SC2440AEVB datasheet(21/28 Pages) SEMTECH

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Applications Information (Continued)

COMP

1.33V

SC2440A

Figure 11. Compensation Network for Each Regulator.

PGOOD pull-up resistor. If the PGOOD output high level

(= V

OUT) is unacceptably low, then tying the power good

pull-up resistor to the input or to a separate power supply

will be the only choice.

Sequencing the Outputs

As mentioned above, pulling either soft-start pin low with

an external transistor shuts off the corresponding regula-

tor (Figure 0). Releasing the soft-start pin enables that

channel and allows it to start. Delaying the release of the

soft-start pin of one channel with respect to the other is

a straightforward way of sequencing the outputs. Figure

0(a) shows this method using two external transistors Q

and Q

2. Q is turned off first, allowing channel to start.

Channel 2 is then enabled after time t

The PGOOD output of one channel can also be used in

conjunction with the soft-start pin of the other channel

to delay start of that regulator. This method is depicted

in Figure 0(b). SS2 is pulled low and channel 2 is kept off

until the channel output rises to 90% of its set voltage.

Loop Compensation

Each step-down switching regulator in the SC2440A re-

quires a simple Type-2 compensation network (Figure )

for stable operation. C

Z and RZ form a compensating zero.

This zero nulls out the effect of two low-frequency poles

in the feedback loop and allows the loop amplitude re-

sponse to cross unity gain at -20 dB/decade. Increasing

SC2440A

Sequencing the Outputs by (a) Delaying Release of one Channel Relative to

the Other and (b) Using the PGOOD of one Channel to Control the Other.

Figure 10.

(b)

(a)

OFF

CONTROL1

OFF

CONTROL2

CONTROL1

CONTROL2

SC2440A

PGOOD1

SS1

SS2

20kW

PGOOD2

CSS2

CSS1

OFF

SC2440A

PGOOD1

SS1

SS2

20kW

CSS1

CSS2

PGOOD2

(a)

(b)

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