Rainbow Electronics MAX16040 User Manual
Page 11
MAX16033–MAX16040
Low-Power Battery Backup
Circuits in Small µDFN Packages
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11
BATTON Indicator
(MAX16035/MAX16039 Only)
BATTON is a push-pull output that asserts high when in
battery-backup mode. BATTON typically sinks 3.2mA
at a 0.4V saturation voltage. In battery-backup mode,
this terminal sources approximately 10µA from OUT.
Use BATTON to indicate battery-switchover status or to
supply base drive to an external pass transistor for
higher current applications (see Figure 3).
RESETIN Comparator
(MAX16036/MAX16040 Only)
An internal 1.235V reference sets the RESETIN thresh-
old voltage. RESET asserts when the voltage at
RESETIN is below 1.235V. Use the RESETIN function to
monitor a secondary power supply.
Use the following equations to set the reset threshold
voltage (V
RTH
) of the secondary power supply (see
Figure 4):
V
RTH
= V
REF
(R1 / R2 + 1)
where V
REF
= 1.235V. To simplify the resistor selection,
choose a value for R2 and calculate R1.
R1 = R2 [(V
RTH
/ V
REF
) - 1]
Since the input current at RESETIN is 25nA (max), large
values (up to 1MΩ) can be used for R2 with no signifi-
cant loss in accuracy.
Power-Fail Comparator
The MAX16033–MAX16040 issue an interrupt (nonmask-
able or regular) to the µP when a power failure occurs.
The power line is monitored by two external resistors con-
nected to the power-fail input (PFI). When the voltage at
PFI falls below 1.235V, the power-fail output (PFO) drives
the processor’s NMI input low. An earlier power-fail warn-
ing can be generated if the unregulated DC input of the
regulator is available for monitoring. The MAX16033–
MAX16040 turn off the power-fail comparator and force
PFO low when V
CC
falls below the reset threshold volt-
age (see Figure 1). The MAX160_ _L devices provide
push-pull PFO outputs. The MAX160_ _P devices provide
open-drain PFO outputs.
2.4V TO 5.5V
CE
(CEIN)
( ) FOR MAX16035 ONLY
A0–A15
µP
CMOS RAM
BATTON
V
CC
BATT
(CEOUT)
GND
RESET
OUT
0.1µF
ADDRESS
DECODE
MAX16035
MAX16039
RESET
Figure 3. MAX16035/MAX16039 BATTON Driving an External Pass Transistor
MAX16036
MAX16040
R1
RESETIN
V
IN
V
CC
R2
Figure 4. Setting RESETIN Voltage for the
MAX16036/MAX16040