SC2738IMSTRT datasheet(7/13 Pages) SEMTECH

7 / 13 page

 2004 Semtech Corp.

www.semtech.com

SC2738

POWER MANAGEMENT

Applications Infomation (Cont.)

supplied to the MOSFET drain should be greater than

the output under-voltage threshold when that

output is enabled. This assumes that the drop through

the pass MOSFET is negligible. If not, then this drop needs

to be taken into account also since:

OUT = VDRAIN - (IOUT x RDS(ON)).

If the supply to the SC2738 IN pin comes up before the

supply to the MOSFET drain, then that output should be

enabled as the supply to the MOSFET drain is applied

- the Power Good signal for this rail would be ideal. If the

power supply to the MOSFET drain comes up before the

power supply to the SC2738 IN pin, then the output can

either be enabled with the supply to the IN pin or after-

wards. Please see the example below.

Example: SC2738 powered from 5V, output 1 powered

from 1.8V set for 1.5V out, output 2 not shown for

simplicity. Worst case under-voltage threshold is 60%

(over temperature) of 1.5V, or 0.9V. The typical enable

threshold is ~1V. See Figure 1 below.

Component Selection

Output Capacitors: low ESR capacitors such as Sanyo

POSCAPs or Panasonic SP-caps are recommended for

bulk capacitance, with ceramic bypass capacitors for

decoupling high frequency transients.

Input Capacitors: placement of low ESR capacitors such

as Sanyo POSCAPs or Panasonic SP-caps at the input to

the MOSFET (V

DRAIN) will help to hold up the power supply

during fast load changes, thus improving overall transient

response. If V

DRAIN is located at the bulk capacitors for

the upstream voltage regulator, additional capacitance

may not be required. In this case a 0.1µF ceramic

capacitor will suffice. The input supply to the SC2738

should be bypassed with a 0.1µF ceramic capacitor.

SC2738 Supply Comes Up Before MOSFET Drain Supply

MOSFET Drain Supply Comes Up Before SC2738 Supply

Figure 1: Power Supply Sequencing

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