6

LT1070/LT1071

10701fe

voltage is obtained by using the output of a voltage

sensing error amplifier to set current trip level. This

technique has several advantages. First, it has immediate

response to input voltage variations, unlike ordinary

switchers which have notoriously poor line transient

response. Second, it reduces the 90

° phase shift at

The LT1070/LT1071 is a current mode switcher. This

means that switch duty cycle is directly controlled by

switch current rather than by output voltage. Referring to

the Block Diagram, the switch is turned “on” at the start

of each oscillator cycle. It is turned “off” when switch

current reaches a predetermined level. Control of output

Switch “Off” Characteristics

SWITCH VOLTAGE (V)

0

600

800

1000

80

1070/71 G19

400

200

500

700

900

300

100

0

20

40

60

10

90

30

50

70

100

3V

15V

40V

55V

FREQUENCY (Hz)

1000

3000

4000

6000

7000

1k

100k

1M

10M

1070/71 G21

–1000

10k

5000

2000

0

150

90

θ

60

0

–30

210

30

120

180

Transconductance of Error

Amplifier

0

– 400

– 300

– 200

–100

300

100

0.5

1.0

200

0

1.5

2.0

2.5

1070/71 G20

(CURRENT INTO

–

+

ERROR

AMP

CURRENT

AMP

0.02

Ω

(0.04

Ω LT1071)

SHUTDOWN

CIRCUIT

MODE

SELECT

40kHz

OSC

LOGIC

DRIVER

ANTISAT

16V

5A, 75V

SWITCH

FB

COMP

GAIN

≈ 6

0.15V

1070/71 BD

1.24V

REF

2.3V

REG

FLYBACK

ERROR

AMP

SWITCH

OUT