SC198 datasheet(7/16 Pages) SEMTECH

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SC198

POWER MANAGEMENT

SC198 Detailed Description

The SC198 is a dual step-down, pulse-width modulated

DC-DC converter with programmable output voltage

and power-save mode. This device has a 1MHz internal

oscillator that can be used during forced PWM mode or

power-save mode (PSAVE).

Operation

During normal operation, the PMOS switch 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. When synchronizing

to an external frequency, the SC198 operates in PWM

mode only. The device has an internal synchronous

NMOS 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.

At light loads the part can enter PSAVE mode to maximize

efﬁciency.

Output Filter

The SC198 converter also has internal loop compensa-

tion. The internal compensation is designed to work with

a certain output ﬁlter corner frequency deﬁned by the

equation:

C = —————

2 √ L×C

———

When selecting output ﬁlter components, the LC product

should not vary over a wide range. Selection of smaller

inductor and capacitor values will move the corner fre-

quency, potentially impacting system stability. Due to

this issue the practical lower limit for the inductor value

is 4.7μH. The internal compensation is designed to oper-

ate with a single-pole output ﬁlter, L = 4.7μH and C

OUT =

10μF.

Programmable Output Voltage

The SC198 has three control pins per output to allow the

user to select a voltage level from eight predetermined

output voltage levels (see Table A, Output Voltage Options

below). The output voltage can be selected at any time,

including while the output is enabled and loaded.

Table A — Output Voltage Options

Programmable Output Voltage

CTLA2/B2 CTLA1/B1 CTLA0/B0

Nominal Output

Voltage (V)

000

1.2

1.375

1.4

1.5

1.575

1.8

1.875

Continuous Conduction and Oscillator Synchronization

The SC198 is designed to operate in a continuous

conduction mode, where it maintains a ﬁxed frequency.

When the SYNC pin is tied high, the part runs under

control of the internal oscillator in PWM mode. The part

can be synchronized to an external clock by connecting a

clock signal to the SYNC pin. The part synchronizes to the

rising edge of the clock.

Power-Save Mode Operation

The SC198 has a power-save mode (PSAVE) which may be

selected by tying the SYNC pin to GND. Selecting PSAVE

mode enables automatic activation of PSAVE mode at

light loads, thus maximizing efﬁciency across the full

load range. The SC198 will automatically detect the load

current at which it should enter PSAVE mode. The SC198

is optimized to track maximum efﬁciency with respect to

IN.

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