SC493 datasheet(13/28 Pages) SEMTECH

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SC493

Over Temperature Protection

When the temperature of the device reaches the over

temperature rising threshold, the OT flag bit is set, turning

off both high side and low side MOSFETs. After the

temperature of the device drops to the over temperature

falling threshold, the converter restarts as if the device

has been enabled. The converter will go through power-

on delay and soft start.

Synchronous Buck Converter Operation, Ben-

efits, and Features

The SC493 employs pseudo-fixed frequency adaptive on-

time control. This control method allows fast transient

response thereby lowering the size of the power

components needed in the system.

The on time is determined by an internal one-shot with a

period proportional to the output voltage and inversely

proportional to the input voltage. The output ripple

voltage generated by the ESR of the output capacitance is

used as the PWM ramp signal. This ripple voltage

determines the off time for the controller.

For the SC493 the operating frequency range is from 250kHz

to 1MHz , programmable via the I2C interface.

Adaptive on-time control has significant advantages over

traditional control methods. Some of the advantages of

the adaptive on-time control are:

No error amplifier, which reduces external com-

ponents used for compensation

Predictable frequency spread because of adap-

tive on-time architecture

Fast transient response — operation with

minimum output capacitance

Overall superior performance compared to fixed

frequency architectures

On-Time One-Shot Generator (TON)

Adaptive on-time controllers like the SC493 have an

internal on-time one-shot generator. The one-shot timer

uses an internal comparator and a capacitor. The positive

input of the comparator is a voltage proportional to the

output voltage and the negative input is connected to the

capacitor charged by a current proportional to the input

voltage. The TON time is the time required to charge this

•

latch off just because this bit is set. The PGD output is

driven low when the FB pin is above 120% of the nominal

voltage and returns to high when the FB pin is below 110%

of the nominal voltage.

Output Over Current Protection

The SC493 features adjustable current limit capability.

The R

DS(ON) of the external low side MOSFET is used as the

current sensing element. The over current limit is set by

ILIM (connected externally). Internally there is a 10μA

current source that feeds the ILIM pin when the low side

MOSFET has turned on. This current flows through the R

ILIM

resistor and creates a voltage drop across it. When the low

side MOSFET turns on, the inductor current flowing

through it creats a voltage across the MOSFET due to its

DS(ON). If this voltage drop exceeds the voltage across the

ILIM resistor, current limit will activate. This prevents the

high side MOSFET from turning on until the voltage drop

across the low side MOSFET falls below the voltage across

the R

ILIM resistor. This effectively sets a valley current limit

of R

ILIM x 10μA/RDS(ON). Please note that RDS(ON) of the MOSFET

is dependent on the V

GS voltage (equals to PVDD applied

to SC493). The ILIM flag bit is set whenever current limit

occurs. The converter does not latch off just because this

bit is set.

Output Under Voltage Protection

The output under voltage condition occurs with or without

current limit. The output under voltage without current

limit is normally a result of low input voltage. The control-

ler will look at the inductor current to differentiate these

two situations and respond accordingly. After PGD is

asserted, if the FB voltage falls below the PGD falling

threshold (80%) and current limit does not happen simul-

taneously, the Brown Out (BO) flag bit is set. This indicates

an output under voltage has happened because of low

input voltage. The converter does not latch off just

because the BO bit is set. After PGD is asserted, if FB

voltage falls below 70% of the nominal voltage for 8 con-

secutive current limited switching cycles, the UVO flag bit

is set. This latches the converter off, with both the high

side and low side MOSFETs turned off.

To restart, either the EN pin or ENSW register bit must be

set low and then back to high.

Applications Information (continued)

SC493 Datasheet(PDF) 13 Page - Semtech Corporation

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