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

LT1170

GND

10V TO

20V

D1

+

D

G

Q1

1170/1/2 TA06

D1

LT1170

GND

R2

R1

500

Ω

C4*

100

µF

12V

2A

1170/1/2 TA07

R3

2.2k

C2

1000

µF

D1

C1

0.22

µF

Q1

FB

LT1170

GND

OPTIONAL

INPUT FILTER

L3

–20V

REQUIRED IF INPUT LEADS

≥ 2"

PULSE ENGINEERING 92114, COILTRONICS 50-2-52

*

**

R2

1.24k

L1**

50

µH

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

FB

LT1170

GND

NOTE THAT THE LT1170

GND PIN IS NO LONGER

R1

1k

+

–

C1

1000pF

Q1

C3*

100

µF

–5.2V

4.5A

1170/1/2 TA09

R3

L1**

50

µH

R1

4.64k

C2

1000

µF

D1

C1

Q1

2N3906

FB

LT1170

GND

OPTIONAL

INPUT FILTER

L3

–20V

LOAD

REQUIRED IF INPUT LEADS

≥ 2"

PULSE ENGINEERING 92114

COILTRONICS 50-2-52

*

**

R2

1.24k

R4

12k

L2

4

µH

OPTIONAL

OUTPUT

FILTER

C4

200

µF

+

+

+

TYPICAL APPLICATIO S