SC4524 datasheet(9/21 Pages) SEMTECH

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ã 2006 Semtech Corp.

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SC4524

POWER MANAGEMENT

Applications Information

Setting the Output Voltage

The regulator output voltage is set with an external

resistive divider (Figure 4) with its center tap tied to the

FB pin.

287

(1)

The percentage error due the input bias current of the

error amplifier is

287

ÔÏ

Example: Determine the output voltage error of a

9287 =

converter with

From (1),

287

ÔÏ

This error is at least an order of magnitude lower than

the ratio tolerance resulting from the use of 1% resistors

in the divider string.

Setting the Switching Frequency

The switching frequency of the SC4524 is set with an

external resistor from the ROSC pin to ground. A graph

of switching frequency against R

OSC is shown in Typical

Performance Characteristics. The switching frequency

is programmable up to 1.5MHz.

The switching frequency is limited by the minimum

controllable on time at low duty cycles. For V

IN > 20V,

setting switching frequency below 500kHz makes

converter output short circuit operation more robust.

These will be described in more details later.

Minimum On Time Consideration

The operating duty cycle of a non-synchronous step-down

switching regulator in continuous-conduction mode (CCM)

is given by

&(6$7

287

(2)

where V

CESAT is the switch saturation voltage and VD is

voltage drop across the rectifying diode.

Duty cycle decreases with increasing

287

ratio. In peak

current-mode control, the PWM modulating ramp is the

sensed current ramp of the power switch. This current

ramp is absent unless the switch is turned on. The

intersection of this ramp with the output of the voltage

feedback error amplifier determines the switch pulse

width. The propagation delay time required to

immediately turn off the switch after it is turned on is

the minimum controllable switch on time (T

ON(MIN)). Closed-

loop measurement of the SC4524 with low

287

ratios

WPVU

A7Ã

T8#$!#

Figure 4. V

OUT is set with a Resistive Divider

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Figure 5. Variation of Minimum On Time with

Ambient Temperature.

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