SC4524CSETRT datasheet(11/22 Pages) SAMTEC

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SC4524C

Applications Information (Cont.)

To allow for transient headroom, the minimum operating

switch on time should be at least 20% to 30% higher than

the worst-case minimum on time.

100

120

140

160

180

200

-50 -25

75 100 125

Temperature (OC)

VO =1.5V,IO =1A,1MHz

Figure 4 — Variation of Minimum On Time

with Ambient Temperature

Minimum Off Time Limitation

The PWM latch in Figure 2 is reset every cycle 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

00ns typically. 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.

Inductor Selection

The inductor ripple current for a non-synchronous step-

down converter in continuous-conduction mode is

)

(

)

(

−

where F

SW is the switching frequency and L is the

inductance.

An inductor ripple current between 20% to 50% of the

maximum load current gives a good compromise among

efficiency, cost and size. Re-arranging the previous

equation and assuming 35% inductor ripple current, the

inductor is given by

)

(

)

(

−

If the input voltage varies over a wide range, then choose

based on the nominal input voltage. Always verify

converter operation at the input voltage extremes.

The peak current limit of SC4524C power transistor is at

least 2.6A. The maximum deliverable load current for the

SC4524C is 2.6A minus one half of the inductor ripple

current.

Input Decoupling Capacitor

The input capacitor should be chosen to handle the RMS

ripple current of a buck converter. This value is given by

)

(

CIN

RMS

−

The input capacitance must also be high enough to keep

input ripple voltage within specification. This is important

in reducing the conductive EMI from the regulator. The

input capacitance can be estimated from

where DV

IN is the allowable input ripple voltage.

Multi-layer ceramic capacitors, which have very low ESR

(a few mW) and can easily handle high RMS ripple current,

are the ideal choice for input filtering. A single 4.7µF

X5R ceramic capacitor is adequate for 500kHz or higher

switching frequency applications, and 0µF is adequate

for 200kHz to 500kHz switching frequency. For high

voltage applications, a small ceramic (µF or 2.2µF) can be

placed in parallel with a low ESR electrolytic capacitor to

satisfy both the ESR and bulk capacitance requirements.

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