RT8288A Datasheet

Datasheet Feedback/Errors

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Pin Configurations

(TOP VIEW)

4A, 21V 500kHz Synchronous Step-Down Converter

General Description

The RT8288A is a synchronous step-down regulator with

an internal power MOSFET. It achieves 4A of continuous

output current over a wide input supply range with excellent

load and line regulation. Current mode operation provides

fast transient response and eases loop stabilization.

Fault condition protection includes cycle-by-cycle current

limiting and thermal shutdown. An internal soft-start

minimizes external parts count and internal compensation

circuitry.

The RT8288A requires a minimal number of readily

available external components, providing a compact

solution.

Features

z4A Output Current

zInternal Soft-Start

z120mΩΩ

ΩΩ

Ω/40mΩΩ

ΩΩ

Ω Internal Power MOSFET Switch

zInternal Compensation Minimizes External Parts

Count

zFixed 500kHz Frequency

zThermal Shutdown Protection

zCycle-by-Cycle Over Current Protection

zWide 4.5V to 21V Operating Input Range

zAdjustable Output from 0.808V to 15V

zAvailable in an SOP-8 (Exposed Pad) Package

zRoHS Compliant and Halogen Free

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.

SOP-8 (Exposed Pad)

Marking Information

Applications

zDistributive Power Systems

zBattery Charger

zDSL Modems

zPre-Regulator for Linear Regulators

RT8288AZSP : Product Number

YMDNN : Date Code

VIN

BOOT

GND

VCC

GND

RT8288A

ZSPYMDNN

Package Type

SP : SOP-8 (Exposed Pad-Option 2)

RT8288A

Lead Plating System

Z : ECO (Ecological Element with

Halogen Free and Pb free)

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Function Pin Description

Pin No. Pin Name Pin Function

1 VIN Supply Input. VIN supplies the power to the IC, as well as the step-down converter

switches. Drive VIN with a 4.5V to 21V power source. Bypass VIN to GND with a

suitably large capacitor to eliminate noise on the input to the IC.

2, 3 SW Switch Node. SW is the switching node that supplies power to the output. Connect

the output LC filter from SW to the output load. Note that a capacitor is required

from SW to BOOT to power the high side switch.

4 BOOT

High Side Gate Drive Boost Input. BOOT supplies the drive for the high side

N-MOSFET switch. Connect a 100nF or greater capacitor from SW to BOOT to

power the high side switch.

5 EN Chip Enable (Active High). For automatic start-up, connect the EN pin to VIN with a

100kΩ resistor.

6 FB Feedback Input. FB senses the output voltage to regulate said voltage. Drive FB

with a resistive voltage divider from the output voltage. The feedback threshold is

0.808V.

7 VCC Bias Supply. Decouple with 0.1μF to 0.22μF capacitor. The capacitance should be

no more than 0.22μF.

9 (Exposed Pad) GND Ground. The exposed pad must be soldered to a large PCB and connected to

GND for maximum power dissipation.

Table 1. Recommended Components Selection

Typical Application Circuit

VOUT (V) R1 (kΩ) R2 (kΩ) RT (kΩ) L (μH) COU T (μF)

5 75 14.46 0 4.7 22 x 2

3.3 75 24.32 0 3.6 22 x 2

2.5 75 35.82 0 3.6 22 x 2

1.8 5 4.07 30 2 22 x 2

1.5 5 5.84 39 2 22 x 2

1.2 5 10.31 47 2 22 x 2

1.05 5 16.69 47 1.5 22 x 2

VIN

GND

BOOT

2, 3

100nF

22µF RT8288A

8, 9 (Exposed Pad)

CBOOT

CIN

COUT

VCC

0.1µF

VIN

VOUT

Chip Enable

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Function Block Diagram

Driver

Current Sense

Amplifier

PWM

Comparator

Oscillator

500kHz

Ramp

Generator

Regulator

Reference

Error

Amplifier

BOOT

VIN

GND

VCC

1.2V

1.7V

1µA

Lockout

Comparator

Shutdown

Comparator

400k

30pF

1pF

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Absolute Maximum Ratings (Note 1)

zSupply Input Voltage, VIN ---------------------------------------------------------------------------------- −0.3V to 26V

zSwitch Voltage, SW ----------------------------------------------------------------------------------------- −0.3V to (VIN + 0.3V)

zBoot Voltage, BOOT ----------------------------------------------------------------------------------------- (SW − 0.3V) to (SW + 6V)

zOther Pins------------------------------------------------------------------------------------------------------ −0.3V to 6V

zPower Dissipation, PD @ TA = 25°C

SOP-8 (Exposed Pad) -------------------------------------------------------------------------------------- 1.333W

zPackage Thermal Resistance (Note 2)

