SC1109CSTR datasheet(11/17 Pages) SEMTECH

11 / 17 page

SYNCHRONOUS PWM CONTROLLER WITH

DUAL LOW DROPOUT REGULATOR

CONTROLLERS

PRELIMINARY - October 16, 2000

SC1109

TEL:805-498-2111 FAX:805-498-3804 WEB:http://www.semtech.com

Mosfet gate drive can be provided by an external 12V

supply that is connected from BST to GND. The

actual gate to source voltage of the upper Mosfet will

approximately equal 7V (12V-VCC). If the external

12V supply is not available, a classical boot strap

technique can be implemented from the VCC supply.

A boot strap capacitor is connected from BST to

Phase while VCC is connected through a diode

(Schottky or other fast low VF diode) to the BST. This

will provide a gate to source voltage approximately to

VCC-Vdiode drop.

Shoot through control circuitry provides a 100ns dead

time to ensure both upper and lower MOSFET will not

turn on simultaneously and cause a shoot through

condition.

OVER CURRENT

Upper Mosfet’s Rdson is used to monitor the drop

across the top FET due to an over current condition.

This Method of current sensing minimizes any unnec-

essary losses due to external sense resistance.

An internal comparator with a 200mV reference moni-

tors the Drop across the upper FET, Once the Vdson

of the Mosfet exceeds the 200mV limit, the low side

gate is turned on and the upper FET is turned off.

Also an internal latch is set and the Soft start capaci-

tor is discharged. Once the lower threshold of the soft

start circuit is crossed, the same Softstart sequence

mentioned previously is repeated. This sequence is

repeated until the over condition is removed.

GATE DRIVERS

The Low side gate driver is supplied from VCC and

provide a peak source/sink current of and 500mA.

The high side gate drive is also capable of sourcing

and sinking peak currents of 500mA. The high side

Lower Gate

PhaseNode

Lower Gate

Upper Gate

Lower Gate

PhaseNode

Vtt Shorted

Upper Gate

Lower Gate

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