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SC2542EVB

High Performance Wide Input Range Dual Synchronous Buck Controller

Semtech Corporation 

POWER MANAGEMENT

Applications Information (Cont.)

SC2542

350

TG

300

BG

250

200

Vo

150

Inductor

100

Current

50

Rosc(KOHM)

(b)

Figure 6. Typical waveforms of SKIP mode.

Figure 7. Switching frequency versus Rosc.

Frequency Setting

The frequency of the SC2542 is user- programmable.

The oscillator of SC2542 can be programmed with an

external resistor from the Rosc pin to ground. The step-

down controller is capable of operating up to 300KHz.

The relationship between oscillation frequency versus

oscillation resistor is shown in Figure 7.

The advantages of using constant frequency operation

include simple passive component selection and ease of

feedback compensation. Before setting the operating fre-

quency, the following trade-offs should be considered.

1) Passive component size

2) Circuitry efficiency

3) EMI condition

4) Minimum switch on time

5) Maximum duty ratio

For a given output power, the sizes of the passive com-

ponents are inversely proportional to the switching

frequency, whereas MOSFETs/Diodes switching losses

are proportional to the operating frequency. Other issues

such as heat dissipation, packaging, and cost also need

to be considered. The frequency bands for signal trans-

mission should be avoided because of EM interference.

Power on Sequence

After applying the input voltage and with STBY and EN

inputs high, Vcc and 5VSTBY will rise. When Vcc rises

above the UVLO threshold both switcher outputs will also

ramp up. By pulling the SS pins low, the two switcher

outputs can be disabled, with Vcc and 5VSTBY maintain-

ing regulation. Vcc will turn off when STBY is pulled low,

with 5VSTBY maintaining regulation. And finally 5VSTBY

will turn off when EN is pulled low, and the device will be

in shutdown mode.

Mode

Logic input

Normal Mode:

Both PWM rails in regulation

5V STBY rail in regulation

VCC rail in regulation

Standby Mode:

5V STBY rail in regulation

Setting EN pin to logic high

Setting STBY pin to logic high

No pull down of SS

Setting EN pin to logic high

Setting STBY pin to logic high

Shutdown Mode:

Setting EN pin to logic low

Soft Start

During start-up, the reference voltage of the error ampli-

fier equals 30% of the voltage on Css (soft-start capaci-

tor) which is connected between the SS pin and ground.

When the controller is enabled (by pulling EN pin or STBY

© 2005 Semtech Corp.

11

www.semtech.com

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