SOP-8 (Exposed Pad), θJA --------------------------------------------------------------------------------- 75°C/W

SOP-8 (Exposed Pad), θJC -------------------------------------------------------------------------------- 15°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 Model)--------------------------------------------------------------------------------- 2kV

Recommended Operating Conditions (Note 4)

zSupply Input Voltage, VIN ---------------------------------------------------------------------------------- 4.5V to 21V

zJunction Temperature Range ------------------------------------------------------------------------------- −40°C to 125°C

zAmbient Temperature Range------------------------------------------------------------------------------- −40°C to 85°C

Electrical Characteristics

Parameter Symbol Test Conditions Min Typ Max Unit

Shutdown Current ISHDN V

EN = 0 -- 0 1 μA

Quiescent Current IQ V

EN = 2V, VFB = 1V -- 0.7 -- mA

Upper Switch On Resistance RDS(ON)1 -- 120 -- mΩ

Lower Switch On Resistance RDS(ON)2 -- 40 -- mΩ

Switch Leakage ILEAK V

EN = 0V, VSW = 0V or 12V -- 0 10 μA

Current Limit ILIMIT V

BOOT − VSW = 4.8V 5.9 7 -- A

Oscillator Frequency fSW V

FB = 0.75V 425 500 575 kHz

Short Circuit Frequency VFB = 0V -- 150 -- kHz

Maximum Duty Cycle DMAX V

FB = 0.8V -- 90 -- %

Minimum On Time tON -- 100 -- ns

Feedback Voltage VFB 4.5V ≤ VIN ≤ 21V 0.796 0.808 0.82 V

Feedback Current IFB -- 10 50 nA

Logic-High VIH 2 -- 5.5

EN Input Threshold

Voltage Logic-Low VIL -- -- 0.4

EN = 2V -- 1 --

Enable Current V

EN = 0V -- 0 -- μA

(VIN = 12V, TA = 25°C unless otherwise specified)

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Parameter Symbol Test Conditions Min Typ Max Unit

Under Voltage Lockout

Threshold VUVLO V

IN Rising 3.8 4 4.2 V

Under Voltage Lockout

Threshold Hysteresis ΔVUVLO -- 400 -- mV

VCC Regulator -- 5 -- V

VCC Load Regulation ICC = 5mA -- 5 -- %

Soft-Start Period tSS -- 2 -- ms

Thermal Shutdown TSD -- 150 -- °C

Thermal Shutdown Hysteresis ΔTSD -- 30 -- °C

Note 1. Stresses beyond those listed “Absolute Maximum Ratings” 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

the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may

affect device reliability.bsolute maximum rating conditions for extended periods may remain possibility to affect device

reliability.

Note 2. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is

measured at the exposed pad of the package.

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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Output Voltage vs. Output Current

1.200

1.205

1.210

1.215

1.220

1.225

1.230

1.235

1.240

00.511.522.533.54

Output Current (A)

Output Voltage (V)

VIN = 12V, VOUT = 1.22V, IOUT = 0A to 4A

Reference Voltage vs. Input Voltage

0.790

0.795

0.800

0.805

0.810

0.815

0.820

0.825

0.830

4 6 8 10 12 14 16 18 20 22

Input Voltage (V)

Reference Voltage (V)

Reference Voltage vs. Temperature

0.76

0.77

0.78

0.79

0.80

0.81

0.82

0.83

0.84

-50 -25 0 25 50 75 100 125

Temperature (°C)

Reference Voltage (V)

Typical Operating Characteristics

Switching Frequency vs. Temperature

350

375

400

425

450

475

500

525

550

-50-25 0 25 50 75100125

Temperature (°C)

Switching Frequency (kHz) 1

VIN = 12V, VOUT = 1.22V, IOUT = 1A

Switching Frequency vs. Input Voltage

350

375

400

425

450

475

500

525

550

4 6 8 10 12 14 16 18 20 22

Input Voltage (V)

Switching Frequency (kHz) 1

VOUT = 1.22V, IOUT = 0.8A

Efficiency vs. Output Current

100

00.511.522.533.54

Output Current (A)

