Richtek RT7277GSP Schematic [ru]

RT7277

3A, 18V, 700kHz ACOTTM Synchronous Step-Down Converter

General Description

The RT7277 is a synchronous step-down DC/DC converter

with Advanced Constant On-Time (ACOT

) mode control.

It achieves high power density to deliver up to 3A output

current from a 4.5V to 18V input supply. The proprietary

ACOTTM mode offers an optimal transient response over a

wide range of loads and all kinds of ceramic capacitors,

which allows the device to adopt very low ESR output

capacitors for ensuring performance stabilization. In

addition, RT7277 keeps an excellent constant switching

frequency under line and load variation and the integrated

synchronous power switches with the ACOTTM mode

operation provides high efficiency in whole output current

load range. Cycle-by-cycle current limit provides an

accurate protection by a valley detection of low side

MOSFET and external soft-start setting eliminates input

current surge during startup. Protection functions also

include output under voltage protection, output over voltage

protection, and thermal shutdown.

Marking Information

RT7277GSP : Product Number

RT7277 GSPYMDNN

YMDNN : Date Code

Features

ACOTTM Mode Enable s Fa st T ra n sient Respon se

z 4.5V to 18V Input Voltage Range

z 3A Output Current

ΩΩ

z 60m

Ω Internal Low Site N-MOSFET

ΩΩ

z Advanced Constant On-Time Control

z Support All Ceramic Capa citors

z Up to 95% Efficiency

z 700kHz Switching Frequency

z Adjustable Output Voltage from 0.765V to 8V

z Adjustable Soft-Start

z Cycle-by-Cycle Current Limit

z Input Under Voltage Lockout

z Thermal Shutdown

z RoHS Compliant and Halogen Free

Applications

z Industrial and Commercial Low Power Systems

z Computer Peripherals

z LCD Monitors and TVs

z Green Electronics/Appliances

z Point of Load Regulation for High-Performance DSPs,

FPGAs, and ASICs

Simplified Application Circuit

RT7277

VIN

C1 C2

Chip Enable

BOOT

PVCC

GND

DS7277-01 March 2013 www.richtek.com

PVCC

OUT

RT7277

Ordering Information

Pin Configurations

RT7277

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

Lead Plating System G : Green (Halogen Free and Pb Free)

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.

Functional Pin Description

Pin No. Pin Name Pin Function

1 EN

2 FB

3 PVCC

4 SS

5, 9

(Exposed Pad)

GND

Enable Input. A logic-high enables the converter; a logic-low forces the IC into shutdown mode reducing the supply current to less than 10μA.

Feedback Input. It is used to regulate the output of the converter to a set value via an external resistive voltage divider. The feedback threshold voltage is 0.765V typically.

Internal Regulator Output. Connect a 1μF capacitor to GND to stabilize output voltage.

Soft-Start Control Input. SS controls the soft-start period. Connect a capacitor from SS to GND to set the soft-start period. A 3.9nF capacitor sets the soft-start period of

to 2.6ms.

OUT

Ground. The Exposed pad should be soldered to a large PCB and connected to GND for maximum thermal dissipation.

(TOP VIEW)

PVCC

GND

SOP-8 (Exposed Pad)

VIN

BOOT

GND

6 SW Switch Node. Connect this pin to an external L-C filter.

7 BOOT

8 VIN

Bootstrap Supply for High Side Gate Driver. Connect a 0.1μF or greater ceramic capacitor from BOOT to SW pins.

Power Input. The input voltage range is from 4.5V to 18V. Must bypass with a suitably large ( ≥10μF x 2) ceramic capacitor.

DS7277-01 March 2013www.richtek.com

Function Block Diagram

RT7277

Operation

SS FB

PVCC

Internal

Regulator

VIBIAS

PVCC

On-Time

2µA

PVCC

REF

Ripple

Gen.

Over Current

Protection

Switch

Controller

Comparator

BOOT

PVCC

Driver

VIN

UGATE

LGATE

GND

In normal operation, the high side N-MOSFET is turned

on when the FB Comparator sets the Switch Controller,

and it is turned off when On-Time Controller resets the

Switch Controller. While the high side N-MOSFET is turned

off, the low side N-MOSFET is turned on and waits for the

FB Comparator to set the beginning of next cycle.

