RT8251_技术文档

®

RT8251

5A, 24V, 570kHz Step-Down Converter

General Description

Features

^zWide Operating Input Voltage Range : 4.75V to 24V

^zAdjustable Output Voltage Range : 0.8V to 15V

^zOutput Current up to 5A

The RT8251 is a monolithic step-down switch mode

converter with a built-in internal power MOSFET. It achieves

5A 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.

^z25μA Low Shutdown Current

^zInternal Power MOSFET : 70mΩ

^zHigh Efficiency up to 95%

^z570kHz Fixed Switching Frequency

^zStable with Low ESR Output Ceramic Capacitors

^zThermal Shutdown Protection

The RT8251 provides protection functions such as

cycle-by-cycle current limiting and thermal shutdown. In

shutdown mode, the regulator draws 25μA of supply

current. Programmable soft-start minimizes the inrush

supply current and the output overshoot at initial startup.

The RT8251 requires a minimum number of external

components. The RT8251 is available in WQFN-16L 3x3

and SOP-8 (Exposed Pad) packages.

^zCycle-By-Cycle Over Current Protection

^zRoHS Compliant and Halogen Free

Applications

z Distributed Power Systems

z Battery Charger

z DSL Modems

Pin Configurations

z Pre-regulator for Linear Regulators

(TOP VIEW)

Ordering Information

16 15 14 13

RT8251

SW

12

11

VIN

1

2

3

4

Package Type

QW : WQFN-16L 3x3 (W-Type)

SP : SOP-8 (Exposed Pad-Option 1)

GND

10 SW

BOOT

VIN

17

9

GND

5

6

7

8

Lead Plating System

G : Green (Halogen Free and Pb Free)

Note :

Richtek products are :

WQFN-16L 3x3

` 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.

8

BOOT

VIN

SS

2

3

4

7

6

5

EN

GND

SW

COMP

FB

9

GND

SOP-8 (Exposed Pad)

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RT8251

Marking Information

RT8251GQW

RT8251GSP

GE= : Product Code

RT8251GSP : Product Number

YMDNN : Date Code

RT8251

GSPYMDNN

YMDNN : Date Code

GE=YM

DNN

Typical Application Circuit

1, 2, 3,

V

15, 16

9

IN

VIN

BOOT

RT8251

4.75V to 24V

C

100nF

C

BOOT

L

IN

4.7µH

10µF x 2

V

OUT

10 to 14

SW

7

8

3.3V/5A

EN

SS

Chip Enable

D

R1

B540C

30.9k

C

5

OUT

C

SS

FB

22µF x 2

C

10nF

4,

R

C

R2

2.2nF

22k

10k

Exposed Pad (17)

6

GND

COMP

C

P

(Open)

Figure 1. Typical Application Circuit for WQFN-16L 3x3

1

3

2

7

V

IN

BOOT

RT8251

VIN

C

4.75V to 24V

IN

C

100nF

BOOT

10µF x 2

L

V

OUT

SW

Chip Enable

3.3V/5A

4.7µH

D

EN

SS

R1

B540C

30.9k

8

C

OUT

22µF x 2

5

6

FB

C

10nF

SS

4,

C

R

C

R2

10k

2.2nF

Exposed Pad(9)

22k

GND

COMP

C

P

NC

Figure 2. Typical Application Circuit for SOP-8 (Exposed Pad)

Table 1. Recommended Component Selection

V

(V)

R1 (kΩ)

182

R2 (kΩ)

10

R (kΩ)

C (nF)

C

L1 (μH)

22

C

(μF)

OUT

C

15

51

43

39

30

22

16

13

1

44

10

8

115

10

1.2

1.5

2.2

10

44

91

10

5

52.3

30.9

21.5

12.4

4.99

10

6.8

4.7

2.2

3.3

2.5

1.8

1.2

10

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RT8251

Functional Pin Description

Pin No.

Pin Name

Pin Function

SOP-8

(Exposed Pad)

WQFN-16L 3x3

Power Input. VIN supplies the power to the IC, as well as the

step-down converter switches. Connect VIN with a 4.75V to 24V

power source. Connect VIN to GND with a capacitor that the

capacitance is large enough to eliminate noise on the input to the

IC.