Efficiency (%)

VIN = 12V

VIN = 21V

VOUT = 1.22V, IOUT = 0A to 4A

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Current Limit vs. Input Voltage

4 6 8 10 12 14 16 18 20 22

Input Voltage (V)

Current Limit (A)

Current Limit vs. Temperature

-50-25 0 25 50 75100125

Temperature (°C)

Current Limit (A)

VIN = 12V, VOUT = 1.22V

Output Voltage Ripple

Time (1μs/Div)

(2A/Div)

VOUT

(50mV/Div)

VSW

(10V/Div)

VIN = 12V, IOUT = 4A

Output Voltage Ripple

Time (1μs/Div)

(2A/Div)

VOUT

(50mV/Div)

VSW

(10V/Div)

VIN = 12V, IOUT = 1A

VIN = 12V, VOUT = 1.22V, IOUT = 1A to 4A

Load Transient Response

Time (100μs/Div)

IOUT

(2A/Div)

VOUT

(200mV/Div)

VIN = 12V, VOUT = 1.22V, IOUT = 0A to 4A

Load Transient Response

Time (100μs/Div)

IOUT

(2A/Div)

VOUT

(200mV/Div)

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Power On from VIN

Time (5ms/Div)

(5A/Div)

VOUT

(1V/Div)

VIN

(10V/Div)

VIN = 12V, VOUT = 1.22V, IOUT = 4A

Power Off from VIN

Time (50ms/Div)

(5A/Div)

VOUT

(1V/Div)

VIN

(10V/Div)

VIN = 12V, VOUT = 1.22V, IOUT = 4A

Power On from EN

Time (2.5ms/Div)

VOUT

(1V/Div)

(5A/Div)

VEN

(5V/Div)

VIN = 12V, VOUT = 1.22V, IOUT = 4A

VSW

(10V/Div)

Power Off from EN

Time (50μs/Div)

VIN = 12V, VOUT = 1.22V, IOUT = 4A

VOUT

(1V/Div)

(5A/Div)

VEN

(5V/Div)

VSW

(10V/Div)

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Application Information

The IC is a synchronous high voltage buck converter that

can support the input voltage range from 4.5V to 21V and

the output current can be up to 4A.

Output Voltage Setting

The output voltage is set by an external resistive divider

according to the following equation :

⎛⎞

⎜⎟

⎝⎠

OUT FB R1

V = V1

Figure 1. Output Voltage Setting

where VFB is the feedback reference voltage 0.808V

(typical).

The resistive divider allows the FB pin to sense a fraction

of the output voltage as shown in Figure 1.

RT8288A

GND

VOUT

External Bootstrap Diode

Connect a 100nF low ESR ceramic capacitor between

the BOOT pin and SW pin as shown in Figure 2. 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 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 IN4148 or BAT54. The external 5V

can be a 5V fixed input from system or a 5V output of the

IC. Note that the external boot voltage must be lower than

5.5V.

Figure 2. External Bootstrap Diode

Soft-Start

The IC contains an internal soft-start function to prevent

large inrush current and output voltage overshoot when

the converter starts up. Soft-Start automatically begins

once the chip is enabled. During soft-start, the internal

soft-start capacitor becomes charged and generates a

linear ramping up voltage across the capacitor. This voltage

clamps the voltage at the internal reference, causing the

duty pulse width to increase slowly and in turn reduce the

output surge current. Finally, the internal 1V reference

takes over the loop control once the internal ramping-up

voltage becomes higher than 1V. The typical soft-start

time for this IC is set at 2ms.

Under Voltage Lockout Threshold

The IC includes an input Under Voltage Lockout Protection

(UVLO). If the input voltage exceeds the UVLO rising

threshold voltage (4.2V), the converter resets and prepares

the PWM for operation. If the input voltage falls below the

UVLO falling threshold voltage (3.8V) during normal

operation, the device stops switching. The UVLO rising

and falling threshold voltage includes a hysteresis to

prevent noise caused reset.

Chip Enable Operation

The EN pin is the chip enable input. Pulling the EN pin

low (<0.4V) will shut down the device. During shutdown

mode, the IC quiescent current drops to lower than 1μA.

Driving the EN pin high (>2V, <5.5V) will turn on the device

again. For external timing control (e.g.RC), the EN pin

can also be externally pulled high by adding a REN* resistor

and CEN* capacitor from the VIN pin, as can be seen from

the Figure 5.

