Document Number: 70818

S11-0975-Rev. D, 16-May-11

www.vishay.com

13

Vishay Siliconix

Si9136

This document is subject to change without notice.

DESCRIPTION OF OPERATION (CONT’D)

Normal Operation: Buck Converters

In normal operation, the buck converter high-side MOSFET

is turned on with a delay (known as break-before-make time

time, the high-side MOSFET is turned off and then after a

rising edge of the clock, or the inductor current reaches zero.

to guarantee the efficiency is not adversely affected at the

high switching frequency and a specified minimum to

account

for

variations

of

possible

MOSFET

gate

capacitances.

During the normal operation, the high-side MOSFET switch

on-time is controlled internally to provide excellent line and

load regulation over temperature. Both buck converters

should have load, line, regulation to within 0.5 % tolerance.

Pulse Skipping: Buck Converters

When the buck converter switching frequency is less than

the internal clock frequency, its operation mode is defined as

pulse skipping mode. During this mode, the high-side

on time reaches its maximum duty ratio. After the high-side

MOSFET is turned off, the low-side MOSFET is turned on

current reaches zero. The output voltage will rise slightly

above the regulation voltage after this sequence, causing the

controller to stay idle for the next one, or several clock cycles.

When the output voltage falls slightly below the regulation

level, the high-side MOSFET will be turned on again at the

next clock cycle. With the converter remaining idle during

some clock cycles, the switching losses are reduced in order

to preserve conversion efficiency during the light output

current condition.

Current Limit: Buck Converters

When the buck converter inductor current is too high, the

voltage across pin CS3(5) and pin FB3(5) exceeds

approximately 120 mV, the high-side MOSFET would be

turned off instantaneously regardless of the input, or output

condition. The Si9136 features clock cycle by clock cycle

current limiting capability.

Flyback Converter Operation:

Designed mainly for PCMCIA or EEPROM programming, the

Si9136 has a 12 V output non-isolated buck boost converter,

called for brevity a flyback.

turned on and off in phase, and two diodes. Similar to the

buck converter, during the light load conditions, the flyback

converter will switch at a frequency lower than the internal

clock frequency, which can be defined as pulse skipping

mode (PSM); otherwise, it is operating in normal PWM mode.

Normal Operation: Flyback Converter

In normal operation mode, the two MOSFETs are turned on

at the rising edge of the clock, and then turned off. The on

time is controlled internally to provide excellent load, line,

and temperature regulation. The flyback converter has load,

line and temperature regulation well within 0.5 %.

Pulse Skipping: Flyback Converter

Under the light load conditions, similar to the buck converter,

the flyback converter will enter pulse skipping mode. The

MOSFETs will be turned on until the inductor current

increases to such a level that the voltage across the pin CSP

and pin CSN reaches 100 mV, or the on time reaches the

maximum duty cycle. After the MOSFETs are turned off, the

inductor current will conduct through two diodes until it

reaches zero. At this point, the flyback converter output will

rise slightly above the regulation level, and the converter will

stay idle for one or several clock cycle(s) until the output falls

back slightly below the regulation level. The switching losses

are reduced by skipping pulses and so the efficiency during

light load is preserved.

Current Limit: Flyback Converter

Similar to the buck converter; when the voltage across pin

CSP and pin CSN exceeds 410 mV typical, the two

MOSFETs will be turned off regardless of the input and

output conditions.

Flyback Lowside Drive

Unlike the gate drive for the two buck converters, the flyback

lowside gate drive DLFY is powered by a voltage that can be

as high as 15 V with 20 V input for the flyback converter. If

resistor-zener circuit can be used: a resistor series with gate

and zener diode across the gate and source to clamp its

voltage. A 100

, 10 V combination works well.