Datasheet

LTC692/LTC693
13
0692fb
APPLICATIONS INFORMATION
10μF
100μF
V
IN
V
OUT
ADJ
692_3 • F08
V
CC
0.1μF
TO μP
PFO
GND
LT1086-5
V
IN
≥ 7.5V
R4
10k
PFI
LTC692
LTC693
R1
51k
R2
10k
R3
300k
5V
++
10μF
10μF
V
IN
V
OUT
ADJ
692_3 • F09
0.1μF
TO μP
LT1086-5
V
IN
≥ 6.5V
R4
10k
R1
27k
R3
2.7M
R2
8.2k
5V
R5
3.3k
V
CC
GND
PFO
PFI
LTC692
LTC693
++
Figure 8. Monitoring Unregulated DC Supply with the
LTC692/LT693 Power-Fail Comparator
Figure 9. Monitoring Regulated DC Supply with the
LTC692/LT693 Power-Fail Comparator
Example 1: The circuit in Figure 8 demonstrates the use
of the power-fail comparator to monitor the unregulated
power supply input. Assuming the the rate of decay of the
supply input V
IN
is 100mV/ms and the total time to execute
a shutdown procedure is 8ms. Also, the noise of V
IN
is
200mV. With these assumptions in mind, we can reason-
ably set V
L
= 7.25V which is 1.25V greater than the sum of
maximum reset voltage threshold and the dropout voltage
of LT
®
1086-5 (4.5V + 1.5V) and V
HYSTERESIS
= 850mV.
V5V
R1
R3
850mV
R3 5.88 R1
HYSTERESIS
==
≈
Choose R3 = 300k and R1 = 51k. Also select R4 = 10k
which is much smaller than R3.
7.25V =1.3V 1+
R2
–
(5V –1.3V)5151
1 3 310
kk
Vk.( )
⎛
⎝
⎜
⎞⎞
⎠
⎟
R2 = 10.1k, Choose nearest 5% resistor 10k and recal-
culate V
L
,
V1.3V1
10 k
–
(5V – 1.3V)51
1.3V(310 k
L
=+
⎛
⎝
⎜
⎞
⎠
51k k
)
⎟⎟
=
=++
⎛
⎝
⎜
⎞
⎠
⎟
=
732.V
V1.3V1
51k
10k
51k
300k
8.151
H
VV
(7.32V – 6.25V)
100mV/ms
10.7ms=
V
HYSTERESIS
= 8.151V – 7.32V = 831mV
The 10.7ms allows enough time to execute shutdown pro-
cedure for microprocessor and 831mV of hysteresis would
prevent PFO from going low due to the noise of V
IN
.