An external MOSFET can be added to implement digital

control on the EN pin when front age system voltage below

2.5V is available, as shown in Figure 3. In this case, a

100kΩ pull-up resistor, REN, is connected between VIN

and the EN pin. MOSFET Q1 will be under logic control to

pull down the EN pin.

To prevent enabling circuit when VIN is smaller than the

VOUT target value, a resistive voltage divider can be placed

between the input voltage and ground and connected to

the EN pin to adjust IC lockout threshold, as shown in

Figure 4. For example, if an 8V output voltage is regulated

from a 12V input voltage, the resistor REN2 can be selected

to set input lockout threshold larger than 8V.

RT8288A

BOOT

100nF

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Figure 3. Enable Control Circuit for Logic Control with Low Voltage

Figure 4. The Resistors can be Selected to Set IC Lockout Threshold

Under Output Voltage Protection-Hiccup Mode

For the IC, Hiccup Mode of Under Voltage Protection (UVP)

is provided. When the FB voltage drops below half of the

feedback reference voltage, VFB, the UVP function will be

triggered and the IC will shut down for a period of time and

then recover automatically. The Hiccup Mode of UVP can

reduce input current in short circuit conditions.

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current ΔIL increases with higher VIN and decreases

with higher inductance.

OUT OUT

LIN

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. Highest efficiency operation is achieved by reducing

ripple current at low frequency, but it requires a large

inductor to attain this goal.

For the ripple current selection, the value of ΔIL = 0.24

(IMAX) will be a reasonable starting point. The largest ripple

current occurs at the highest VIN. To guarantee that the

ripple current stays below a specified maximum, the

inductor value should be chosen according to the following

equation :

OUT OUT

L(MAX) IN(MAX)

L = 1

fI V

⎡⎤⎡ ⎤

×−

⎢⎥⎢ ⎥

×Δ

⎣⎦⎣ ⎦

The inductor's current rating (caused a 40°C temperature

rising from 25°C ambient) should be greater than the

maximum load current and its saturation current should

be greater than the short circuit peak current limit. Please

see Table 2 for the inductor selection reference and it is

highly recommended to keep inductor value as close as

possible to the recommended inductor values for each

VOUT as shown in Table 1.

Component Supplier Series Dimensions (mm)

TDK VLF10045 10 x 9.7 x 4.5

TDK SLF12565 12.5 x 12.5 x 6.5

TAIYO YUDEN NR8040 8 x 8 x 4

Table 2. Suggested Inductors for Typical

Application Circuit

Input and Output Capacitors Selection

The input capacitance, CIN, is needed to filter the

trapezoidal current at the source of the high side 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 :

VIN

VCC

BOOT

2, 3

VOUT

VIN

RT8288A

CBOOT

COUT

CIN

GND 8, 9 (Exposed Pad)

REN

100k

Chip Enable

VIN

VCC

BOOT

2, 3

VOUT

VIN

RT8288A

CBOOT

COUT

CIN

GND 8, 9 (Exposed Pad)

REN

100k

REN2

OUT IN

RMS OUT(MAX) IN OUT

I = I 1

−

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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.

Table 3. Suggested Capacitors for CIN and COUT

Location Component Supplier Part No. Capacitance (μF) Case Size

CIN MURATA GRM32ER71C226M 22 1210

CIN TDK C3225X5R1C226M 22 1210

COUT MURATA GRM31CR60J476M 47 1206

COUT TDK C3225X5R0J476M 47 1210

COUT MURATA GRM32ER71C226M 22 1210

COUT TDK C3225X5R1C226M 22 1210

The selection of COUT is determined by the required 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.

The output ripple, ΔVOUT, is determined by :

OUT L OUT

VIESR

8fC

⎡⎤

Δ≤Δ +

⎢⎥

⎣⎦

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.

Thermal Shutdown

Thermal shutdown is implemented to prevent the chip from

operating at excessively high temperatures. When the

junction temperature is higher than 150°C, the chip will

shut down the switching operation. The chip will

automatically resume switching, once the junction

temperature cools down by approximately 30°C.