1, 2, 3, 15, 16

2

VIN

Ground. This pin is the voltage reference for the regulated output

voltage. For this reason, care must be taken in its layout. This

4,

node should be placed outside of the D1 to C ground path to

IN

GND

17 (Exposed Pad) 9 (Exposed Pad)

prevent switching current spikes from inducing voltage noise into

the part. The exposed pad must be soldered to a large PCB and

connected to GND for maximum power dissipation.

Feedback Input. An external resistor divider from the output to

GND, tapped to the FB pin, sets the output voltage.

5

6

5

6

FB

Compensation Node. This node is the output of the

transconductance error amplifier and the input to the current

comparator. Frequency compensation is done at this node by

connecting a series R-C to ground.

COMP

Enable Input. EN is a digital input that turns the regulator on or

off. Drive EN higher than 1.4V to turn on the regulator, lower

than 0.4V to turn it off. For automatic startup, leave EN

unconnected.

Soft-Start Control Input. SS controls the soft start period.

Connect a capacitor (≧10nF) from SS to GND to set the

soft-start period. A 10nF capacitor sets the Soft-Start period to

1ms.

7

8

7

8

EN

SS

Bootstrap. This capacitor C

is needed to drive the power

BOOT

switch’s gate above the supply voltage. It is connected between

the SW and BS pins to form a floating supply across the power

9

1

3

BOOT

SW

switch driver. The voltage across C

is about 5V and is

BOOT

supplied by the internal +5V supply when the SW pin voltage is

low.

Power Switching Output. 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.

10, 11, 12, 13,

14

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RT8251

Function Block Diagram

VIN

Current Sense

V

CC

Slope Comp

Internal

Regulator

Amplifier

Oscillator

570kHz

VA

+

-

1µA

10k

V

VA

-

CC

Foldback

Control

BOOT

SW

EN

+

1.1V

0.4V

+

-

Shutdown

Comparator

Logic

3V

+

-

UV

V

CC

Comparator

GND

Current

Comparator

10µA

0.8V

+

-

EA

SS

Gm = 820µA/V

FB

COMP

Absolute Maximum Ratings (Note 1)

z Supply Voltage, V_IN------------------------------------------------------------------------------------------−0.3V to 26V

z Switching Voltage, V_SW-------------------------------------------------------------------------------------−0.3V to (V_IN+ 0.3V)

z BOOT Voltage, V_BOOT---------------------------------------------------------------------------------------(V_SW− 0.3V) to (V_SW+ 6V)

z All Other Pins -------------------------------------------------------------------------------------------------−0.3V to 6V

z Power Dissipation, P_D@ T_A= 25°C

WQFN-16L 3x3 -----------------------------------------------------------------------------------------------1.471W

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

z Package Thermal Resistance (Note 2)

WQFN-16L 3x3, θ_JA------------------------------------------------------------------------------------------68°C/W

WQFN-16L 3x3, θ_JC-----------------------------------------------------------------------------------------7.5°C/W

SOP-8 (Exposed pad), θ_JA---------------------------------------------------------------------------------75°C/W

SOP-8 (Exposed Pad), θ_JC--------------------------------------------------------------------------------15°C/W

z Junction Temperature ----------------------------------------------------------------------------------------150°C

z Lead Temperature (Soldering, 10 sec.)------------------------------------------------------------------260°C

z Storage Temperature Range -------------------------------------------------------------------------------−65°C to 150°C

z ESD Susceptibility (Note 3)

HBM (Human Body Model)---------------------------------------------------------------------------------2kV

Recommended Operating Conditions (Note 4)

z Supply Voltage, V_IN------------------------------------------------------------------------------------------4.75V to 24V

z Enable Voltage, V_EN-----------------------------------------------------------------------------------------0V to 5.5V

z Junction Temperature Range-------------------------------------------------------------------------------−40°C to 125°C

z Ambient Temperature Range-------------------------------------------------------------------------------−40°C to 85°C

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RT8251

Electrical Characteristics

(V_IN= 12V, T_A= 25°C unless otherwise specified)

