Richtek RT8251GQW, RT8251GSP Schematic

®

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

RT8251

General Description

The RT8251 is a monolithic step-down switch mode converter with a built-in internal power MOSFET . It a chieves 5A continuous output current over a wide input supply range with excellent load a nd line regulation. Current mode operation provides fast tra nsient response a nd ea ses 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 WQ F N-16L 3x3 and SOP-8 (Exposed Pad) pa ckages.

Pin Configurations

(TOP VIEW)

Features

zz

Wide Operating Input Voltage Range : 4.75V to 24V

z

zz

z Adjustable Output Voltage Range : 0.8V to 15V

zz

z Output Current up to 5A

zz

μμ

zz

z 25

μA Low Shutdown Current

zz

μμ

zz

z Internal Power MOSFET : 70m

zz

z High Efficiency up to 95%

zz

z 570kHz Fixed Switching Frequency

zz

z Stable with Low ESR Output Ceramic Capacitors

zz

z Thermal Shutdown Protection

zz

z Cycle-By-Cycle Over Current Protection

zz

z RoHS Compliant and Halogen Free

zz

ΩΩ

Ω

ΩΩ

Applications

z Distributed Power Systems z Battery Charger z DSL Modems z Pre-regulator for Linear Regulators

VIN

VINSWSW

VIN

1 2

VIN VIN

GND

3 4

FB

COMP

WQFN-16L 3x3

BOOT

VIN SW

GND

2

GND

3 4

SOP-8 (Exposed Pad)

17

EN

13141516

SW

12 11

SW

10

SW BOOT

9

8765

SS

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 require-

8

SS

7

EN

6

9

COMP

5

FB

ments of IPC/JEDEC J-STD-020.

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

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DS8251-04 February 2013 www.richtek.com

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RT8251

Marking Information

RT8251GQW

GE= : Product Code

GE=YM

DNN

Typical Application Circuit

YMDNN : Date Code

RT8251GSP

RT8251 GSPYMDNN

RT8251GSP : Product Number YMDNN : Date Code

4.75V to 24V

V

Chip Enable

4.75V to 24V

Chip Enable

IN

C

10µF x 2

C

SS

10nF Exposed Pad (17)

V

IN

C

SS

10nF

Exposed Pad(9)

1, 2, 3,

15, 16

IN

VIN

RT8251

7

EN

8

SS

4,

GND

BOOT

SW

FB

COMP

9

10 to 14

5

2.2nF

6

C

(Open)

C

C 100nF

D

B540C

R

22k

P

BOOT

C

L

4.7µH

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

C

IN

10µF x 2

2

VIN

RT8251

7

EN

8

SS

4,

GND

BOOT

SW

FB

COMP

1

C

BOOT

100nF

3

D B540C

5

C

2.2nF

6

4.7µH

R

C

22k

L

R1

30.9k

R2 10k

R1

30.9k

R2

10k

C

OUT

22µF x 2

V

3.3V/5A

C

OUT

22µF x 2

V

OUT

3.3V/5A

OUT

C

P

NC

Figure 2. T ypical Application Circuit for SOP-8 (Exposed Pa d)

Table 1. Recommended Component Selection

V

(V) R1 (kΩ) R2 (kΩ) RC (kΩ) CC (nF) L1 (μH) C

OUT

(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

2

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DS8251-04 February 2013www.richtek.com

Functional Pin Description

Pin No.

WQFN-16L 3x3

1, 2, 3, 15, 16 2 VIN

4,

17 (Exposed Pad )

5 5 FB

6 6 COMP

7 7 EN

8 8 SS

9 1 BOOT

10, 11, 12, 13,

14

SOP-8

(Exposed Pad)

4,

9 (Exposed Pad)

3 SW

Pin Name Pin Function

GND

RT8251

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.

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 C 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.

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.

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. Bootstrap. This capacitor C switch’s gate above the supply voltage. It is connected between the SW and BS pins to form a floating supply across the power switch driver. The voltage across C 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.

is needed to drive the power

BOOT

ground path to

IN

is about 5V and is

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RT8251

Function Block Diagram

VIN

EN

SS

1µA

10k

3V

V

CC

1.1V

Regulator

VA

-

+

Shutdown

Comparator

V

CC

10µA

Internal

V

CC

0.4V

Comparator

0.8V

Gm = 820µA/V

FB

Oscillator

570kHz

Foldback

Control

+

-

UV

+

EA

+

-

COMP

Slope Comp

+

Current

Comparator

Current Sense

Amplifier

Logic

+

-

VA

Absolute Maximum Ratings (Note 1)

z Supply Voltage, V z Switching Voltage, V z BOOT V oltage, V z All Other Pins -------------------------------------------------------------------------------------------------−0.3V to 6V z Power Dissipation, P

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

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

z Package Thermal Re sistance (Note 2)

