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SC4603

POWER MANAGEMENT

Applications Information - (Cont.)

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 SC4603 is

controlled by an internal timing circuit which is used dur-

ing start up and over current to set the hiccup time. The

soft start time can be calculated by:

S

START

_

SOFT

f

1020

T

=

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

frequency dependant. For example, if f

T

T

The SC4603 implements its soft start by ramping up the

error amplifier reference voltage providing a controlled

slew rate of the output voltage, then preventing over-

shoot and limiting inrush current during its start up.....

Over Current Protection

Over current protection for the SC4603 is implemented

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

during conduction, also known as high side R

tion. This loss-less detection eliminates the sense resis-

tor and its loss. The overall efficiency is improved and

the number of components and cost of the converter

are reduced. R

is mainly used to protect the power supply during a fault

case. The over current trigger point will vary from unit to

unit as the R

unit, the over current trigger point will vary as the junc-

tion temperature of P-MOSFET varies. The SC4603 pro-

vides a built-in 50

µA current source, which is combined

with R

the current limit threshold. The value of R

erly selected according to the desired current limit point

I

the I

A

50

R

I

R

)

ON

(

DS

MAX

SET

µ

•

=

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 50

µA current source has a

positive coefficient of about 0.3%/C° providing first or-

der 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 SC4603 due to the compact layout of

the power supply.

When the converter detects an over current condition

(I > I

takes is to enter cycle by cycle protection mode (Point B

to Point C), which responds to minor over current cases.

Then the output voltage is monitored. If the over current

and low output voltage (set at 70% of nominal output

voltage) occur at the same time, the Hiccup mode op-

eration (Point C to Point D) of the SC4603 is invoked

and the internal soft start capacitor is discharged. This is

like a typical soft start cycle.

nom

O

⋅

6875

.

0

nom

O

O

V

O

I

A

B

C

D

Figure 1. Over current protection characteristic of

SC4603

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