Parameter

Symbol

Test Conditions

4.75V ≦ V ≦ 24V

Min

Typ

Max Unit

Feedback Reference Voltage

High-Side Switch-On Resistance

Low-Side Switch-On Resistance

High-Side Switch Leakage

Current Limit

V

0.784

--

0.8

70

0.816

--

V

mΩ

Ω

FB

IN

R

DS(ON)1

DS(ON)2

--

15

--

V

EN

= 0V, V

= 0V

--

10

--

μA

SW

I

Duty = 85%; V

= 4.8V

--

6.8

4.6

920

570

185

85

A

LIM

BOOT−SW

Current Sense Transconductance

Error Amplifier Tansconductance

Oscillator Frequency

G

CS

Output Current to V

--

A/V

μA/V

kHz

%

COMP

gm

ΔI = ±10μA

C

--

f

420

--

720

--

SW

Short Circuit Oscillation Frequency

Maximum Duty Cycle

V

= 0V

FB

D

MAX

= 0.7V

--

FB

Minimum On-Time

t

--

100

4.1

200

--

ns

ON

UVLO Threshold Rising

--

V

UVLO Threshold Hysteresis

--

mV

Logic Low

EN Input Voltage

V

--

0.4

5.5

--

IL

V

Logic High

1.4

--

IH

Enable Pull Up Current

Shutdown Current

Quiescent Current

Soft-Start Current

Soft-Start Period

V

= 0V

1

μA

mA

μA

ms

°C

EN

I

--

25

--

SHDN

Q

EN

SS

I

V

= 2V, V = 1V

--

0.8

10

1

FB

= 0V

--

SS

C

= 10nF

--

1

--

SS

Thermal Shutdown

T

--

150

--

SD

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.

Note 2. θ_JAis measured at T_A= 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θ_JCis

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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RT8251

Typical Operating Characteristics

Efficiency vs. Load Current

100

90

80

70

60

50

40

30

20

10

0

V_IN= 12V

V_IN= 24V

V_IN= 12V

90

80

V_IN= 24V

70

60

50

40

30

20

10

V_OUT= 3.3V

V_OUT= 5V

4.5 5

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0

0.5

1

1.5

2

2.5

3

3.5

4

Load Current (A)

Output Voltag Deviation vs. Load Current

Output Voltage Deviation vs. Input Voltage

2

1.5

1

I_OUT= 5A

_OUT= 3A

I_OUT= 0A

V_IN= 24V

1.5

1

I

V_IN= 12V

V_IN= 5V

0.5

0

0.5

0

-0.5

-1

-0.5

-1

-1.5

-2

-1.5

V_OUT= 3.3V

19 21.5 24

V_OUT= 3.3V

-2

4

6.5

9

11.5

14

16.5

0.001

0.01

0.1

1

10

Input Voltage (V)

Reference Voltage vs. Temperature

Load Current (A)

Quiescent Current vs. Temperature

0.816

0.811

0.806

0.801

0.796

0.791

0.786

1.2

1

0.8

0.6

0.4

0.2

0

V_IN= 12V

100 125

V_IN= 12V

100 125

-50

-25

0

25

50

75

-50

-25

0

25

50

75

Temperature ( C)

°

Temperature ( C)

°

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RT8251

Switching Frequency vs. Temperature

Switching Frequency vs. Input Voltage

610

600

590

580

570

560

550

630

610

590

570

550

530

510

V_IN= 12V

V_IN= 24V

V_OUT= 3.3V, I_OUT= 1A

-50

-25

0

25

50

75

100

125

4

6.5

9

11.5

14

16.5

19

21.5

24

Input Voltage (V)

Temperature ( C)

°

Output Ripple

Current Limit vs. Duty Cycle

9.3

8.7

8.1

7.5

6.9

6.3

5.7

V_OUT

(10mV/Div)

V_SW

(10V/Div)

V_IN= 12V

V_OUT= 3.3V

I_SW

(2A/Div)

I_OUT= 5A

0

10

20

30

40

50

60

70

80

90

100

Time (1μs/Div)

Duty Cycle (%)

Load Transient Response

V_IN= 12V, V_OUT= 3.3V

_OUT= 0A to 5A

V_IN= 12V, V_OUT= 3.3V

I_OUT= 2.5A to 5A

I

V_OUT

(200mV/Div)

V_OUT

(200mV/Div)

I_OUT

(2A/Div)

I_OUT

(2A/Div)

Time (100μs/Div)

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RT8251

Power On from EN

Power Off from EN

V_EN

(5V/Div)

V_EN

(5V/Div)

V_OUT

(2V/Div)

V_OUT

(2V/Div)

V_IN= 12V, V_OUT= 3.3V, I_OUT= 5A

Time (250μs/Div)

Time (25μs/Div)

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RT8251

Application Information

Soft-Start

The RT8251 is an asynchronous high voltage buck

converter that can support the input voltage range from

4.75V to 24V and the output current can be up to 5A.

