ASM802LAEPA ALSC [Alliance Semiconductor Corporation], ASM802LAEPA Datasheet - Page 6

• Manufacturer Part Number: ASM802LAEPA
• Description: UP Power Supply Supervisor With Battery Backup Switch
• Manufacturer: ALSC [Alliance Semiconductor Corporation]

October 2003
Operation without a Backup Power Source
When operating without a back-up power source, the V
pin should be connected to GND and V
connected to V
occur. Connecting V
due to the ON-resistance of the PMOS switch.
Power-Fail Comparator
The Power Fail feature is an independent voltage monitoring
function that can be used for any number of monitoring
activities. The PFI function can provide an early sensing of
power supply failure by sensing the voltage of the
unregulated DC ahead of the regulated supply sensing seen
by the backup-battery switchover circuitry. The PFI pin is
compared to a 1.25V internal reference. If the voltage at the
PFI pin is less than this reference voltage, the PFO pin goes
low. By sensing the voltage of the raw DC power supply, the
microprocessor system can prepare for imminent power-loss,
especially if the battery backup supply is not enabled. The
input voltage at the PFI pin results from a simple resistor
voltage divider as shown in Figure 6.
rev 1.0
Figure 6: Simple Voltage divider sets PFI trip point
A
=
+5V
------------------- -
R
PFO
1
5R 2
0V
+
R
V
2
IN
<
1.25V
CC
R2
R1
, since power source switchover will not
+5V
A
OUT
B
V
PFI
=
to V
CC
------------------- -
R 1
µP Power Supply Supervisor With Battery Backup Switch
5R
+
CC
2
GND
R 2
>
ASM802M
eliminates the voltage drop
ASM692A
ASM690A
ASM802L
ASM805L
Notice: The information in this document is subject to change without notice
1.25V
B
PFO
OUT
should be
BATT
Power Fail Hysteresis
A noise margin can be added to the simple monitoring circuit
of Figure 6 by adding positive feedback from the PFO pin.
The circuit of Figure 7 adds this positive “latching” effect by
means of an additional resistor R3 connected between PFO
and PFI which helps in pulling PFI in the direction of PFO and
eliminating an indecision at the trip point. Resistor R3 is
normally about 10 times higher in resistance than R2 to keep
the hysteresis band reasonable and should be larger than
10kΩ to avoid excessive loading on the PFO pin. The
calculations for the correct values of resistors to set the
hysteresis thresholds are given in Figure 7. A capacitor can
be added to offer additional noise rejection by low-pass
filtering.
R
PFO
R
+5V
Figure 7: Hysterisis Added To PFI Pin
1
2
0V
V
IN
V
TRIP
C1*
0V
=
+5V
-------------------------
⎛
⎜
⎝
------------------- -
R 2
1.25
V
---------------------- -
R 2
+
L
R 2
R
To µP
–
R
3
⎞
⎟
⎠
1
1.25
PFO
V
PFI
CC
+
5 1.25
------------------ -
–
ASM690A / 692A
ASM802L / 802M
V
R
GND
L
ASM690A
ASM692A
ASM802L
ASM802M
ASM805L
3
V
H
V
=
TRIP
=
1.25
--------- -
----------------------------------- -
⎛
⎝
R
----------------------------- -
R
* Optional
2
1
R
ASM805L
V
+
1.25
2
H
R
||
2
R
||
3
R
3
⎞
⎠
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