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SC4510

High Performance Synchronous Buck Controller With Reference Tracking

Semtech Corporation 

SC4510

POWER MANAGEMENT

Application Information (Contd.)

For simplicity Rs2 can be omitted and the current limit can

be estimated from

I LM

Req

= 75

mV

×

Rs + Rs1

Rs1

In the application circuit the current feedback signal had

to be halved to account for variations explained above and

ensure that the current limit is at least 20% above nominal

at minimum input.

a) The time it takes to discharge the capacitor from 3.4V to

0.4V

tssf

= CSS ×

(3.4 − 0.4)V

1 µA

.

In the application circuit, CSS= 47 nF and tssf is calculated

as 140 mS.

b) When the required current limit value ILM is less than

ILMcp remove Rs1 solve

for Rs and solve

RsC s

=

L

R eq

I LM

Req

+

Rs

Rs3

Vo

=

75

mV

to set the current limit.

Similar steps and equations apply to the current limit setting

and scaling for current sinking mode.

OVERCURRENT PROTECTION AND HICCUP MODE

In addition to the current limit capabilty, SC4510 provides

overcurrent protection in case the convertor output is

shorted to ground. If this fault condition happens, the

controller senses the output voltage via voltage feedback

pin IN-. When the sensed voltage is below 70% of the

normal feedback voltage, the controller shuts down both

top and bottom MOSFETs. At the same time, a current sink

of 1 µA discharges the soft start capacitor CSS connected

to the SS/EN pin.

When the capacitor is discharged until its voltage reaches

0.4V, the controller initiates the soft start process. If the

short circuit fault persists, the controller shuts down the

convertor again when the voltage across the soft start

capacitor reaches 3.4V. This hiccup process repeats until

the fault condition is removed. Under this situation, it is

important to make sure that the convertor does not fail.

One important parameter is the convertor thermal condition

which is directly related to the effective inductor and

MOSFETs current. The effective currents in inductor and

MOSFETs can be estimated using the following equations.

b) The time interval of the soft start process from 0.4V to

3.4V

tssr

=

CSS

×

(3.4 − 0.4) V

2 µA

.

When CSS = 47 nF, tssr is calculated as 70 mS.

Notice that during the soft start process, the convertor only

starts switching when the voltage at SS/EN exceeds 1.3V.

Then,

c) The effective operation time interval

t sso

= C SS

×

(3.4 − 1.3) V

2 µA

.

This is the interval where the gate drive outputs are active

and current builds up in the inductor.

The effective inductor current is then

I Leff

= I LMcp

tsso .

tssf + tssr

It turns out that ILeff is independent of the soft start

capacitor value and is determined as 0.3ILMcp. This should

lead to a reasonable thermal condition in the convertor

hiccup operation. Note that ILeff calculated is not the true

average value of the inductor current since the convertor

is still in the soft start mode during the tsso interval.

© 2004 Semtech Corp.

14

www.semtech.com

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