EMI Consideration

Since parasitic inductance and capacitance effects in PCB

circuitry would cause a spike voltage on SW pin when

high side MOSFET is turned-on/off, this spike voltage on

SW may impact on EMI performance in the system. In

order to enhance EMI performance, there are two methods

to suppress the spike voltage. One way is by placing an

R-C snubber (RS*, CS*) between SW and GND and locating

them as close as possible to the SW pin, as shown in

Figure 5. Another method is by adding a resistor in series

with the bootstrap capacitor, CBOOT, but this method will

decrease the driving capability to the high side MOSFET.

It is strongly recommended to reserve the R-C snubber

during PCB layout for EMI improvement. Moreover,

reducing the SW trace area and keeping the main power

in a small loop will be helpful on EMI performance. For

detailed PCB layout guide, please refer to the section

Layout Considerations.

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.

For the input capacitor, one 22μF low ESR ceramic

capacitors are recommended. For the recommended

capacitor, please refer to Table 3 for more detail.

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Figure 5. Reference Circuit with Snubber and Enable Timing Control

Thermal Considerations

For continuous operation, do not exceed absolute

maximum junction temperature. The maximum power

dissipation depends on the thermal resistance of the IC

package, PCB layout, rate of surrounding airflow, and

difference between junction and ambient temperature. The

maximum power dissipation can be calculated by the

following formula :

PD(MAX) = (TJ(MAX) − TA) / θJA

where TJ(MAX) is the maximum junction temperature, TA is

the ambient temperature, and θJA is the junction to ambient

thermal resistance.

For recommended operating condition specifications, the

maximum junction temperature is 125°C. The junction to

ambient thermal resistance, θJA, is layout dependent. For

SOP-8 (Exposed Pad) package, the thermal resistance,

θJA, is 75°C/W on a standard JEDEC 51-7 four-layer

thermal test board. The maximum power dissipation at TA

= 25°C can be calculated by the following formula :

PD(MAX) = (125°C − 25°C) / (75°C/W) = 1.333W for

SOP-8 (Exposed Pad) package

The maximum power dissipation depends on the operating

ambient temperature for fixed TJ(MAX) and thermal

resistance, θJA. The derating curve in Figure 6 allows the

designer to see the effect of rising ambient temperature

on the maximum power dissipation.

Layout Considerations

Follow the PCB layout guidelines for optimal performance

of the IC.

`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).

`SW node is with high frequency voltage swing and

should be kept at small area. Keep analog components

away from the SW node to prevent stray capacitive noise

pickup.

`Connect feedback network behind the output capacitors.

Keep the loop area small. Place the feedback

components near the IC.

Figure 6. Derating Curve of Maximum Power Dissipation

VIN

VCC

BOOT

2, 3

VOUT

VIN

RT8288A

CBOOT

COUT

CIN

GND 8, 9 (Exposed Pad)

5CS*

RS*

REN*

CEN*

* : Optional

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 25 50 75 100 125

Ambient Temperature (°C)

Maximum Power Dissipation (W) 1

Four-Layer PCB

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Figure 7. PCB Layout Guide

CIN

CBOOT

VOUT

COUT GND

VOUT

RTR2

GND

Place the input and

output capacitors

as close to the IC

as possible.

SW should be connected

to inductor by wide and

short trace and keep

sensitive components

away from this trace.

Place the feedback

as close to the IC as

possible.

VIN

BOOT

GND

VCC

GND

`Connect all analog grounds to a common node and then

connect the common node to the power ground behind

the output capacitors.

`An example of PCB layout guide is shown in Figure 7

for reference.

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Richtek Technology Corporation

5F, No. 20, Taiyuen Street, Chupei City

Hsinchu, Taiwan, R.O.C.

Tel: (8863)5526789

Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should

obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot

assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be

accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third

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Outline Dimension

EXPOSED THERMAL PAD

(Bottom of Package)

8-Lead SOP (Exposed Pad) Plastic Package

Dimensions In Millimeters Dimensions In Inches

Symbol Min Max Min Max

A 4.801 5.004 0.189 0.197

B 3.810 4.000 0.150 0.157

C 1.346 1.753 0.053 0.069

D 0.330 0.510 0.013 0.020

F 1.194 1.346 0.047 0.053

H 0.170 0.254 0.007 0.010

I 0.000 0.152 0.000 0.006

J 5.791 6.200 0.228 0.244

M 0.406 1.270 0.016 0.050

X 2.000 2.300 0.079 0.091

Option 1 Y 2.000 2.300 0.079 0.091

X 2.100 2.500 0.083 0.098

Option 2 Y 3.000 3.500 0.118 0.138

RT8288A Datasheet

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