Datasheet

8
LTC1540
sn1540 1540fas
TYPICAL APPLICATIONS
U
3.3V Output Low Dropout Linear Regulator
The LTC1540 can be connected as a micropower (I
Q
=
5.5µA at V
IN
= 5V) low dropout linear regulator (Figure 7).
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
input to C1. R4, R1 and Q2’s V
BE
also provide current
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
load, it typically provides a voltage of (V
BAT
– 0.17V). The
whole circuit draws a mere 0.8µA of quiescent current with
V
BAT
= 5V. The three resistor voltage divider programs
50mV of hysteresis for the comparator, and sets the IN
–
voltage at 200mV. This gives an IN
+
trip threshold of
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
the capacitor and the input current at the IN
+
pin.
Figure 7. 3.3V Output Low Dropout Linear Regulator
V
+
V
–
1
C3
2
7
8
R1
47k
GND
OUT
1540 F07
IN
+
IN
–
HYST
REF
3
4
5
6
–
+
R3
750k
1%
R2
430k
1%
R4
10Ω
Q1
TP0610L
V
IN
= 5V
Q2
2N3906
C1
10µF
V
OUT
3.3V
C2
2.2nF
LTC1540
V
+
IN
+
OUT
432k
V
BAT
V
–
21
7
8
3
GND
CR
(V
BAT
– 0.17V)
10mA
MOMENTARY
SWITCH
LTC1540
1540 F08
IN
–
HYST
REF
4
5
6
121k
2M
–
+
Figure 8. Auto Power-Off Switch Operates
on 0.8µA Quiescent Current