SC412AEVB datasheet(11/22 Pages) SEMTECH

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SC412A

POWER MANAGEMENT

Applications Information (continued)

Soft-Start

The soft-start is accomplished by ramping the FB com-

parator’s internal reference from zero to 0.75V in 30mV

increments. Each 30mV step typically lasts for eight clock

cycles.

During the soft-start period, the Zero Cross Detector is

active to monitor the voltage across the lower MOSFET

while DL is high. If the inductor current reaches zero, the

FB comparator’s internal ramp reference is immediately

overridden to match the voltage at the FB pin. This soon

causes the FB comparator to trip which forces DL to turn

off and a DH on-time will begin. This prevents the inductor

current from going too negative which would cause droop

in the VOUT start-up waveform. The next 30mV step on the

internal reference ramp occurs from the new point at the

FB pin. Since any of the internal 30mV steps can be over-

ridden by the FB waveform, the start-up time is therefore

dependent upon operating conditions. This override feature

will stop when the FB pin reaches approximately 660mV.

At start-up, during the ﬁrst 32 switching cycles, the over-

current threshold is reduced by 50%, to reduce overshoot

caused by the ﬁrst set of switching pulses.

MOSFET Gate Drivers

The DH and DL drivers are optimized for driving moderate

high-side and larger low-side power MOSFETs. An adaptive

dead-time circuit monitors the DL output and prevents the

high-side MOSFET from turning on until DL is fully off, and

conversely, monitors the DH output and prevents the low-

side MOSFET from turning on until DH is fully off. Be sure

there is low resistance and low inductance between the DH

and DL outputs to the gate of each MOSFET.

The SC412A utilizes SmartDriveTM to achieve fast switching

with reduced noise. At the start of the DH on-time when

LX is typically below GND, the DH output drives the high-

side MOSFET through a pull-up resistance of 10 ohms,

which results in a soft reverse-recovery of the low-side

diode. The high-side MOSFET conducts and causes LX to

rise; when LX reaches 1.5volts, the DH drive resistance is

reduced to 2 ohms to provide fast switching and reduce

switching loss.

Design Procedure

Prior to designing a switch mode supply, the input voltage,

load current, switching frequency and inductor ripple cur-

rent must be speciﬁed.

For notebook systems the maximum input voltage (VIN

MAX)

is determined by the highest AC adaptor voltage, and the

minimum input voltage (VIN

MIN) is determined by the lowest

battery voltage after accounting for voltage drops due to

connectors, fuses and battery selector switches.

In general, four parameters are needed to deﬁne the

design:

1) Nominal output voltage (VOUT)

2) Static or DC output tolerance

3) Transient response

4) Maximum load current (IOUT)

There are two values of load current to consider: continu-

ous load current and peak load current. Continuous load

current is concerned with thermal stresses which drive

the selection of input capacitors, MOSFETs and commuta-

tion diodes. Peak load current determines instantaneous

component stresses and ﬁltering requirements such as

inductor saturation, output capacitors and design of the

current limit circuit.

Design example:

VBAT = 10V min, 20V max

VOUT = 1.15V +/- 4%

Load = 20A maximum

Inductor Selection

Low inductor values result in smaller size, but create high-

er ripple current and are less efﬁcient because of the high

AC current ﬂowing in the inductor. Higher inductor values

will reduce the ripple current and are more efﬁcient, but

are larger and more costly. The inductor selection is gen-

erally based on the ripple current which is typically set

between 20% to 50% of the maximum load current. Cost,

size, output ripple and efﬁciency all play a part in the se-

lection process.

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