SC4612HSTRT datasheet(10/21 Pages) SEMTECH

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SC4612H

POWER MANAGEMENT

Applications Information

INTRODUCTION

The SC4612H is a versatile voltage mode synchronous

rectified buck PWM convertor, with an input supply (VIN)

ranging from 4.5V to 40V designed to control and drive

N-channel MOSFETs.

The power dissipation is controlled by allowing high speed

and integration with the high drive currents to ensure low

MOSFET switching loss. The synchronous buck configu-

ration also allows converter sinking current from load with-

out losing output regulation.

The internal reference is trimmed to 500mV with ± 1%

accuracy, and the output voltage can be adjusted by an

external resistor divider.

A fixed oscillator frequency (up to 1.2MHz) can be

programmed by an external capacitor for design

optimization.

Other features of the SC4612H include:

Wide input power voltage range (from 4.5V to 40V), low

output voltages, externally programmable soft-start, hiccup

over current protection, wide duty cycle range, thermal

shutdown, and -40 to 125°C junction operating

temperature range.

THEORY OF OPERATION

SUPPLIES

Two pins (VDD and DRV) are used to power up the

SC4612H. If input supply (Vin) is less than 10V, tie DRV

and VDD together.

This DRV supply should be bypassed with a low ESR 2.2uF

(or greater) ceramic capacitor directly at the DRV to GND

pins of the SC4612H.

The DRV supply also provides the bias for the low and the

high side MOSFET gate drive.

The maximum rating for DRV supply is 10V and for

applications where input supply is below 10V, it should be

connected directly to VDD.

The internal pass transistor will regulate the DRV from an

external supply connected to VDD to produce 7.8V typical

at the DRV pin.

Soft Start / Shut down

The SC4612H performs a “pre-bias” type startup. This

ensures that a pre-charged output capacitor will not

cause the SC4612H to turn on the bottom FET during

startup to discharge it, as a normal synchronous buck

controller would do. An external capacitor on the SS/EN

pin is used to set the Soft Start duration.

−

•

≈

Startup is inhibited until VDD input reaches the UVLO

threshold (typically 4.5V). Once VDD rises above UVLO,

the external soft start capacitor begins to charge from an

internal 25uA current source. When the SS/EN pin reaches

approximately 0.8V, top side switching is enabled.

However, a top side pulse will not occur until SS/EN has

charged up to the level appropriate for the existing output

voltage (a pre bias condition). Once the first top side gate

pulse actually occurs, the bottom side driver is enabled

and the remainder of the startup is fully synchronous.

In the event of an over current during startup, the SC4612H

behaves in the same manner as an over current in steady

state (see Over Current Protection).

Oscillator Frequency Selection

The internal oscillator sawtooth signal is generated by

charging an external capacitor with a current source of

100µA charge current.

See Table 1 “Frequency vs. C

OSC” to determine oscillator

frequency.

Frequency, vs. C

OSC

100

200

300

400

500

600

700

800

900

1000

1100

1200

100

200

300

400

500

600

700

800

900

1000

1100

1200

Frequency, (kHz)

Table 1

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