The RT8251 contains an external soft-start clamp that

gradually raises the output voltage. The soft-start timming

can be set by the external capacitor between SS pin and

GND. The chip provides a 10μA charge current for the

external capacitor. If 10nF capacitor is used to set the

soft-start time, its period will be 1ms (typ.).

Output Voltage Setting

The resistive divider allows the FB pin to sense the output

voltage as shown in Figure 3.

V

OUT

Chip Enable Operation

The EN pin is the chip enable input. Pull the EN pin low

(<0.4V) will shutdown the device.During shutdown mode,

the RT8251 quiescent current drops to lower than 25μA.

Drive the EN pin to high ( >1.4V, < 5.5V) will turn on the

device again. If the EN pin is open, it will be pulled to high

by internal circuit. For external timing control (e.g.RC),

the EN pin can also be externally pulled to High by adding

a100kΩ or greater resistor from the VIN pin (see Figure 5).

R1

FB

RT8251

GND

R2

Figure 3. Output Voltage Setting

The output voltage is set by an external resistive divider

according to the following equation :

R1

R2

⎛

⎝

⎞

⎟

⎠

V_OUT= V_FB1+

Inductor Selection

⎜

The inductor value and operating frequency determine the

ripple current according to a specific input and output

voltage. The ripple current ΔI_Lincreases with higher V_IN

and decreases with higher inductance.

Where V_FBis the feedback reference voltage (0.8V typ.).

External Bootstrap Diode

Connect a 100nF low ESR ceramic capacitor between

the BOOT pin and SW pin. This capacitor provides the

gate driver voltage for the high side MOSFET.

V

V_OUT

⎡

_OUT⎤ ⎡

× 1−

⎥ ⎢

⎤

ΔI_L=

⎢

⎣

⎥

⎦

f ×L

V

IN

⎦ ⎣

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.

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

cycle is higher than 65%. The bootstrap diode can be a

low cost one such as 1N4148 or BAT54.

For the ripple current selection, the value of ΔI_L= 0.24(I_MAX

)

The external 5V can be a 5V fixed input from system or a

5V output of the RT8251.

will be a reasonable starting point. The largest ripple current

occurs at the highest V_IN. To guarantee that the ripple

current stays below the specified maximum, the inductor

value should be chosen according to the following

equation :

5V

⎡

⎤ ⎡

⎤

V

f × ΔI

V

OUT

V

IN(MAX)

OUT

L =

× 1−

BOOT

⎢

⎥ ⎢

⎥

L(MAX)

⎣

⎦ ⎣

⎦

100nF

RT8251

SW

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.

Figure 4. External Bootstrap Diode

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RT8251

Table 2. Suggested Inductors for Typical

Application Circuit

The output ripple, ΔV_OUT, is determined by :

1

⎡

⎤

ΔV_OUT≤ ΔI_LESR +

⎢

⎣

⎥

⎦

8fC_OUT

Component

Supplier

TDK

Series

Dimensions (mm)

The output ripple will be highest at the maximum input

SLF10165

NR10050

VLF12060

10.1x10.1x7

10x9.8x5

voltage since ΔI_Lincreases 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.

TAIYO YUDEN

TDK

12x11.7x6

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.

C_INand C_OUTSelection

The input capacitance, C_IN, 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 :

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, V_IN. 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 V_INlarge enough to damage the

part.

V

IN

V

OUT

I

= I

−1

RMS

OUT(MAX)

IN

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 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.

For the input capacitor, two 10μF low ESR ceramic

capacitors are recommended. For the recommended

capacitor, please refer to table 3 for more detail.

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, V_OUTimmediately shifts by an amount

equal to ΔI_LOAD(ESR) also begins to charge or discharge

C_OUTgenerating a feedback error signal for the regulator

The selection of C_OUTis determined by the required ESR

to minimize voltage ripple.

Moreover, the amount of bulk capacitance is also a key

for C_OUTselection 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.

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RT8251

to return V_OUTto its steady-state value. During this

recovery time, V_OUTcan be monitored for overshoot or

ringing that would indicate a stability problem.

snubber between SW and GND and make them as close

as possible to the SW pin (see Figure 5). Another method

is to add a resistor in series with the bootstrap capacitor,

C_BOOT. 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 of Layout Consideration.