WQF N-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 T emperature----------------------------------------------------------------------------------------150°C z Lead T e mperature (Soldering, 10 sec.)------------------------------------------------------------------260 °C z Storage T emperature Range -------------------------------------------------------------------------------−65°C to 150°C z ESD Susceptibility (Note 3)

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

------------------------------------------------------------------------------------------−0.3V to 26V

IN

-------------------------------------------------------------------------------------−0.3V to (VIN + 0.3V)

SW

---------------------------------------------------------------------------------------(V

BOOT

@ TA = 25°C

D

− 0.3V) to (V

SW

BOOT

SW

GND

SW

+ 6V)

Recommended Operating Conditions (Note 4)

z Supply Voltage, V z Enable Voltage, V z Junction T emperature Range-------------------------------------------------------------------------------−40°C to 125°C z Ambient T emperature Range-------------------------------------------------------------------------------−40°C to 85°C

©

4

------------------------------------------------------------------------------------------4.75V to 24V

IN

-----------------------------------------------------------------------------------------0V to 5.5V

EN

DS8251-04 February 2013www.richtek.com

RT8251

Electrical Characteristics

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

Parameter Symbol Test Conditions Min Typ Max Unit

Feedb ack Ref eren ce V olt age VFB 4.75V ≦ VIN ≦ 24V 0.784 0.8 0.816 V High-Side Switch-On Resistance R Low -Side Swi tch -On Res is tanc e R

DS(ON)1 DS(ON)2

High-Side Switch Leakage V Current Limit I

LIM

Current Sense Transconductance GCS Output Current to V Erro r Amplifi er T ansc onduct ance gm ΔIC = ±10μA -- 920 -- μA/V Oscillator Frequency fSW 420 570 720 kHz Short Circuit Oscillation Frequency V Maximum Duty Cycle D

MAX

Mini mum On- Ti me tON -- 100 -- ns UVLO Threshold Rising -- 4.1 -- V UVL O Thresh old Hyst eresis -- 200 -- mV

Logic Low VIL -- -- 0.4

EN Input Voltage

Logic High V

IH

Enable Pu l l Up Cu r r en t V Shutdown Current I

SHDN

Quies cen t Curr ent IQ V Soft-Start Current ISS V Soft-Start Period C 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. θ

Note 3. Devices are ESD sensitive. Handling precaution is recommended. Note 4. The device is not guaranteed to function outside its operating conditions.

is measured at T

JA

measured at the exposed pad of the package.

= 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is

A

-- 70 -- mΩ

-- 15 -- Ω = 0V, V

EN

Duty = 85%; V

= 0V -- 185 -- kHz

FB

V

= 0.7V -- 85 -- %

FB

= 0V -- -- 10 μA

SW

BOOT−SW

= 4.8V -- 6.8 -- A

-- 4.6 -- A/V

COMP

1.4 -- 5.5 = 0V -- 1 -- μA

EN

VEN = 0V -- 25 -- μA

= 2V, VFB = 1V -- 0.8 1 mA

EN

= 0V -- 10 -- μA

SS

= 10nF -- 1 -- ms

SS

V

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RT8251

)

Typical Operating Characteristics

Efficiency vs. Load Current

100

90 80 70 60 50 40

Eff iciency (%)

30 20 10

0

00.511.522.533.544.55

VIN = 24V

V

Load Current (A)

Output Voltag Deviation vs. Load Current

2

VIN = 24V

1.5

0.5

1

= 12V

V

IN

VIN = 5V

VIN = 12V

= 3.3V

OUT

Efficiency vs. Load Current

100

90 80 70 60 50 40

Efficiency(%)

30 20 10

0

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

VIN = 24V

V

OUT

Load Current (A)

Output Voltage Deviation vs. Input Voltage

2

I

= 5A

OUT

1.5

I

= 3A

OUT

= 0A

I

OUT

1

0.5

VIN = 12V

= 5V

Reference Volt age (V)

0

-0.5

-1

-1.5

Output Voltage Deviati on (%) 1

-2 4 6.5 9 11.5 14 16.5 19 21.5 24

V

Inpu t Volt age (V)

Reference Voltage vs. Temperature

0.816

0.811

0.806

0.801

0.796

0.791

0.786

-50 -25 0 25 50 75 100 125

Temperature (°C)

= 3.3V

OUT

VIN = 12V

0

-0.5

-1

-1.5

Output V oltage Deviati on (%) 1

-2

0.001 0.01 0.1 1 10

V

Load Current (A)

Quiescent Current vs. Temperature

1.2

1

0.8

0.6

0.4

Quiescent C urrent (mA

0.2

0

-50 -25 0 25 50 75 100 125

Temperature (°C)

= 3.3V

OUT

VIN = 12V

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Richtek RT8251GQW, RT8251GSP Schematic [ru]

Specifications and Main Features

Frequently Asked Questions

User Manual