SC194AEVB datasheet(7/17 Pages) SEMTECH

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SC194A

PRELIMINARY

POWER MANAGEMENT

Applications Information

SC194A Detailed Description

The SC194A 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 a 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 uses 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 SC194A has an internal synchronous NMOS rectiﬁer

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

Programmable Output Voltage

The SC194A has four pre-determined output voltage

values which can be individually selected by the correct

programmingoftheVID0andVID1pins(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.

Continuous Conduction & Oscillator Synchronization

The SC194A 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 SC194A provides the following protection features:

• Thermal shutdown

• Current limit

• Over-voltage protection

• Soft-start

Thermal Shutdown

The device has a thermal shutdown feature to protect the

SC194A 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 SC194A. 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 SC194A

to automatically activate/deactivate operation at light

loads maximizing efﬁciency across the full load range. The

SC194A automatically detects the load current at which it

should enter PSAVE mode. The SC194A is optimized to

track maximum efﬁciency with respect to V

IN.

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