SC2542 datasheet(9/27 Pages) SEMTECH

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SC2542

POWER MANAGEMENT

Applications Information

General Description

The SC2542 is a fixed frequency dual voltage mode step-

down PWM controller for adapter or battery operated

systems. The two channels of the controller operate 180

degrees out of phase, which results in lower input cur-

rent ripple and reduces the amount of input filtering ca-

pacitance needed.

Synchronous and “pulse skip” mode of operation are

adopted in both switching channels to increase overall

efficiency. The internal light-load current detection cir-

cuit and minimum pulse setting circuit determine the

mode of operation and the “pulse skip” duration.

To extend battery life, the SC2542 features shutdown

mode, where the quiescent current is 100nA (typical),

and standby mode, where the 5V linear standby regula-

tor is active and quiescent current is typically 200uA.

The functional block diagram is shown in Figure 1.

VO1

INPUT

Regulator

Channel 2

STBY

Power in

Sequence

VO2

PWG

Linear

SMPS

Power

Channel 1

Good

5VSBY

Figure 1. The functional block diagram.

Skip Mode Operation

When the RMIN pin is left open, the SC2542 operates

as a synchronous PWM (SPWM) controller with the bot-

tom MOSFET ON whenever the top MOSFET is OFF. Fig-

ure 2 shows the typical SPWM waveforms.

Figure 2. Typical waveforms of SPWM mode.

To enhance light-load efficiency, “pulse skip” mode (SKIP)

can be enabled by placing a resistor from the RMIN pin

to ground. For normal loads the controller will operate in

SPWM mode, but when the load is light enough, the pulse

widths of the top and bottom MOSFETs will be adjusted

by the load condition. The SC2542 employs a simple

circuit to detect the load condition by monitoring the

PHASE node of the circuit. The PHASE node will start to

turn positive if the inductor current begins to reverse (I

< 0) signaling a light-load condition.

If a light-load condition is detected the SC2542 will op-

erate in SKIP mode. Similar to a buck converter in dis-

continuous conduction mode (DCM), the converter will

try to reduce its duty cycle as the load decreases. For

example, it would start out with continuous conduction

mode (CCM) duty cycle values, but as the load decreases,

the duty cycle would try to shrink to zero as shown in

Figure 3. However, the SC2542 clamps the DCM duty

cycle to a minimum percentage (set by the RMIN resistor)

of the CCM value. If the DCM duty cycle falls below this

percentage, SKIP mode will be active.

DRVH 1

DRVL 1

PHASE 1

IL 1

DRVH 1

DRVL 1

PHASE 1

IL 1

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