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 2006 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC4520

Application Information

The SC4520 is a current mode buck converter regulator.

SC4520 has an internal fixed-frequency clock. The

SC4520 uses two feedback loops that control the duty

cycle of the internal power switch. The error amplifier

functions like that of the voltage mode converter. The

output of the error amplifier works as a switch current

reference. This technique effectively removes one of the

double poles in the voltage mode system. With this, it is

much simpler to compensate a current mode converter

to have better performance. The current sense amplifier

in the SC4520 monitors the switch current during each

cycle. Overcurrent protection (OCP) is triggered when the

current limit exceeds the upper limit of 3A, detected by a

voltage on COMP greater than about 2V. When an OCP

fault is detected, the switch is turned off and the external

C

Once the COMP voltage has fallen below 250mV,

the part enters a normal startup cycle. C

capacitance value attached to COMP. In the case of

sustained overcurrent or dead-short, the part will

continually cycle through the retry sequence as described

above, at a rate dependent on the value of Ccomp. During

start up, the voltage on COMP rises roughly at the rate of

Therefore, the retry time for a

sustained overcurrent can be approximately calculated

as:

uA

3

V

2

C

120uA

2V

C

T

comp

comp

retry

•

+

•

=

Figure 1 shows the voltage on COMP during a sustained

overcurrent condition.

Figure 1. Voltage on COMP for Startup and OCP

Enable

Pulling and holding the EN pin below 0.4V activates the

shut down mode of the SC4520 which reduces the input

mode, the switch is turned off. The SC4520 is turned on

if the EN pin is pulled high.

Oscillator

The external resistor connected to the FSET pin sets the

PWM frequency from 100kHz to 600kHz.

0

100

200

300

400

500

600

700

0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

UVLO

When the EN pin is pulled and held above 1.8V, the voltage

on Pin IN determines the operation of the SC4520. As

V

V

4.4V.

Load Current

The peak current I

For a specific application, the allowed load current I

OMAX

will change if the input voltage drifts away from the original

design as given for continuous current mode:

s

O

OMAX

f

L

2

)

D

1

(

V

3

I

⋅

⋅

−

⋅

−

=

Where:

f

V

D = duty ratio, V

V