Datasheet

10
LT1170/LT1171/LT1172
Flyback Converter
LCD Contrast Supply
1170/1/2 TA03
D1
C1
2000µF
C4*
100µF
C2
0.15µF
R1
3.74k
R2
1.24k
V
IN
20V TO 30V
R3
1.5k
*REQUIRED IF INPUT LEADS ≥ 2"
LT1170
V
IN
V
SW
FB
V
C
OPTIONAL
FILTER
L2
5µH
C4
100µF
V
OUT
5V
6A
V
SNUB
CLAMP TURN-ON
SPIKE
PRIMARY FLYBACK VOLTAGE =
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
PRIMARY CURRENT
SECONDARY CURRENT
LT1170 SWITCH CURRENT
SNUBBER DIODE CURRENT
V
OUT
+ Vf
N
0V
V
IN
a
b
0V
c
d
V
OUT
+ V
f
N • V
IN
∆I
I
PRI
0
I
PRI
/N
I
PRI
I
PRI
t =
(I
PRI
)(L
L
)
V
SNUB
0
0
0
1
N* = 1/3
N*
D3
25V
1W
D2
MUR110
GND
+
+
C4
0.047µF
V
OUT
–10V TO –26V
1170/1/2 TA04
R3
15k
OPTIONAL
SHUTDOWN
R2
100k
R1
200k
C1
1µF
TANTALUM
D1
1N914
C3
0.0047µF
C2***
2µF
TANTALUM
D2
VN2222
5V*
L1**
50µH
V
BAT
*
3V TO 20V
V
IN
V
C
FB
V
SW
LT1172
E2
E1
GND
D3
*
**
***
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.
+
+
TYPICAL APPLICATIO S
U