Datasheet

11
LT1170/LT1171/LT1172
Negative-to-Positive Buck-Boost Converter
†
Driving High Voltage FET
(for Off-Line Applications, See AN25)
(Note that maximum output currents are divided by 2 for LT1171, by 4 for LT1172.)
Negative Buck Converter
External Current Limit
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
C4*
100µF
V
OUT
12V
2A
1170/1/2 TA07
R3
2.2k
C2
1000µF
D1
C1
0.22µF
Q1
V
IN
V
C
FB
V
SW
LT1170
GND
OPTIONAL
INPUT FILTER
L3
V
IN
–20V
REQUIRED IF INPUT LEADS ≥ 2"
PULSE ENGINEERING 92114, COILTRONICS 50-2-52
*
**
R2
1.24k
L1**
50µH
OPTIONAL
OUTPUT
FILTER
C3
L2
R1
11.3k
THIS CIRCUIT IS OFTEN USED TO CONVERT –48V TO 5V. TO GUARANTEE
FULL SHORT-CIRCUIT PROTECTION, THE CURRENT LIMIT CIRCUIT SHOWN
IN AN19, FIGURE 39, SHOULD BE ADDED WITH C1 REDUCED TO 200pF.
†
+
+
1170/1/2 TA08
R2
C2
V
IN
V
C
FB
V
SW
LT1170
GND
V
IN
NOTE THAT THE LT1170
GND PIN IS NO LONGER
COMMON TO V
IN
–
.
R1
1k
+
–
C1
1000pF
Q1
R
S
C3*
100µF
–5.2V
4.5A
1170/1/2 TA09
R3
L1**
50µH
R1
4.64k
C2
1000µF
D1
C1
Q1
2N3906
V
IN
V
C
FB
V
SW
LT1170
GND
OPTIONAL
INPUT FILTER
L3
V
IN
–20V
LOAD
REQUIRED IF INPUT LEADS ≥ 2"
PULSE ENGINEERING 92114
COILTRONICS 50-2-52
*
**
R2
1.24k
R4
12k
L2
4µH
OPTIONAL
OUTPUT
FILTER
C4
200µF
+
+
+
TYPICAL APPLICATIO S
U