The FB Comparator sets the Switch Controller by

comparing the feedback signal (FB) from output voltage

with the internal 0.765V reference. When load transient

induces VOUT drop, the FB voltage will be less than its

threshold voltage. This means that the high side N-MOSFET

will turn on again immediately after minimum off-time

expired. The switching frequency will vary during the

transient period thus can provide a very fast transient

response. After the load transient finished, the RT7277

will be back to steady state with a constant switching

frequency.

Enable

Activate internal regulator once EN input level is higher

than the target level. Force IC to enter shutdown mode

when the EN input level is lower than 0.4V

Internal Regulator

Provide internal power for logic control and switch gate

drivers.

On-Time Controller

Control on-time according to VIN and SW to obtain

constant switching frequency.

DS7277-01 March 2013 www.richtek.com

RT7277

Absolute Maximum Ratings (Note 1)

z Supply Voltage, VIN ----------------------------------------------------------------------------------------------- −0.3V to 21V

z Switch Voltage, SW ----------------------------------------------------------------------------------------------- −0.8V to (V

<10ns ----------------------------------------------------------------------------------------------------------------- −5V to 25V

z BOOT to SW, PVCC ---------------------------------------------------------------------------------------------- −0.3V to 6V

z Other Pins Voltage ------------------------------------------------------------------------------------------------- −0.3V to 21V

z Power Dissipation, P

@ TA = 25°C

SOP-8 (Exposed Pad) -------------------------------------------------------------------------------------------- 2.041W

z Package Thermal Resistance (Note 2)

SOP-8 (Exposed Pad), θJA--------------------------------------------------------------------------------------- 49°C/W

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

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

+ 0.3V)

Recommended Operating Conditions

z Supply Voltage, VIN ----------------------------------------------------------------------------------------------- 4.5V to 18V

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

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

(Note 4)

Electrical Characteristics

(VIN = 12V, T

Supply Current

Shutdown Current I

Quiescent Current IQ V

Logic Threshold

EN Voltage

VFB Voltage and Discharge Resistance

Feedback Threshold Voltage VFB 4.5V ≤ V

Feedback Input Current IFB V

Output

PVCC

Output Voltage V

PVCC

Line Regulation 6V ≤ V Load Regulation 0 < I

Output Current I

= 25°C, unless otherwise specified)

Parameter Symbol Test Conditions Min Typ Max Unit

SHDN

= 0V -- 1.5 10 μA

= 3V, VFB = 1V -- 0.7 -- mA

Logic-High 2 -- 18

Logic-Low -- -- 0.4

≤ 18V 0.757 0.765 0.773 V

= 0.8V −0.1 0 0.1 μA

6V ≤ V

PVCC

≤ 18V, 0 < I

≤ 18V, I

< 5mA -- -- 100 mV

PVCC

= 6V, V

PVCC

= 4V -- 110 -- mA

PVCC

< 5mA 4.7 5.1 5.5 V

PVCC

= 5mA -- -- 20 mV

DS7277-01 March 2013www.richtek.com

RT7277

Parameter Symbol Test Conditions Min Typ Max Unit

Switch On Resistance

DS(ON)

High Side

DS(ON)_H

-- 90 --

mΩ

Low Side

DS(ON)_L

-- 60 --

Current Limit

Current Limit I

3.5 4.1 5.7 A

LIM

Thermal Shutdown

Thermal Shutdown Threshold TSD -- 150 -- °C Thermal Shutdown Hysteresis ΔTSD -- 20 -- °C

On-Time Timer Control

On-Time tON V

Minimum On-Time t

Minimum Off-Time t

ON(MIN)

OFF(MIN)

-- 60 -- ns

= 12V, V

= 1.05V -- 145 -- ns

OUT

-- 230 -- ns

Soft-Start

SS Charge Current VSS = 0V 1.4 2 2.6 μA

SS Discharge Current VSS = 0.5V 0.05 0.1 -- mA

UVLO

UVLO Threshold VIN Rising to Wake up V

3.55 3.85 4.15

PVCC

Hysteresis -- 0.3 --

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

measured at the exposed pad of the package. The PCB copper area of exposed pad is 70mm

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

DS7277-01 March 2013 www.richtek.com

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