MAX16016_0812 Datasheet, PDF (17/21 Pages) Maxim Integrated Products – Low-Power μP Supervisory Circuits with Battery-Backup Circuit and Chip-Enable Gating

Low-Power µP Supervisory Circuits with
Battery-Backup Circuit and Chip-Enable Gating
Adding Hysteresis to PFI
The power-fail comparators have a typical input hys-
teresis of V_PFT-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 50kΩ 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.
V_IN
R1
R2 _C1*
+5V
0.1µF
V_CC
PFI
R3
MAX16016L
MAX16020L
PFO MAX16021L
TO µP
*OPTIONAL
+5V
PFO
0
GND
(—) ^R1+R2
V_TRIP = V_PFT x _R2
(– – ) ^VH ^{= (V}PFT ^{+ V}PFT-HYS^{) x RR12+ RR13+ 1}
(— —) ^VL ^{= R1 x}
V_{PFT +} V_CC - V_PFT
R2
R3
+ V_PFT
WHERE V_PFT IS THE POWER-FAIL THRESHOLD VOLTAGE
V_{L VTRIP} V_H
V_IN
Figure 4. Adding Hysteresis to the Power-Fail Comparator
V_CC
0.1µF
1µF
V_CC BATTON
OUT
BATT
CEOUT
CE
MAX16020L
CEIN
MR
RESET
GND
ADDRESS
DECODE
CMOS RAM
A0–A15
µP
RESET
Figure 5. BATTON Driving an External Pass Transistor
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