EMI Consideration

Since parasitic inductance and capacitance effects in PCB

circuitry would cause a spike voltage on the 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 is to place an R-C

R

*

BOOT

1

3

2

V

IN

BOOT

RT8251

VIN

4.75V to 24V

C

IN

10µF x 2

C

BOOT

L

4.7µH

100nF

R

*

EN

V

OUT

SW

Chip Enable

3.3V/5A

7

EN

D

R *

S

B540C

C

*

EN

R1

30.9k

C

OUT

22µF x 2

C *

S

8

SS

5

6

FB

C

SS

10nF

4,

C

R

C

R2

10k

2.2nF

Exposed Pad(9)

22k

GND

COMP

C

P

* : Optional

NC

Figure 5. Reference Circuit with Snubber and Enable Timing Control

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 :

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

PSOP-8

P_D(MAX)= (125°C − 25°C) / (68°C/W) = 1.471W for

WQFN

(min.copper area PCB layout)

P_D(MAX)= (125°C − 25°C) / (49°C/W) = 2.04W for

PSOP-8 (70mm²copper area PCB layout)

P_D(MAX)= (T_J(MAX)− T_A) / θ_JA

The thermal resistance θ_JAof SOP-8 (Exposed Pad) is

determined by the package architecture design and the

PCB layout design. However, the package architecture

design had been designed. If possible, it's useful to

increase thermal performance by the PCB layout copper

design. The thermal resistance θ_JAcan be decreased by

adding copper area under the exposed pad of SOP-8

(Exposed Pad) package.

Where T_J(MAX)is the maximum operation junction

temperature , T_Ais the ambient temperature and the θ_JAis

the junction to ambient thermal resistance.

For recommended operating conditions specification of

RT8251, the maximum junction temperature is 125°C. The

junction to ambient thermal resistance θ_JAis layout

dependent. For PSOP-8 and WQFNpackages, the thermal

resistance θ_JAare 75°C/W and 68°C/W on the standard

JEDEC 51-7 four-layers thermal test board. The maximum

power dissipation at T_A= 25°C can be calculated by

following formula :

As shown in Figure 6, the amount of copper area to which

the SOP-8 (Exposed Pad) is mounted affects thermal

performance. When mounted to the standard

©

is a registered trademark of Richtek Technology Corporation.

DS8251-04 February 2013

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11

RT8251

SOP-8 (Exposed Pad) pad (Figure 6a), θ_JAis 75°C/W.

Adding copper area of pad under the SOP-8 (Exposed

Pad) (Figure 6.b) reduces the θ_JAto 64°C/W. Even further,

increasing the copper area of pad to 70mm²(Figure 6.e)

reduces the θ_JAto 49°C/W.

The maximum power dissipation depends on operating

ambient temperature for fixed T_J(MAX)and thermal

resistance θ_JA. For RT8251 packages, the derating curves

in Figure 7 and Figure 8 allow the designer to see the

effect of rising ambient temperature on the maximum power

dissipation allowed.

(a) Copper Area = (2.3 x 2.3) mm²,θ_JA= 75°C/W

2.2

Four Layer PCB

2.0

1.8

(b) Copper Area = 10mm²,θ_JA= 64°C/W

Copper Area

1.6

2

70mm

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

2

50mm

30mm

10mm

Min.Layout

(c) Copper Area = 30mm²,θ_JA= 54°C/W

0

25

50

75

100

125

(°C)

Ambient Temperature

Figure 7. Derating Curves for PSOP-8 Package

1.6

Four Layer PCB

1.4

1.2

(d) Copper Area = 50mm²,θ_JA= 51°C/W

1.0

WQFN-16L 3x3

0.8

0.6

0.4

0.2

0.0

0

15

30

45

60

75

90 105 120 135

(e) Copper Area = 70mm²,θ_JA= 49°C/W

Ambient Temperature (°C)

Figure 8.Derating Curves for WQFNPackage

Figure 6. Themal Resistance vs. CopperArea Layout

Design

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12

DS8251-04 February 2013

RT8251

Layout Consideration

` Connect feedback network behind the output capacitors.

Keep the loop area small. Place the feedback

components near the RT8251.

Follow the PCB layout guidelines for optimal performance

of the RT8251.

` Connect all analog grounds to a command node and

then connect the command node to the power ground

behind the output capacitors.

` 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

` Examples of PCB layout guide are shown in Figure 9

pins (VINandGND).

and Figure 10 for reference.

