LT1764A View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT1764A

3A, Fast Transient Response, Low Noise, LDO Regulators

Linear Technology 

LT1764A Series

APPLICATIONS INFORMATION

In situations where the ADJ pin is connected to a resistor

divider that would pull the ADJ pin above its 7V clamp

voltage if the output is pulled high, the ADJ pin input

current must be limited to less than 5mA. For example, a

resistor divider is used to provide a regulated 1.5V output

from the 1.21V reference when the output is forced to 20V.

The top resistor of the resistor divider must be chosen to

limit the current into the ADJ pin to less than 5mA when the

ADJ pin is at 7V. The 13V difference between OUT and ADJ

pins divided by the 5mA maximum current into the ADJ pin

yields a minimum top resistor value of 2.6k.

In circuits where a backup battery is required, several

different input/output conditions can occur. The output

voltage may be held up while the input is either pulled to

ground, pulled to some intermediate voltage, or is left

open circuit. Current flow back into the output will follow

the curve shown in Figure 5.

When the IN pin of the LT1764A-X is forced below the OUT

pin or the OUT pin is pulled above the IN pin, input current

will typically drop to less than 2µA. This can happen if the

input of the device is connected to a discharged (low

voltage) battery and the output is held up by either a

backup battery or a second regulator circuit. The state of

the SHDN pin will have no effect on the reverse output

current when the output is pulled above the input.

5.0

4.5

LT1764A-1.5

4.0

LT1764A-1.8

3.5

3.0

LT1764A

2.5

2.0

1.5

TJ = 25°C

VIN = OV

CURRENT FLOWS INTO

OUTPUT PIN

VOUT = VADJ (LT1764A)

VOUT = VFB (LT1764A-1.5

LT1764A-1.8, LT1764A-2.5,

LT1764A-3.3)

LT1764A-2.5

1.0

LT1764A-3.3

0.5

0

0 1 2 3 4 5 6 7 8 9 10

OUTPUT VOLTAGE (V)

1764 F12

Figure 5. Reverse Output Current

TYPICAL APPLICATIO S

SCR Preregulator Provides Efficiency Over Line Variations

L2

90V AC

TO 140V AC

NTE5437

L1

500µH

LT1764A-3.3

10V AC

AT 115VIN

10V AC

AT 115VIN

1N4148

1k

+

10000µF

IN

OUT

SHDN FB

+

GND

34k*

12.1k*

NTE5437

1N4002 1N4002

“SYNC”

V+

TO

ALL “V+”

POINTS

1N4002 2.4k

+

22µF

750Ω

+

C1A

1/2 LT1018

–

1N4148 200k

0.1µF

L1: COILTRONICS CTX500-2-52

L2: STANCOR P-8560

*1% FILM RESISTOR

V+

0.033µF

750Ω

C1B

1/2 LT1018

1N4148

10k

V+

V+

A1

LT1006

1µF

VOUT

3.3V

3A

22µF

10k 10k

V+

LT1004

1.2V

1764 TA03

1764afb

17

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