SC198A

10

Detailed Description

The SC198A 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 oscil-

lator 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 feed-

back for the switching regulator is through the PMOS

current path, and it is amplified and summed with the

internal slope compensation network. The voltage feed-

back loop uses an internal feedback divider. The on-time

is determined by comparing the summed current feed-

back and the output of the error amplifier. The period is

set by the onboard oscillator or by an external clock

attached to the MODE/SYNC pin. When synchronizing to

an external frequency, the SC198A operates in PWM mode

only. The device has an internal synchronous NMOS recti-

fier 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 SC198A converter also has internal loop compensa-

tion. The internal compensation is designed to work with

a specific output filter corner frequency defined by the

equation

OUT

C

C

L

2

1

f

When selecting output filter components, the LC product

should not vary over a wide range. The practical lower

limit for the inductor value is 4.7μH to ensure system sta-

bility. Selection of smaller inductor and capacitor values

will shift the corner frequency higher, which may impact

stability. The internal compensation is designed to

operate with a single-pole output filter where L = 4.7μH

and COUT = 10μF.

Programmable Output Voltage

The SC198A has three control pins per output to allow

selection of a voltage level from eight predetermined

output voltage levels (see Table 1). CAUTION — All CTL

pins much be pulled high or low for proper operation

(they must not be allowed to float). The output voltage

can be selected while the device is enabled and loaded.

Table 1 — Output Voltage Options

Programmable Output Voltage

CTLA2/B2

CTLA1/B1

CTLA0/B0

Nominal Output

Voltage (V)

AB

0

0

0

1.0

1.8

0

0

1

1.1

2.2

0

1

0

1.2

2.5

0

1

1

1.3

2.7

1

0

0

1.4

2.8

1

0

1

1.5

2.9

1

1

0

1.6

3.0

1

1

1

1.8

3.3

Continuous Conduction and Oscillator

Synchronization

The SC198A is designed to operate in continuous con-

duction mode, where it maintains a fixed frequency.

When the MODE/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 con-

necting a clock signal to the MODE/SYNC pin. The part

synchronizes to the rising edge of the clock.

Power-Save Mode Operation

The SC198A has a power-save mode (PSAVE) which may

be selected by tying the MODE/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 device will automatically

detect the load current at which it should enter PSAVE

mode. The device is optimized to track maximum

efficiency.

Application Information