SC2440AEVB datasheet(22/28 Pages) SEMTECH

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Applications Information (Continued)

Z increases the mid-band loop gain and the crossover

frequency. However the converter becomes less stable.

Using a linear equivalent model and setting the loop gain

crossover frequency to one-tenth the switching frequen-

cy, R

Z can be calculated:

(2)

where R

Z is in W.

Z can be determined by setting the zero frequency to

one-fifth of the loop gain crossover frequency:

(3)

where C

Z is Farads.

The capacitor C

P from COMP to ground rolls off the loop

gain at high frequency. C

P is generally not required for sta-

bility. In some cases, the addition of a small capacitor (0

to 22pF) from the COMP pin to ground eliminates SW fall-

ing edge jitter.

Z and CZ calculated above are based upon a linear equiv-

alent circuit which does not model the non-linear nature

of switching regulators very well. It is imperative to verify

loop compensation by checking regulator load transient

response. With the largest load step pertinent to the ap-

plication applied, the regulator output voltage and the in-

ductor current are observed. These transient waveforms

should not show any ringing or excessive overshoot (see

Figures 2(a) and 2(b) for examples of stable load tran-

sient waveforms.). If necessary, adjust R

Z until a stable

transient is obtained. The R

ZCZ product is to be kept con-

stant during tuning.

Board Layout Considerations

In a step-down switching regulator, the input bypass

capacitor, the main power switch and the freewheel-

ing diode carry pulse currents with high

(Figure 3).

For jitter-free operation, the size of the loop formed by

these components should be minimized. Since the power

switches are already integrated within the SC2440A, con-

SC2440A

VOUT

VIN

Fast Switching Current Paths in a Step-

down Converter. The input capacitor and

the freewheeling diode should be placed

close to the part for improved switching

performance.

Figure 13.

Figure 12. Load Transient Response of the Converter in

Figure 1.

(a) Channel 1 (3.3V) LoadTransient Response,

OUT1 is switched between 0.3A and 1.8A.

(d) Channel 2 (5V) Load Transient Response,

OUT2 is switched between 0.3A and 1.8A.

(a)

OUT1

0.5V/div

AC Coupled

0.5A/div

40ms/div

(b)

40ms/div

OUT1

0.5V/div

AC Coupled

0.5A/div

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