LT1
170/LT1
171/LT1172
TYPICAL APPLICATIO S
Flyback Converter
V
IN
20V TO 30V
V
IN
C4*
100µF
D2
MUR110
V
SW
+
LT1170
FB
GND
V
C
R3
1.5k
C2
0.15µF
*REQUIRED IF INPUT LEADS
≥
2"
R2
1.24k
0
I
PRI
0
I
PRI
E2
LT1172
E1
GND
V
C
OPTIONAL
SHUTDOWN
VN2222
C4
0.047µF
D2, D3 = ER82.004 600mA SCHOTTKY. OTHER FAST SWITCHING TYPES MAY BE USED.
* V
IN
AND BATTERY MAY BE TIED TOGETHER. MAXIMUM VALUE FOR V
BAT
IS EQUAL TO THE
NEGATIVE
OUTPUT + 1V. WITH HIGHER
BATTERY VOLTAGES, HIGHEST EFFICIENCY IS OBTAINED BY RUNNING THE LT1172 V
IN
PIN FROM 5V. SHUTTING OFF THE 5V SUPPLY
WILL AUTOMATICALLY TURN OFF THE LT1172. EFFICIENCY IS ABOUT 80% AT I
OUT
= 25mA.
R1, R2, R3 ARE MADE LARGE TO MINIMIZE BATTERY DRAIN IN SHUTDOWN, WHICH IS APPROXIMATELY V
BAT
/(R1 + R2 + R3).
** FOR HIGH EFFICIENCY, L1 SHOULD BE MADE ON A FERRITE OR MOLYPERMALLOY CORE. PEAK INDUCTOR CURRENTS ARE ABOUT
600mA AT P
OUT
= 0.7Ω. INDUCTOR SERIES RESISTANCE SHOULD BE LESS THAN 0.4Ω FOR HIGH EFFICIENCY.
*** OUTPUT RIPPLE IS ABOUT 200mV
P-P
TO 400mV
P-P
WITH C2 = 2µF TANTALUM. IF LOWER RIPPLE IS DESIRED, INCREASE C2, OR ADD
A 10Ω , 1µF TANTALUM OUTPUT FILTER.
1170/1/2 TA04
10
+
U
L2 OPTIONAL
FILTER
5µH
C4
100µF
V
IN
N* = 1/3
D3
25V
1W
1 N*
D1
V
OUT
5V
6A
V
SNUB
CLAMP TURN-ON
SPIKE
a
b
0V
c
V
OUT
+ V
f
N • V
IN
d
R1
3.74k
0
I
PRI
/N
∆I
I
PRI
+ Vf
V
PRIMARY FLYBACK VOLTAGE =
OUT
N
LT1170 SWITCH VOLTAGE
AREA “a” = AREA “b” TO MAINTAIN
ZERO DC VOLTS ACROSS PRIMARY
SECONDARY VOLTAGE
AREA “c” = AREA “d” TO MAINTAIN
ZERO DC VOLTS ACROSS SECONDARY
+
C1
2000µF
0V
PRIMARY CURRENT
SECONDARY CURRENT
LT1170 SWITCH CURRENT
SNUBBER DIODE CURRENT
0
(I
)(L )
t = PRI L
V
SNUB
1170/1/2 TA03
LCD Contrast Supply
5V*
V
IN
V
SW
L1**
50µH
V
BAT
*
3V TO 20V
+
R2
100k
R1
200k
D1
1N914
C1
1µF
TANTALUM
V
OUT
–10V TO –26V
D2
FB
R3
15k
C3
0.0047µF
D3
C2***
2µF
TANTALUM
LINER [ LINEAR TECHNOLOGY ]
