Richtek RT8251GQW, RT8251GSP Schematic [ru]

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RT8251

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

General Description

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.

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.

Pin Configurations

(TOP VIEW)

 

VIN

VIN

SW

SW

 

 

 

16

15

14

13

 

 

VIN

1

 

 

 

12

SW

VIN

2

GND

 

11

SW

VIN

3

 

10

SW

 

17

GND

4

 

9

BOOT

 

 

 

 

5

6

7

8

 

 

 

FB

COMP

EN

SS

 

 

 

WQFN-16L 3x3

 

BOOT

 

8

SS

VIN

2

7

EN

SW

3

GND

COMP

6

GND

4

9

FB

5

SOP-8 (Exposed Pad)

Features

zWide Operating Input Voltage Range : 4.75V to 24V

zAdjustable Output Voltage Range : 0.8V to 15V

zOutput Current up to 5A

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

zCycle-By-Cycle Over Current Protection

zRoHS Compliant and Halogen Free

Applications

zDistributed Power Systems

zBattery Charger

zDSL Modems

zPre-regulator for Linear Regulators

Ordering Information

RT8251

Package Type

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

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

Lead Plating System

G : Green (Halogen Free and Pb Free)

Note :

Richtek products are :

`RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.

`Suitable for use in SnPb or Pb-free soldering processes.

Copyright ©2013 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

DS8251-04 February 2013

www.richtek.com

 

1

RT8251

Marking Information

RT8251GQW

 

RT8251GSP

 

 

 

GE= : Product Code

 

 

RT8251GSP : Product Number

 

 

 

RT8251

 

GE=YM

YMDNN : Date Code

 

YMDNN : Date Code

 

 

GSPYMDNN

 

DNN

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Typical Application Circuit

 

 

 

VIN

 

 

 

 

 

 

 

 

 

 

 

1, 2, 3,

 

 

 

 

 

 

 

 

 

 

 

 

 

 

15, 16

4.75V to 24V

 

 

 

 

 

 

 

 

 

 

CIN

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Chip Enable

 

 

 

 

 

 

 

 

 

 

10µF x 2

7

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

8

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CSS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

10nF

4,

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Exposed Pad (17)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

VIN

BOOT

9

 

CBOOT

L

 

RT8251

 

 

 

10 to 14

100nF

4.7µH

VOUT

EN

SW

D

 

 

 

 

R1

3.3V/5A

 

 

 

 

B540C

 

SS

 

5

 

30.9k

COUT

FB

 

 

 

 

 

 

 

 

CC

RC

R2

22µF x 2

 

 

 

GND

COMP 6

2.2nF

22k

10k

 

 

 

 

 

 

 

 

CP

 

 

 

 

 

(Open)

 

 

 

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

VIN

2

VIN

BOOT 1

CBOOT

 

4.75V to 24V

CIN

 

 

 

 

 

10µF x 2

 

RT8251

3

100nF

L

VOUT

Chip Enable

 

 

SW

 

 

7

EN

 

D

4.7µH

3.3V/5A

 

 

 

R1

 

8

 

 

 

B540C

30.9k

CSS

SS

FB 5

 

 

COUT

4,

 

 

 

CC

RC

22µF x 2

10nF

Exposed Pad(9)

 

 

 

R2

 

GND

COMP 6

2.2nF

22k

10k

 

 

 

 

 

 

 

 

 

 

CP

 

 

 

 

 

 

 

NC

 

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

Table 1. Recommended Component Selection

 

VOUT (V)

R1 (kΩ)

R2 (kΩ)

RC (kΩ)

CC (nF)

L1 (μH)

 

COUT (μF)

 

 

 

15

182

10

51

1

22

 

44

 

 

 

 

 

 

 

 

 

 

 

 

 

 

10

115

10

43

1.2

10

 

44

 

 

 

8

91

10

39

1.5

10

 

44

 

 

 

5

52.3

10

30

1.5

6.8

 

44

 

 

 

3.3

30.9

10

22

2.2

4.7

 

44

 

 

 

2.5

21.5

10

16

2.2

4.7

 

44

 

 

 

1.8

12.4

10

13

2.2

2.2

 

44

 

 

 

1.2

4.99

10

13

2.2

2.2

 

44

 

 

 

 

 

 

 

 

 

 

 

 

 

Copyright ©2013 Richtek Technology Corporation. All rights reserved.

 

is a registered trademark of Richtek Technology Corporation.

www.richtek.com

 

 

 

 

 

DS8251-04 February 2013

2

RT8251

Functional Pin Description

Pin No.

 

 

WQFN-16L 3x3

SOP-8

Pin Name

Pin Function

(Exposed Pad)

 

 

 

 

 

 

 

 

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

1, 2, 3, 15, 16

2

VIN

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

 

 

 

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

 

 

 

IC.

