Adding hysteresis to pfi, Battery-on indicator (push-pull version), Operation without a backup power source – Rainbow Electronics MAX16021 User Manual
Page 17
MAX16016/MAX16020/MAX16021
Low-Power µP Supervisory Circuits with
Battery-Backup Circuit and Chip-Enable Gating
______________________________________________________________________________________
17
MAX16020L
CE
CEIN
GND
CMOS RAM
V
CC
BATTON
MR
BATT
0.1
µF
1
µF
CEOUT
OUT
RESET
V
CC
ADDRESS
DECODE
µP
A0–A15
RESET
Figure 5. BATTON Driving an External Pass Transistor
Adding Hysteresis to PFI
The power-fail comparators have a typical input hys-
teresis of V
PFI-HYS
. This is sufficient for most applica-
tions where a power-supply line is being monitored
through an external voltage-divider (see the
Monitoring
an Additional Supply
section). Figure 4 shows how to
add hysteresis to the power-fail comparator. Select the
ratio of R1 and R2 so that PFI sees V
PFT
when V
IN
falls
to the desired trip point (V
TRIP
). Resistor R3 adds hys-
teresis. R3 is typically an order of magnitude greater
than R1 or R2. R3 should be larger than 50k
Ω to pre-
vent it from loading down PFO. Capacitor C1 adds
additional noise rejection.
Battery-On Indicator (Push-Pull Version)
BATTON goes high when in battery-backup mode. Use
BATTON to indicate battery-switchover status or to sup-
ply base drive to an external pass transistor for higher
current applications (Figure 5).
Operation Without a Backup Power Source
The MAX16016/MAX16020/MAX16021 provide a bat-
tery-backup function. If a backup power source is not
used, connect BATT to GND and OUT to V
CC
.
MAX16016L
MAX16020L
MAX16021L
PFO
GND
V
CC
PFI
0.1
µF
R2
R1
R3
C1*
PFO
0
+5V
V
L
V
H
V
TRIP
V
IN
0.1
µF
+5V
V
IN
TO
µP
R1+R2
R2
V
TRIP
= V
PFT
x
(—)
*OPTIONAL
R1
R3
R1
R2
V
H
= (V
PFT
+ V
PFT_HYS
) x
(–
+
–
+ 1
)
V
CC
- V
PFT
R3
V
PFT
R2
V
L
= R1 x
(—
+
—)
+ V
PFT
WHERE V
PFT
IS THE POWER-FAIL THRESHOLD VOLTAGE
Figure 4. Adding Hysteresis to the Power-Fail Comparator