SC1403ITSTR datasheet(21/30 Pages) SEMTECH

21 / 30 page

 2002 Semtech Corp.

www.semtech.com

SC1403

POWER MANAGEMENT

PRELIMINARY

Operation below 6V input

The SC1403 will operate below 6V input voltage with careful

design, but there are limitations. The first limitation is the

maximum available duty cycle from the SC1403, which limits the

obtainable output voltage. The design should minimize all circuit

losses through the system in order to deliver maximum power to

the output.

A second limitation with operation below 6V is transient response.

When load current increases rapidly, the output voltage drops

slightly; the feedback loop normally increases duty cycle briefly to

bring the output voltage back up. If duty cycle is already near the

maximum limit, the duty cycle cannot increase enough to meet

the demand, and the output voltage sags more than normal. This

problem can not be solved by changing the feedback

compensation, it is a function of the input voltage, duty cycle, and

inductor and capacitor values.

If an application requires 5V output from an input voltage below

6V, the following guidelines should be used:

1 - Set the switching frequency to 200 kHz (Tie SYNC to

GND). This increases the maximum duty cycle

compared to 300 kHz operation.

2 - Minimize the resistance in the power train. Select

MOSFETs, inductor, and current sense resistor to

provide the lowest resistance as is practical.

3 - Minimize the pcb resistance for all traces carrying

high current. This includes traces to the input

capacitors, MOSFETS and diodes, inductor, current

sense resistor, and output capacitor.

4 - Minimize the resistance between the SC1403

circuit and the power source (battery, battery charger,

AC adaptor).

5 - Use low ESR capacitors on the input to prevent the

input voltage dropping during on-time.

6 - If large load transients are expected, high

capacitance and low ESR capacitors should be used on

both the input and output.

Overvoltage Test

Measuring the overvoltage trip point can be problematic. Any

buck converter with synchronous MOSFETS can act as a boost

converter, 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 SC1403 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, ON3 both 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 SC1403 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 the input lab supply 1 in-

creases, 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

SC1403

Evaluation

Board

Lab

Output

test

under

Supply

Lab

to DVM

1/2W

ON5

1/2W

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