SC1404 datasheet(18/25 Pages) SEMTECH

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 2004 Semtech Corp.

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SC1404

PRELIMINARY

POWER MANAGEMENT

Overvoltage Test

Measuring the overvoltage trip point can be problematic. Any

buck converter with synchronous mosfets can act as a boost con-

verter, sending energy from output to input. In some cases the

energy sent to the input is enough to drive the input voltage be-

yond normal levels, causing input overvoltage. To prevent this,

enable the SC1404 PSAVE# feature, which effectively disables

the low side mosfet drive so that little energy, if any, is transferred

back to the input.

Semtech recommends the following circuit for measuring the ov-

ervoltage trip point. D1 prevents the output voltage from damag-

ing lab supply 1. R1 limits the amount of energy that can be cycled

from the output to the input. R2 absorbs the energy that might

flow from output to input, and D2 protects lab supply from pos-

sible damage. The ON5 signal is monitored to indicate when

overvoltage occurs.

Initial conditions:

Both lab supplies set to zero volts

No load connected to 3V or 5V

PSAVE# enabled (PSAVE# tied to GND)

ON5 enabled

ON3 enabled

DVMs monitoring ON5 and the output under test.

Oscilloscope probe connected to Phase Node

of the output under test (not strictly required).

Set Lab Supply 2 to provide 10V at the SC1404 input. The phase

node of the output being tested should show some switching ac-

tivity. The ON5 pin should be above 4V.

Slowly increase Lab Supply 1 until the output under test rises

slightly above it’s normal DC level. As Lab Supply 1 increases,

switching activity at the phase node will cease. The ON5 pin should

remain above 4V.

Increase Lab Supply 1 in very small increments, monitoring both

ON5 and the output under test. The overvoltage trip point is the

highest voltage seen at the output before ON5 pulls low (approxi-

mately 0.3V). Do not record the voltage seen at the output after

ON5 has pulled low; when ON5 pulls low, the current flowing in D1

changes, corrupting the voltage seen at the output.

e.g. 1N4004

to DVM

e.g. 1N4004

Vin

Supply

470

SC1404

Evaluation

Board

Lab

Output

test

under

Supply

Lab

to DVM

1/2W

ON5

1/2W

3.3VEfficiency

100

0.01

0.1

LoadCurrent (A)

10V

19V

5VEfficiency

100

0.01

0.1

LoadCurrent (A)

10V

19V

Typical Characteristics

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