
8fCOUT ⎦⎥
⎣
DS8259-03 March 2011
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RT8259
Thermal Considerations
Layout Consideration
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 :
Follow the PCB layout guidelines for optimal performance
of RT8259.
PD(MAX) = (TJ(MAX) − TA) / θJA
`
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).
`
LX node is with high frequency voltage swing and should
be kept at small area. Keep sensitive components away
from the LX node to prevent stray capacitive noise pickup.
`
Place the feedback components to the FB pin as close
as possible.
`
Connect GND to a ground plane for noise reduction and
thermal dissipation.
where T J(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
RT8259, 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 120°C/W on standard JEDEC 51-7 four-
V OUT
layers thermal test board. The maximum power dissipation
at TA = 25°C can be calculated by following formula :
C OUT
L1
CB
D1
P D(MAX) = (125°C − 25°C) / (250°C/W) = 0.4W for
T/SOT-23-6 packages
BOOT
1
6
PHASE
GND
2
5
VIN
FB
3
4
EN
C IN
The maximum power dissipation depends on operating
ambient temperature for fixed TJ(MAX) and thermal resistance
θJA . For RT8259 packages, the Figure 3 of derating curves
allows the designer to see the effect of rising ambient
temperature on the maximum power allowed.
R2
V OUT
R1
GND
Figure 4. PCB Layout Guide
Maximum Power Dissipation (W)
0.50
Single Layer PCB
0.45
0.40
0.35
0.30
T/SOT-23-6
0.25
0.20
0.15
0.10
0.05
0.00
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 3. Derating Curves for RT8259 Packages
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DS8259-03 March 2011
RT8259
Table 2. Suggested Inductors for L1
Component Supplier
Series
Inductance (µH)
TDK
TAIYO YUDEN
GOTERND
GOTERND
SLF7045
NR8040
GTSD53
GSSR2
DCR (mΩ)
4.7
4.7
4.7
4.7
Current Rating (A) Dimensions (mm)
30
18
45
18
2
4.7
1.87
5.7
7 x 7 x 4.5
8x8x4
5 x 5 x 2.8
10 x 10 x 3.8
Table 3. Suggested Capacitors for CIN and COUT
Component Supplier
Part No.
Capacitance ( µF)
Case Size
MURATA
GRM31CR61E106K
10
1206
TDK
C3225X5R1E106K
10
1206
TAIYO YUDEN
TMK316BJ106ML
10
1206
MURATA
GRM31CR61C226M
22
1206
TDK
C3225X5R1C226M
22
1206
TAIYO YUDEN
EMK316BJ226ML
22
1206
Table 4. Suggested Diode for D1
Component Supplier
Series
VR RM (V)
IOUT (A)
Package
DIODES
DIODES
PANJIT
PANJIT
B230A
B330A
SK23
SK33
30
30
30
30
2
3
2
3
DO-214AC
DO-214AC
DO-214AC
DO-214AB
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RT8259
Outline Dimension
H
D
L
C
B
b
A
A1
e
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
SOT-23-6 Surface Mount Package
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DS8259-03 March 2011
RT8259
H
D
L
C
B
b
A
A1
e
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
TSOT-23-6 Surface Mount Package
Richtek Technology Corporation
Richtek Technology Corporation
Headquarter
Taipei Office (Marketing)
5F, No. 20, Taiyuen Street, Chupei City
5F, No. 95, Minchiuan Road, Hsintien City
Hsinchu, Taiwan, R.O.C.
Taipei County, Taiwan, R.O.C.
Tel: (8863)5526789 Fax: (8863)5526611
Tel: (8862)86672399 Fax: (8862)86672377
Email: marketing@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.
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