SC9301 datasheet(21/29 Pages) SEMTECH

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SC9301

Applications Information (continued)

Example

LOAD

This would cause the output current to move from 10A to

0A in 4μs, giving the minimum output capacitance

requirement shown in the following equation.

OUT

OUT = 216 μF

Note that C

OUT

is much smaller in this example, 216μF

compared to 357μF based on a worst-case load release. To

meet the two design criteria of minimum 357μF and

maximum 8.1mΩ ESR, select one capacitor of 330μF and

6mΩ ESR.

It is recommended that an additional small capacitor be

placed in parallel with C

OUT

in order to filter high frequency

switching noise.

Stability Considerations

Unstable operation is possible with adaptive on-time con-

trollers, and usually takes the form of double-pulsing or

ESR loop instability.

Double-pulsing occurs due to switching noise seen at the

FB input or because the FB ripple voltage is too low. This

causes the FB comparator to trigger prematurely after the

250ns minimum off-time has expired. In extreme cases

the noise can cause three or more successive on-times.

Double-pulsing will result in higher ripple voltage at the

output, but in most applications it will not affect opera-

tion. This form of instability can usually be avoided by

providing the FB pin with a smooth, clean ripple signal

that is at least 10mVp-p, which may dictate the need to

increase the ESR of the output capacitors. It is also impera-

tive to provide a proper PCB layout as discussed in the

Layout Guidelines section.

Another way to eliminate doubling-pulsing is to add a

small capacitor across the upper feedback resistor, as

shown in Figure 9. This capacitor should be left unpopu-

lated unless it can be confirmed that double-pulsing

exists. Adding the C

TOP

capacitor will couple more ripple

into FB to help eliminate the problem. An optional con-

nection on the PCB should be available for this capacitor.

VOUT

To FB pin

CTOP

Figure 9 — Capacitor Coupling to FB Pin

ESR loop instability is caused by insufficient ESR. The

details of this stability issue are discussed in the ESR

Requirements section. The best method for checking sta-

bility is to apply a zero-to-full load transient and observe

the output voltage ripple envelope for overshoot and

ringing. Ringing for more than one cycle after the initial

step is an indication that the ESR should be increased.

One simple way to solve this problem is to add trace resis-

tance in the high current output path. A side effect of

adding trace resistance is decreased load regulation.

ESR Requirements

A minimum ESR is required for two reasons. One reason is

to generate enough output ripple voltage to provide

10mVp-p at the FB pin (after the resistor divider) to avoid

double-pulsing.

The second reason is to prevent instability due to insuffi-

cient ESR. The on-time control regulates the valley of the

output ripple voltage. This ripple voltage is the sum of the

two voltages. One is the ripple generated by the ESR, the

other is the ripple due to capacitive charging and dis-

charging during the switching cycle. For most applica-

tions the minimum ESR ripple voltage is dominated by the

output capacitors, typically SP or POSCAP devices. For

stability the ESR zero of the output capacitor should be

lower than approximately one-third the switching fre-

quency. The formula for minimum ESR is shown by the

following equation.

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