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POWER MANAGEMENT

SC191

SC191 Detailed Description

The SC191 is a step-down, pulse-width-modulated (PWM)

DC-DC converter with a Low Dropout (LDO) pre-regulator.

The device has an internal synchronous rectifier and does

not require a Schottky diode on the LX pin. The device is

designed to operate as a buck converter in PWM mode

with a fixed-frequency of 1MHz. The SC191 operates as

a fixed frequency current mode regulator with the input

supply for the switching regulator pre-regulated by a front-

end LDO regulator. This technique reduces the supply

voltage ripple from 10mV, typically seen from a switching

converter, to less than 1mV for the SC191. The supply to

the switcher is regulated to the supply voltage minus ap-

proximately 300mV. Current feedback for the switching

regulator is through the PMOS current path, and it is am-

plified and summed with the internal slope compensation

network and level shifted. The voltage feedback loop is

through an internal feedback divider.

The on-time is determined by comparing the summed cur-

rent feedback and the output of the error amplifier. The

period is set by the onboard oscillator.

Efficiency at moderate to high loads can be improved by

shorting the VIN and BP pins together. This bypasses the

front-end LDO, reducing voltage drop and improving effi-

ciency at the expense of increasing input supply ripple.

Protection Features

The SC191 provides the following protection features:

• Thermal Shutdown

• Current Limit

• Over-Voltage Protection

• Soft-Start

Thermal Shutdown

The thermal shutdown feature protects the device from

exceeding a junction temperature of 150°C. In thermal

shutdown the PWM drive is disabled, the LX output is tri-

stated, and the front-end LDO is disabled. The device will

not be enabled again until the temperature reduces by

10°C. If during this time the output falls by greater than

60% of its regulation voltage, a soft-start will be invoked.

Current Limit

The part has a number of current limit functions. The

front-end LDO regulator has a current limit set at ap-

proximately 500mA, which will protect it in the event of a

pulsed short circuit. The PMOS and NMOS power devices

of the buck switcher stage are also protected by current

limit functions. In the case of a short to ground on the

output, the part enters frequency foldback mode which

causes the switching frequency to divide by a factor deter-

mined by the output voltage, which prevents the inductor

current from “stair stepping”.

Over-Voltage Protection

Over-voltage protection is provided on the SC191. In the

event of an over-voltage on the output, the PWM drive is

disabled, tri-stating the LX output and disabling the front-

end LDO.

Soft-Start

The soft-start mode is enabled after every shutdown cycle

to limit in-rush current. In conjunction with the frequency

foldback this controls the maximum current during start-

up. The switcher’s PMOS current limit is stepped from

25%, to 50%, to 75%, and then 100% of its typical value

by a timer driven by the internal oscillator. The oscillator

frequency is stepped by 1/8, 1/4, 1/2 and 1 under the

control of 4 output voltage thresholds. As soon as the

part reaches regulation, soft start mode is disabled.

Output Filter

The SC191 series of synchronous step-down converters

have internal loop compensation. The internal compen-

sation is designed to work with a certain output filter cor-

ner frequency defined by the equation:

1

f

2 √ L×C

———

The internal compensation is optimized to operate with

an output filter, L = 4.7μH and C

ing output filter components the LC product should not

vary over a wide range.

Applications Information