SC4524 datasheet(10/21 Pages) SEMTECH

10 / 21 page

ã 2006 Semtech Corp.

www.semtech.com

SC4524

POWER MANAGEMENT

shows that the minimum on time is about 105ns at room

temperature (Figure 5). The power switch in the SC4524

is either not turned on at all or for at least T

ON(MIN). If the

required switch on time (= I

) is shorter than the minimum

on time, the regulator will either skip cycles or it will jitter.

Example: Determine the maximum operating frequency

of a 24V to 1.2V switching regulator using the SC4524.

Assuming that V

D = 0.45V, VCESAT = 0.25V and VIN = 26.4V

(10% high line), the duty ratio can be calculated using

(2).

To allow for transient headroom, the minimum operating

switch on time should be at least 30% higher than the

worst-case minimum on time exhibited in Figure 5.

Designing for a switch on time of 150ns at

9,1 =

the maximum operating frequency is

.+]

Minimum Off Time Limitation

The PWM latch in Figure 2 is reset every period by the

clock. The clock also turns off the power transistor to

refresh the bootstrap capacitor. This minimum off time

limits the attainable duty cycle of the regulator at a given

switching frequency. The measured minimum off time is

120ns. For a step-down converter, D increases with

increasing

287

ratio. If the required duty cycle is higher

than the attainable maximum, then the output voltage

will not be able to reach its set value in continuous-

conduction mode.

Example: Determine the maximum operating frequency

of a 5V to 4V switching regulator using the SC4524.

Assuming that V

D = 0.45V, VCESAT = 0.25V and VIN = 4.5V

(10% low line), the duty ratio can be calculated using (2).

Applications Information

The maximum operating channel frequency of the

converter is therefore

.+]

Transient headroom requires that channel frequency be

lower than 410kHz.

Inductor Selection

The inductor ripple current DI

L for a non-synchronous

step-down converter in continuous-conduction mode is

&(6$7

287

(3)

where f is the switching frequency and L is the

inductance.

In current-mode control, the slope of the modulating

(sensed switch current) ramp should be steep enough

to lessen jittery tendency but not so steep that large

flux swing decreases efficiency. Inductor ripple current

L between 25-40% of the peak inductor current limit is

a good compromise. Inductors so chosen are optimized

in size and DCR. Setting

9' =

and

9&(6$7 =

in (3),

287

(4)

where L is in mH and f is in MHz.

Equation (3) shows that for a given

9287

D increases

as D decreases. If ,1

9 varies over a wide range, then

choose L based on the nominal input voltage. Always

verify converter operation at the input voltage extremes.

The peak current limits of both SC4524 power transistors

are internally set at 3.2A. The peak current limits are

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