 

 

 

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

 

 

 

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

4,

4,

GND

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

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.

5

5

FB

Feedback Input. An external resistor divider from the output to

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

 

 

 

 

 

 

Compensation Node. This node is the output of the

6

6

COMP

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.

 

 

 

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

7

7

EN

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.

8

8

SS

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

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

 

 

 

 

 

 

1ms.

 

 

 

Bootstrap. This capacitor CBOOT is needed to drive the power

 

 

 

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

9

1

BOOT

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

switch driver. The voltage across CBOOT is about 5V and is

 

 

 

 

 

 

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

 

 

 

low.

 

 

 

Power Switching Output. SW is the switching node that supplies

10, 11, 12, 13,

3

SW

power to the output. Connect the output LC filter from SW to the

14

output load. Note that a capacitor is required from SW to BOOT

 

 

 

 

 

to power the high-side switch.

Copyright ©2013 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

DS8251-04 February 2013

www.richtek.com

3

RT8251

Function Block Diagram

VIN

 

 

 

 

 

Current Sense

 

VCC

 

 

 

 

 

Internal

 

Oscillator

Slope Comp

Amplifier

 

 

 

 

 

1µA

 

Regulator

570kHz

 

 

+

VA

 

VA VCC

 

 

 

 

-

 

10k

 

 

Foldback

 

 

 

BOOT

 

-

 

Control

 

 

 

EN

 

 

 

 

 

 

1.1V

+

0.4V

+

 

 

 

SW

 

 

Shutdown

 

Logic

3V

 

 

-

+

Comparator

 

 

 

 

UV

 

 

 

 

VCC

 

 

-

 

 

 

 

 

Comparator

 

 

GND

 

 

10µA

 

 

Current

 

 

 

 

0.8V

+

Comparator

 

 

 

SS

 

 

 

 

 

 

 

 

 

+EA

 

 

 

 

 

 

 

 

-

 

 

 

 

 

 

 

Gm = 820µA/V

 

 

 

 

 

 

 

FB

COMP

 

 

 

 

Absolute Maximum Ratings (Note 1)

z Supply Voltage, VIN ------------------------------------------------------------------------------------------

−0.3V to 26V

z Switching Voltage, VSW -------------------------------------------------------------------------------------

−0.3V to (VIN + 0.3V)

z BOOT Voltage, VBOOT ---------------------------------------------------------------------------------------

(VSW − 0.3V) to (VSW + 6V)

z All Other Pins -------------------------------------------------------------------------------------------------

−0.3V to 6V

z Power Dissipation, PD @ TA = 25°C

 

WQFN-16L 3x3 -----------------------------------------------------------------------------------------------

1.471W

SOP-8 (Exposed Pad) --------------------------------------------------------------------------------------

1.333W

z Package Thermal Resistance (Note 2)

 

WQFN-16L 3x3, θJA ------------------------------------------------------------------------------------------

68°C/W

WQFN-16L 3x3, θJC -----------------------------------------------------------------------------------------

7.5°C/W

SOP-8 (Exposed pad), θJA ---------------------------------------------------------------------------------

75°C/W

SOP-8 (Exposed Pad), θJC --------------------------------------------------------------------------------

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, VIN ------------------------------------------------------------------------------------------

4.75V to 24V

z Enable Voltage, VEN -----------------------------------------------------------------------------------------

0V to 5.5V

z Junction Temperature Range -------------------------------------------------------------------------------

40°C to 125°C

z Ambient Temperature Range -------------------------------------------------------------------------------

40°C to 85°C

Copyright ©2013 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

www.richtek.com

DS8251-04 February 2013

4

RT8251

Electrical Characteristics

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

Parameter

Symbol

Test Conditions

Min

Typ

Max

Unit

 

 

 

 

 

 

 

 

Feedback Reference Voltage

VFB

4.75V VIN 24V

0.784

0.8

0.816

V

High-Side Switch-On Resistance

RDS(ON)1

 

--

70

--

Low-Side Switch-On Resistance

RDS(ON)2

 

--

15

--

Ω

High-Side Switch Leakage

 

VEN = 0V, VSW = 0V

--

--

10

μA

Current Limit

 

ILIM

Duty = 85%; VBOOT−SW = 4.8V

--

6.8

--

A

Current Sense Transconductance

GCS

Output Current to VCOMP

--

4.6

--

A/V

Error Amplifier Tansconductance

gm

IC = ±10μA

--

920

--

μA/V

Oscillator Frequency

 

fSW

 

420

570

720

kHz

Short Circuit Oscillation Frequency

 

VFB = 0V

--

185

--

kHz

Maximum Duty Cycle

 

