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SS9175/6

OPERATION

The SS9175/6 series controllers integrate two sets

of synchronous MOSFET driver circuits with current

sharing capability. The following descriptions highlight

the advantages of the SS917x designs.

Soft-start

A 25uA start-up current is provided by the SS/EN pin

for the start-up sequence. During this start-up

sequence, the SS917x is disabled when the SS/EN

pin is less than 1.0V. From 1.0V to 3.0V, PWM output

duty cycle is gradually and smoothly increased to its

desired value. During this time, the current sharing

circuit is disabled for smooth soft start. After 3.0V, the

current sharing circuit is enabled and the whole circuit

operates normally.

Oscillator operation

The SS9175/6 series have three versions with

different

oscillation

frequencies.

The

oscillation

frequency is fixed at 300 kHz, 600 kHz or 1 MHz. The

voltage amplitude of the internal saw tooth oscillator is

from 1.2V to 2.8V.

Error amplifier

The error amplifier’s inverting input is connected to the

IN pin, and the output is connected to the COMP pin.

The

COMP

output

is

available

for

external

compensation, allowing designers to control the

feedback-loop

frequency-response.

Non-inverting

input is not wired out to a pin, but it is internally biased

to a fixed 0.7V ± 1% voltage.

Over-current protection

The over-current protection (OCP) is implemented by

adding a resistor from the MOSFET supply voltage to

the CL+ pin, which sinks a 100uA current source. An

internal comparator senses the voltage difference

between CL+ and CL- pin. If the CL- pin voltage is

lower than the CL+ pin voltage, meaning there is a

larger current flowing through the high-side MOSFET,

the comparator will trigger the OCP protection. The

OCP function also has a 50% fold-back circuit to limit

the MOSFET current within the desired over-current

value.

Output driver

The high-side driver uses an external bootstrap circuit

to provide the required boost supply voltage. The

external bootstrap circuit uses the BSTC output

voltage for providing the diode voltage. For the

low-side driver, the supply voltage is coming from the

BSTC output voltage, which is roughly 6V if VCC is

larger than 6.5V.The output stage is designed to

ensure zero cross-conduction current.

Current Sharing

The

dual-phase

controller

has

current-sharing

capability to match both channels to within 5%.

APPLICATION INFORMATION

Rev.2.02 12/29/2003

PRELIMINARY