` LX node is with high frequency voltage swing and should

be kept at small area. Keep analog components away

from the LX node to prevent stray capacitive noise pick-

up.

Input capacitor must be placed

as close as to the IC as possible.

C

OUT

C

D

S

GND

C

IN

R

S

L

VOUT

16 15 14 13

SW

VIN

12

11

10

9

1

2

3

4

SW should be connected

GND

17

to inductor by Wide and

short trace. Keep sensitive

components away from

this trace.

VIN

C

BOOT

GND

5

6

7

8

R1

R2

C

SS

R

C

VOUT

C

P

C

GND

The feedback components must be connected

as close to the device as possible.

Figure 9. PCB Layout Guide for WQFN Package

is a registered trademark of Richtek Technology Corporation.

©

DS8251-04 February 2013

www.richtek.com

13

RT8251

The feedback components

must be connected as close

to the device as possible.

SW

V

IN

GND

C

BOOT

C

SS

C

Input capacitor must

be placed as close

to the IC as possible.

C

IN

8

7

6

5

BOOT

VIN

SS

2

3

4

R

C

EN

C

P

GND

D

SW

COMP

FB

9

C

R1

S

R

S

GND

R2

V

OUT

L

C

OUT

V

OUT

GND

SW should be connected to inductor by

wide and short trace. Keep sensitive

components away from this trace.

Figure 10. PCB Layout Guide for PSOP-8 Package

Table 3. Suggested Capacitors for C_INand C_OUT

Location Component Supplier Part No.

Capacitance (μF)

Case Size

1210

MURATA

TDK

GRM31ER61E226K

C4535X5R1E226M

TMK325BJ226MM

GRM32ER61C476M

GRM31CR60J476M

C3216X5R0J476M

LMK316BJ476MM

22

47

C_IN

1812

TAIYO YUDEN

MURATA

TDK

1210

C_IN

1210

C_OUT

1206

TAIYO YUDEN

1206

Table 4. Suggested Diode

Component Supplier

DIODES

Part No.

B540C

V

(V)

I

(A)

OUT

Package

SMC

RRM

40

5

ON

MBRS540T3

40

5

SMC

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14

DS8251-04 February 2013

RT8251

Outline Dimension

SEE DETAIL A

D

D2

L

1

E

E2

1

2

1

2

e

b

DETAILA

A

A3

Pin #1 ID and Tie Bar Mark Options

A1

Note : The configuration of the Pin #1 identifier is optional,

but must be located within the zone indicated.

Dimensions In Millimeters

Dimensions In Inches

Symbol

Min

Max

Min

Max

A

A1

A3

b

0.700

0.000

0.175

0.180

2.950

1.300

2.950

1.300

0.800

0.050

0.250

0.300

3.050

1.750

3.050

1.750

0.028

0.000

0.007

0.116

0.051

0.116

0.051

0.031

0.002

0.010

0.012

0.120

0.069

0.120

0.069

D

D2

E

E2

e

0.500

0.020

L

0.350

0.450

0.014

0.018

W-Type 16L QFN 3x3 Package

©

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DS8251-04 February 2013

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15

RT8251

H

A

Y

M

EXPOSED THERMAL PAD

(Bottom of Package)

J

B

X

F

C

I

D

Dimensions In Millimeters Dimensions In Inches

Symbol

Min

Max

Min

Max

0.197

0.157

0.069

0.020

0.053

0.010

0.006

0.244

0.050

0.091

0.098

0.138

A

B

C

D

F

H

I

4.801

3.810

1.346

0.330

1.194

0.170

0.000

5.791

0.406

2.000

2.100

3.000

5.004

4.000

1.753

0.510

1.346

0.254

0.152

6.200

1.270

2.300

2.500

3.500

0.189

0.150

0.053

0.013

0.047

0.007

0.000

0.228

0.016

0.079

0.083

0.118

J

M

X

Y

X

Y

Option 1

Option 2

8-Lead SOP (Exposed Pad) Plastic Package

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

parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.

www.richtek.com

16

DS8251-04 February 2013


型号：	RT8251
厂家：	RICHTEK TECHNOLOGY CORPORATION
描述：	5A, 24V, 570kHz Step-Down Converter 5A ， 24V ， 570kHz降压转换器转换器
文件：	总16页 (文件大小：482K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

RT8251 [RICHTEK]

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