DMAX

VFB = 0.7V

--

85

--

%

Minimum On-Time

 

tON

 

--

100

--

ns

UVLO Threshold Rising

 

 

--

4.1

--

V

UVLO Threshold Hysteresis

 

 

--

200

--

mV

EN Input Voltage

Logic Low

VIL

 

--

--

0.4

V

Logic High

VIH

 

1.4

--

5.5

 

 

 

Enable Pull Up Current

 

VEN = 0V

--

1

--

μA

Shutdown Current

 

ISHDN

VEN = 0V

--

25

--

μA

Quiescent Current

 

IQ

VEN = 2V, VFB = 1V

--

0.8

1

mA

Soft-Start Current

 

ISS

VSS = 0V

--

10

--

μA

Soft-Start Period

 

 

CSS = 10nF

--

1

--

ms

Thermal Shutdown

 

TSD

 

--

150

--

°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.

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.

Copyright ©2013 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

DS8251-04 February 2013

www.richtek.com

5

Richtek RT8251GQW, RT8251GSP Schematic

RT8251

Typical Operating Characteristics

Efficiency vs. Load Current

 

100

 

 

 

 

 

 

 

 

 

 

 

90

 

 

 

 

 

 

 

VIN = 12V

 

 

 

 

 

 

 

 

 

 

 

 

 

80

 

 

 

 

 

 

 

VIN = 24V

 

(%)

70

 

 

 

 

 

 

 

 

 

 

60

 

 

 

 

 

 

 

 

 

 

Efficiency

 

 

 

 

 

 

 

 

 

 

50

 

 

 

 

 

 

 

 

 

 

40

 

 

 

 

 

 

 

 

 

 

30

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

20

 

 

 

 

 

 

 

 

 

 

 

10

 

 

 

 

 

 

 

VOUT = 3.3V

 

 

0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Efficiency vs. Load Current

 

100

 

 

 

 

 

 

 

VIN = 12V

 

90

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

VIN = 24V

 

 

80

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Efficiency(%)

70

 

 

 

 

 

 

 

 

 

 

60

 

 

 

 

 

 

 

 

 

 

50

 

 

 

 

 

 

 

 

 

 

40

 

 

 

 

 

 

 

 

 

 

30

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

20

 

 

 

 

 

 

 

 

 

 

 

10

 

 

 

 

 

 

 

VOUT = 5V

 

 

0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Load Current (A)

Load Current (A)

Output Voltag Deviation vs. Load Current

Output Voltage Deviation vs. Input Voltage

 

2

VIN = 24V

 

 

 

 

 

2

IOUT = 5A

 

 

 

 

 

 

 

(%)1

 

 

 

 

 

(%)1

 

 

 

 

 

 

 

 

1.5

VIN = 12V

 

 

 

 

1.5

IOUT = 3A

 

 

 

 

 

 

 

 

VIN = 5V

 

 

 

 

 

IOUT = 0A

 

 

 

 

 

 

 

Deviation

1

 

 

 

 

Deviation

1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.5

 

 

 

 

 

0.5

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Voltage

0

 

 

 

 

 

Voltage

0

 

 

 

 

 

 

 

 

-0.5

 

 

 

 

 

-0.5

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Output

-1

 

 

 

 

 

Output

-1

 

 

 

 

 

 

 

 

-1.5

 

 

 

VOUT = 3.3V

 

-1.5

 

 

 

 

 

VOUT = 3.3V

 

 

 

 

 

 

 

 

 

 

 

 

 

 

-2

 

 

 

 

 

-2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.001

0.01

0.1

1

10

 

4

6.5

9

11.5

14

16.5

19

21.5

24

Load Current (A)

Input Voltage (V)

 

Quiescent Current vs. Temperature

 

1.2

 

 

 

 

 

 

 

(mA)

1

 

 

 

 

 

 

 

0.8

 

 

 

 

 

 

 

Current

 

 

 

 

 

 

 

0.6

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Quiescent

0.4

 

 

 

 

 

 

 

0.2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

 

 

 

 

 

VIN = 12V

 

 

 

 

 

 

 

 

 

-50

-25

0

25

50

75

100

125

Temperature (°C)

Reference Voltage vs. Temperature

 

0.816

 

 

 

 

 

 

 

 

0.811

 

 

 

 

 

 

 

(V)

0.806

 

 

 

 

 

 

 

Voltage

 

 

 

 

 

 

 

0.801

 

 

 

 

 

 

 

Reference

0.796

 

 

 

 

 

 

 

0.791

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.786

 

 

 

 

 

VIN = 12V

 

 

 

 

 

 

 

 

 

-50

-25

0

25

50

75

100

125

Temperature (°C)

Copyright ©2013 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

www.richtek.com

DS8251-04 February 2013

6

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