8

LTC1540

sn1540 1540fas

3.3V Output Low Dropout Linear Regulator

When the output is low, Q1 turns on, allowing current to

charge output capacitor C1. Local feedback formed by R4,

Q1 and Q2 creates a constant-current source from the 5V

limiting in the case of an output short-circuit to ground. C2

reduces output ripple, while the R2-R3 feedback voltage

divider establishes the output voltage.

Auto Power-Off Source

Figure 8 shows the circuit for a 30mA power supply that

has a timed auto power-off function. The comparator

output is the switched power supply output. With a 10mA

whole circuit draws a mere 0.8µA of quiescent current with

approximately 150mV

The RC time constant determines the maximum power-on

time of the OUT pin before power down occurs. This period

can be approximated by:

t = 4.6RC (seconds)

The actual time will vary with both the leakage current of

Figure 7. 3.3V Output Low Dropout Linear Regulator

1

C3

2

7

8

R1

47k

GND

OUT

1540 F07

HYST

REF

3

4

5

6

–

+

R3

750k

1%

R2

430k

1%

R4

10Ω

Q1

TP0610L

Q2

2N3906

C1

10µF

3.3V

C2

2.2nF

LTC1540

OUT

432k

21

7

8

3

GND

CR

10mA

MOMENTARY

SWITCH

LTC1540

1540 F08

HYST

REF

4

5

6

121k

2M

–

+

Figure 8. Auto Power-Off Switch Operates

on 0.8µA Quiescent Current