Datasheet

6
LTC1522
APPLICATIONS INFORMATION
WUU
U
Figure 2. Ultralow Quiescent Current (<2.1µA) Regulated Supply
1
2
3
4
8
7
6
5

NC
SHDN
GND
C
–
NC
V
IN

V
OUT

C
+
LTC1522
FROM MPU
10µF
10µF
V
OUT

5V ±4%
SHDN PIN WAVEFORMS:
LOW I
Q
MODE (2Hz TO 100Hz, 95% TO 98% DUTY CYCLE)
I
OUT
≤ 100µA
V
OUT
LOAD ENABLE MODE
(I
OUT
= 100µA TO 20mA)
0.22µF
V
IN
2.7V TO 5V

+
+
1522 F02
INPUT VOLTAGE (V)
0.0
2.0
4.0
6.0
SUPPLY CURRENT (µA)
5.0
1522 F03a
2.0 3.0 4.0
Figure 3a. No-Load I
CC
vs Input Voltage for Circuit in Figure 3
OUTPUT CURRENT (µA)
1
10
100
1000
MAXIMUM SHDN OFF TIME (ms)
1000
1522 F03b
1 10 100
SHDN ON PULSE WIDTH = 200µs
C
OUT
= 10µF
The LTC1522 must be out of shutdown for a minimum
duration of 200µs to allow enough time to sense the output
and keep it in regulation. A 2Hz, 98% duty cycle signal will
keep V
OUT
in regulation under no-load conditions. As the
V
OUT
load current increases, the frequency with which the
part is taken out of shutdown must also be increased to
prevent V
OUT
from drooping below 4.8V during the OFF
phase (see Figure 3b). A 100Hz 98% duty cycle signal on
the SHDN pin ensures proper regulation with load currents
as high as 100µA. When load current greater than 100µA
is needed, the SHDN pin must be forced low as in normal
operation. The typical no-load supply current for this
circuit with V
IN
= 3V is only 2.1µA.
Each time the LTC1522 comes out of shutdown, the part
delivers a minimum of one clock cycle worth of charge to
the output. Under high V
IN
(> 3.3V) and/or low I
OUT
(< 10µA)
conditions, this behavior may cause a net excess of charge
to be delivered to the output capacitor if a high frequency
signal is used on the SHDN pin (e.g., 50Hz to 100Hz).
Under such conditions, V
OUT
will slowly drift positive and
may even go out of regulation. To avoid this potential
problem in the low I
Q
mode, it is necessary to switch the
part in and out of shutdown at the minimum allowable
frequency (refer to Figure 3b) for a given output load.
Figure 3b. Maximum SHDN OFF Time vs Output Load Current
for Ultralow I
Q
Operation