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 2004 Semtech Corp.

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POWER MANAGEMENT

SC1405B

To eliminate the effect of the ringing on the boost ca-

pacitor voltage, place a 4.7 - 10 Ohm resistor between

boost Schottky diode and Vcc to filter the negative spikes

on DRN Pin. Alternately, a Silicon diode, such as the

commonly available 1N4148 can substitute for the

Schottky diode and eliminate the need for the series re-

sistor.

Proper layout will guarantee minimum ringing and elimi-

nate the need for external components. Use of SO-8 or

other surface mount MOSFETs will reduce lead induc-

tance and their parasitic effects.

ASYNCHRONOUS OPERATION

The SC1405B can be configured to operate in Asynchro-

nous mode by pulling S-MOD to logic LOW, thus disabling

the bottom FET drive. This has the effect of saving power

at light loads since the bottom FET’s gate capacitance

does not have to charged at the switching frequency.

There can be a significant savings since the bottom driver

can supply up to 2A pulses to the FET at the switching

frequency.

There is an additional efficiency benefit to

operating in asynchronous mode. When operating in syn-

chronous mode, the inductor current can go negative

and flow in reverse direction when the bottom FET is on

and the DC load is less than 1/2 inductor ripple current.

Applications Information (Cont.)

At that point, the inductor core and wire losses, depend-

ing on the magnitude of the ripple current, can be quite

significant. Operating in asynchronous mode at light loads

effectively only charges the inductor by as much as

needed to supply the load current, since the inductor

never completely discharges at light loads. DC regula-

tion can be an issue when operating in asynchronous

mode, depending on the type of controller used and mini-

mum load required to maintain regulation. If there are

no Shottkey diodes used in parallel with bottom FET, the

FET’s body diode will need to conduct in asynchronous

mode. The high voltage drop of this diode must be con-

sidered when determining the criteria for this mode of

operation.

DSPS DR

This pin produces an output which is a logical duplicate

of the bottom FET’s gate drive, if S-MOD is held LOW.

OVP_S/OVER TEMP SHUTDOWN

Output over-voltage protection may be implemented on

the SC1405 independent of the PWM controller . A volt-

age divider from the output is compared with the inter-

nal bandgap voltage of 1.2V (typical). Upon exceeding

this voltage, the overvoltage comparator disables the top

FET, while turning on the bottom FET to allow discharge

of the output capacitors excessive voltage through the

output inductor. There should be sufficient RC time con-

stant as well as voltage headroom on the OVP_S pin to

assure it does not enter overvoltage mode inadvertently.

The SC1405 will shutdown if its Tj exceeds 165°C.