SC2440AEVB datasheet(14/28 Pages) SEMTECH

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Applications Information (Continued)

2X Frequency Synchronization

An external clock with frequency f

SYNC ranging from .8X

to 2.7X f

FREE-RUN will synchronize the individual channels

with 80o out of phase switching. The nominal

free-running channel frequency should be programmed

to between 0.475f

SYNC and 0.5fSYNC when synchronizing us-

ing a 2X frequency clock.

Example: Detemine the value of the frequency setting

resistor if the SC2440A is to be synchronized to run at

.03MHz per channel.

The required external clock is a TTL-compatible pulse train

running at either .03MHz (for X frequency synchroniza-

tion) or 2.06MHz (for 2X frequency synchronization).

Using the guideline given above, set the nominal free-

running channel frequency to MHz (0.97 × .03MHz for

X frequency synchronization or 0.485 × 2.06MHz for 2X

frequency synchronization).

From Table , a 9.6kΩ resistor sets the channel free-run-

ning frequency to MHz.

Inductor Selection and Output Current

The inductor ripple current ΔI

L for a non-synchronous

step-down converter in continuous-conduction mode is

(3)

where f is the switching frequency and L is the induc-

tance.

In current-mode control, the slope of the modulating

(sensed switch current) ramp should be steep enough

to lessen jitter tendency but not so steep that the large

flux swing decreases efficiency. An inductor ripple current

ΔI

L between 20-30% of the peak inductor current limit is

a good compromise. Inductors so chosen are not only

small but also have low core losses. Setting DI

L = 0.25(2.6)

= 0.65A, V

D = 0.45V and VCESAT = 0.3V in (3), the inductance

can be calculated as:

SC2440A

(4)

where L is in μH and f is in MHz.

Equation (3) shows that for a given V

OUT, DIL increases as

D decreases. If V

IN varies over a wide range, then choose

L based on the nominal input voltage. Always verify con-

verter operation at the input voltage extremes.

The channel current limit is at least 2A. The maximum

load current of a step-down converter is the switch cur-

rent limit I

LIM minus

. The maximum channel output

current is slightly less than 2A.

(5)

The available output current can be made to approach I

LIM

by using a larger inductor.

The saturation current of the inductor should be 20-30%

higher than the peak current limit. Low-cost powder iron

cores are not suitable for high-frequency switching power

supplies due to their high core losses. Inductors with fer-

rite cores are recommended.

Interleaved Switching and Input Capacitor

A step-down converter draws a pulsed current with peak-

to-peak amplitude equal to its output current I

OUT from the

input power supply. An input capacitor placed between

the supply and the buck converter filters the AC current

and keeps the current drawn from the supply constant.

The input capacitance C

IN should be high enough to filter

the pulsed input current. Its equivalent series resistance

(ESR) should be low so that power dissipated in the ca-

pacitor does not result in significant temperature rise and

degrade reliability.

For a single channel step-down converter, the RMS ripple

current in the input capacitor is

(6)

Power dissipated in the input capacitor is

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