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 2004 Semtech Corp.

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SC338(A)

POWER MANAGEMENT

Applications Infomation (Cont.)

SC338(A) Supply Comes Up Before MOSFET Drain Supply

MOSFET Drain Supply Comes Up Before SC338(A) Supply

Figure 1: Power Supply Sequencing

supplied to the MOSFET drain should be greater than

the output undervoltage 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:

V

If the supply to the SC338(A) 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 SC338(A) IN pin, then the output

can either be enabled with the supply to the IN pin or

afterwards. Please see the example below.

Example: SC338(A) powered from 5V, output 1 powered

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

simplicity. Worst case undervoltage 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

during fast load changes, thus improving overall transient

response. If V

the upstream voltage regulator, additional capacitance