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SC4602A/B

POWER MANAGEMENT

Applications Information

Enable

Pulling and holding the SYNC/SLEEP pin below 0.8V ini-

tializes the SLEEP mode of the SC4602A/B with its typi-

cal SLEEP mode supply current of 10uA. During the SLEEP

mode, the high side and low side MOSFETs are turned

off and the internal soft start voltage is held low.

Oscillator

The SC4602A/B is a constant frequency, voltage mode,

and synchronous step down controller ideal for low volt-

age, high efficiency, precisely regulated output DC/DC

converters. Its internal free running oscillator sets the

PWM frequency at 300kHz for the SC4602A and 550kHz

for the SC4602B without any external components to

set the frequency. A 100% maximum duty cycle allows

the SC4602A/B to operate as a low dropout regulator in

the event of a low battery condition. An external clock

connected to SYNC/SLEEP activates its synchronous

mode and the frequency of the clock can be up to

450kHz for the SC4602A and 700kH for the SC4602B.

UVLO

When the SYNC/SLEEP pin is pulled and held above 2V,

the voltage on the Vcc pin determines the operation of

the SC4602A/B. As Vcc increases during start up, the

UVLO block senses Vcc and keeps the high side and low

side MOSFETs off and the internal soft start voltage low

until Vcc reaches 2.75V. If no faults are present, the

SC4602A/B will initiate a soft start when Vcc exceeds

2.75V. A hysteresis (150mV) in the UVLO comparator

provides noise immunity during its start up.

Soft Start

The soft start function is required for step down control-

lers to prevent excess inrush current through the DC bus

during start up. Generally this can be done by sourcing a

controlled current into a timing capacitor and then using

the voltage across this capacitor to slowly ramp up the

error amp reference. The closed loop creates narrow

width driver pulses while the output voltage is low and

allows these pulses to increase to their steady state duty

cycle as the output voltage reaches its regulated value.

With this, the inrush current from the input side is con-

trolled. The duration of the soft start in the SC4602A/B

is controlled by an internal timing circuit which is used

during start up and over current to set the hiccup time.

The soft start time can be calculated by:

s

START

_

SOFT

f

720

T

=

As can be seen here, the soft start time is switching fre-

quency dependant. For example, if f

= 720/300k = 2.4ms. But if f

720/600k = 1.2ms.

The SC4602A/B implements its soft start by ramping up

the error amplifier reference voltage providing a con-

trolled slew rate of the output voltage, then preventing

overshoot and limiting inrush current during its start up.

Over Current Protection

Over current protection for the SC4602A/B is imple-

mented by detecting the voltage drop of the high side P-

MOSFET during conduction, also known as high side R

DS(ON)

detection. This loss-less detection eliminates the sense

resistor and its loss. The overall efficiency is improved,

and the number of components and cost of the con-

verter are reduced. R

rate and is mainly used to protect the power supply dur-

ing a fault case. The over current trigger point will vary

from unit to unit as the R

for the same unit, the over current trigger point will vary

as the junction temperature of P-MOSFET varies. The

SC4602A/B provides a built-in 300mV voltage source.

The over current trigger point can be determined based

on the internal 300mV voltage source and the R

P-MOSFET as follows:

)

ON

(

DS

trigger

R

mV

300

I

=

Kelvin sensing connections should be used at the drain

and source of P-MOSFET.

The R

tional feature that enhances the performance of the over

current protection. Because the R

temperature coefficient, the 300mV voltage source has

a positive coefficient of about 0.4%/C° providing first

order correction for current sensing vs temperature. This

compensation depends on the high amount of thermal

transferring that typically exists between the high side P-

MOSFET and the SC4602A/B due to the compact layout

of the power supply.