Datasheet

LT1170/LT1171/LT1172
12
117012fg
TYPICAL APPLICATIONS
(Note that maximum output currents are divided by 2 for LT1171, by 4 for LT1172.)
LCD Contrast Supply
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.
+
+
Driving High Voltage FET
(for Off-Line Applications, See AN25)
External Current Limit
1170/1/2 TA05
V
IN
V
SW
LT1170
GND
10V TO
20V
D1
+
D
G
Q1
1170/1/2 TA06
V
X
D1
LT1170
GND
R2
≈ 2V
V
C
R1
500Ω