SC1102

SYNCHRONOUS DC/DC CONTROLLER FOR

DISTRIBUTED POWER SUPPLY APPLICATIONS

© 2000 SEMTECH CORP.

652 MITCHELL ROAD NEWBURY PARK CA 91320

September 5, 2000

8

THEORY OF OPERATION (CON’T)

An over-current condition occurs when the high-side

drive is turned on, but the PHASE node does not

reach the voltage level set at the OCSET pin. The

PHASE node is sampled only once per cycle during the

valley of the triangular oscillator. Once an over-current

occurs, the high-side drive is turned off and the low-

side drive turns on and the SS/SHDN pin begins to

sink 2uA. The soft-start voltage will begin to decrease

as the 2uA of current discharges the external capaci-

tor. When the soft-start voltage reaches 0.8V, the SS/

SHDN pin will begin to source 10uA and begin to

charge the external capacitor causing the soft-start

voltage to rise again. Again, when the soft-start volt-

age reaches the level of the internal oscillator, switch-

ing will occur.

If the over-current condition is no longer present, nor-

mal operation will continue. If the over-current condi-

tion is still present, the SS/SHDN pin will again begin to

sink 2uA. This cycle will continue indefinitely until the

over-current condition is removed.

In conclusion, below is shown a typical “12V Applica-

tion Circuit” which has a BSTH voltage derived by boot-

strapping input voltage to the PHASE node through

diode D1. This circuit is very useful in cases where only

input power of 12V is available.

In order to prevent substrate glitching, a small-signal

diode should be placed in close proximity to the chip

with cathode connected to PHASE and anode con-

nected to PGND.

APPLICATION CIRCUIT

Typical 12V Application Circuit with Bootstrapped BSTH

VCC

1

PWRGD

2

OVP

3

OCSET

4

PHASE

5

DRVH

6

PGND

7

DRVL

8

BSTL

9

BSTH

10

SENSE

11

VREF

12

SS/SHDN

13

GND

14

U1

SC1102

Q1

STP40NE

Q2

STP40NE

R5

3.9

D3

MBRD1035

R4

10

L1

4uH

D2

MBRA130

C9

1.0

C10

180/4V

C11

180/4V

C12

180/4V

C13

180/4V

C7

270/16V

C6

270/16V

R1

1k

R3

1k

C2

0.1

C3

0.1

C1

0.1

R2

1.74k

PWRGD

OVP

SHDN

VREF

+

_

+

_

Vin 12V

Vout=3.3V*

R8

127

R9

205*

NOTE:

*) Vout = 1.265 x (1+R9/R8)

+5V

D1

MBR0520

R6

2.2

C4

1.0

C5

10.0

Optional

C14

180/4V

C8

270/16V