SC1102 Semtech Corporation, SC1102 Datasheet - Page 8

• Manufacturer Part Number: SC1102
• Description: SYNCHRONOUS DC/DC CONTROLLER FOR DISTRIBUTED POWER SUPPLY APPLICATIONS
• Manufacturer: Semtech Corporation

September 5, 2000
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.
APPLICATION CIRCUIT
© 2000 SEMTECH CORP.
PWRGD
OVP
R1
1k
+5V
1.74k
R2
C1
0.1
C2
0.1
R3
1k
MBR0520
D1
R4
10
1
2
3
4
5
6
7
VCC
PWRGD
OVP
OCSET
PHASE
DRVH
PGND
Typical 12V Application Circuit with Bootstrapped BSTH
SC1102
U1
SS/SHDN
SENSE
DRVL
VREF
BSTH
BSTL
GND
14
13
12
11
10
9
8
SYNCHRONOUS DC/DC CONTROLLER FOR
DISTRIBUTED POWER SUPPLY APPLICATIONS
C3
0.1
C4
1.0
SHDN
VREF
R6
2.2
R5
3.9
MBRA130
D2
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.
STP40NE
STP40NE
652 MITCHELL ROAD NEWBURY PARK CA 91320
Q1
Q2
C9
1.0
Optional
D3
MBRD1035
4uH
L1
C5
10.0
NOTE:
*) Vout = 1.265 x (1+R9/R8)
R8
127
C10
180/4V
C6
270/16V
C11
180/4V
R9
205*
C7
270/16V
C12
180/4V
C8
270/16V
C13
180/4V
Vin 12V
+
_
C14
180/4V
SC1102
Vout=3.3V*
+
_
8