Rainbow Electronics MAX16040 - Manual (Page 12)

MAX16033–MAX16040
Low-Power Battery Backup
Circuits in Small µDFN Packages
12 ______________________________________________________________________________________
RESET
A µP’s reset input puts the µP in a known state. The
MAX16033–MAX16040 µP supervisory circuits assert a
reset to prevent code-execution errors during power-
up, power-down, and brownout conditions. RESET
asserts when V
CC is below the reset threshold voltage
and for at least 140ms (t RP ) after V CC rises above the
reset threshold. RESET also asserts when MR is low
(MAX16033/MAX16037) or when RESETIN is below
1.235V (MAX16036/MAX16040). The MAX16034/
MAX16038 watchdog function causes RESET to assert in
pulses following a watchdog timeout (Figure 2). The
MAX160_ _L devices provide push-pull RESET outputs.
The MAX160_ _P devices provide open-drain RESET
outputs.
Applications Information
Operation Without a Backup Power Source
The MAX16033–MAX16040 provide a battery backup
function. If a backup power source is not used, connect
BATT to GND and OUT to V
CC .
Using a Super Cap as a
Backup Power Source
Super caps are capacitors with extremely high capaci-
tance, such as 0.47F. Figure 5 shows two methods to
use a super cap as a backup power source. Connect
the super cap through a diode to the 3V input (Figure
5a) or connect the super cap through a diode to 5V
(Figure 5b) if a 5V supply is available. The 5V supply
charges the super cap to a voltage close to 5V, allow-
ing a longer backup period. Since V
BATT can be higher
than V CC while V CC is above the reset threshold volt-
age, there are no special precautions required when
using these µP supervisors with a super cap.
(a)
3V OR 3.3V
0.47F
1N4148
V CC
BATT
MAX16033–
MAX16040
(b)
3V OR 3.3V
5V
0.47F
1N4148
V CC
BATT
MAX16033–
MAX16040
Figure 5. Using a Super Cap as a Backup Source