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© 2006 Semtech Corp.

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SC194B

POWER MANAGEMENT

Applications Information

SC194B Detailed Description

The SC194B is a synchronous step-down Pulse Width

Modulated (PWM), DC-DC converter utilizing a 1MHz

fixed-frequency current mode architecture. The device is

designed to operate in fixed-frequency PWM mode across

the full load range and can enter power save mode (PSAVE)

utilizing Pulse Frequency Modulation (PFM) at light loads

to maximize efficiency.

Operation

During normal operation the PMOS MOSFET is activated

on each rising edge of the internal oscillator. Current

feedback for the switching regulator is through the

PMOS current path, and it is amplified and summed with

the internal slope compensation network. The voltage

feedback loop uses an internal feedback divider. The on-

time is determined by comparing the summed current

feedback and the output of the error amplifier. The period

is set by the onboard oscillator or by an external clock

attached to the SYNC/PWM pin.

The SC194B has an internal synchronous NMOS rectifier

and does not require a Schottky diode on the LX pin.

Programmable Output Voltage

The SC194B has four pre-determined output voltage

values which can be individually selected by the

correct programming of the VID0 and VID1 pins (see

Programmable Output Voltage table on Page 4). This

eliminates the need for external programming resistors

saving PCB area and inventory. The VID pins can be

statically tied to GND or VIN for fixed output configurations

or they may be driven by a microprocessor enabling

the possibility of dynamic voltage adjustment for host

equipment "sleep" states.

Oscillator Synchronization Opotions

The SC194B is designed to operate in continuous

conduction, fixed-frequency mode. When the SYNC/PWM

pin is tied high the part runs in PWM mode using the

internal oscillator. The part can be synchronized to an

external clock by driving a clock signal into the SYNC/

PWM pin. The part synchronizes to the rising edge of the

clock.

Protection Features

The SC194B provides the following protection features:

Thermal shutdown

Current limit

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Over-voltage protection

Soft-start

Thermal Shutdown

The device has a thermal shutdown feature to protect the

SC194B if the junction temperature exceeds 145°C. In

thermal shutdown the on-chip power devices are disabled,

tri-stating the LX output. Switching will resume when the

temperature drops by 10°C. During this time if the output

voltage decreases by more than 60% of its programmed

value, a soft-start will be invoked.

Current Limit

The PMOS and NMOS power devices of the buck switcher

stage are protected by current limit functions. In the case of

a short to ground on the output, the part enters frequency

foldback mode that causes the switching frequency to

divide by a factor determined by the output voltage. This

prevents the inductor current from "stair-casing".

Over-Voltage Protection

Over-voltage protection is provided on the SC194B. In the

event of an over-voltage on the output in switcher mode,

the PWM drive is disabled, tri-stating the LX output. The

part will not resume switching until the output voltage has

fallen below 2% of the regulation voltage.

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 PMOS current limit is stepped up through seven

soft-start levels to the full value by a timer driven from

the internal oscillator. During soft-start, the switching

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

oscillator frequency up to the full value, under control of

three output voltage thresholds. As soon as the output

voltage is within 2% of the regulation voltage, soft-start

mode is disabled.

Power Save Mode Operation

The PSAVE mode may be selected by tying the SYNC/PWM

pin to GND. Selecting PSAVE mode will enable the SC194B

to automatically activate/deactivate operation at light

loads maximizing efficiency across the full load range. The

SC194B automatically detects the load current at which

it should enter PSAVE mode. The SC194B is optimized to

track maximum efficiency with respect to V

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