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SC4612HMLTRT

40V Synchronous Buck Controller

Semtech Corporation 

SC4612H

Applications Information

General Description

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 configuration also allows converter sinking

current from load without 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.

EN pin is used to set the Soft Start duration.

t SS

≈

0.5 ⋅ CSS

25 ⋅10−6

. . . . . . . . . . . . . . . . . . . . . (1)

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 an internal

100μA current source.

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/

Under Voltage Lock Out

Under Voltage Lock Out (UVLO) circuitry senses

the VDD through a voltage divider. If this signal

falls below 4.5V (typical) with a 400mV hysteresis

(typical), the output drivers are disabled. During the

thermal shutdown, the output drivers are disabled.



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