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 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC190

Applications Information

SC190 Detailed Description

The SC190 is a step-down, pulse-width-modulated (PWM)

DC-DC converter with a low dropout (LDO) pre-regulator

and a low current LDO regulator for operation in low power

modes.

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, but at loads below 5mA,

the part can be operated as an ultra-low current LDO

regulator to minimize supply current.

Programmable Output Voltage

The SC190 has four device variants (SC190A, SC190B,

SC190C, SC190D) each with a distinct range of output

voltages. The output voltage of each variant has four

predetermined values which can be individually selected

by the correct programming of the VID0 and VID1 pins.

(See “Programmable Output Voltage” table on page 5).

Note that the SC190C has much higher output voltage

settings than the other three variants. This device was

intended for use with higher input voltages, so some

performance over the entire input voltage range cannot

be guaranteed.

Switcher Mode with less than 1mV supply ripple

This is a fixed frequency current mode architecture with

the input supply for the switching regulator pre-regulated

by a front-end LDO regulator. This technique reduces the

supply voltage ripple from 20mV, typically seen from a

switching converter, to approximately 1mV for the SC190.

The supply to the switcher is regulated to the supply

voltage minus approximately 300mV.

Current feedback for the switching regulator is through

the PMOS current path and it is amplified and summed

with the internal slope compensation network and level

shift. The voltage feedback loop is through an internal

feedback divider. The ON time is determined by compar-

ing 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.

Efficiency at moderate to high loads can be improved by

shorting the VIN to BP pins at the expense of higher input

voltage ripple.

Continuous Conduction & Oscillator Synchronization

TheSC190isdesignedtooperateincontinuousconduction

mode thereby maintaining a fixed frequency. When the

SYNC/PWM pin is tied high the part runs under control

of 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.

Back End LDO Mode

The SC190 ultra-low current linear regulator regulates

the same output as the switching regulator. The linear

regulator minimizes the supply current drawn at light

loads consuming only 10µA when supporting a 100µA

load. The SC190 can swap between switching regulator

and linear regulator mode under control of the MODE

pin (see “handover” section). In this way the part either

operates at a fixed frequency or DC output, thereby never

generating load dependant frequencies that are typically

seen with lightly loaded switching regulators.

Handover (Switcher Mode to LDO Regulator Mode, LDO

Regulator Mode to Switcher Mode)

The device can be switched between Switcher mode and

Linear regulator mode and back to Switcher mode without

having to disable and re-enable the part. The output

maintains regulation during the switch over.

Wait Time

200ì s

< 5mA

< 300mA

Time

Linear Regulator Mode to Switching Mode

Transition

Wait Time

100ì s

< 5mA

< 300mA

Time

Switching Mode to Linear Regulator Mode

Transition

Applies

Applies

µs

µs