Rainbow Electronics MAX16040 - Manual (Page 15)

Monitoring a Negative Voltage
Connect the circuit as shown in Figure 9 to use the
power-fail comparator to monitor a negative supply rail.
PFO stays low when V- is good. When V- rises to cause
PFI to be above +1.235V, PFO goes high. Ensure V
CC
comes up before the negative supply.
Negative-Going V
CC
Transients
The MAX16033–MAX16040 are relatively immune to
short-duration, negative-going V
CC transients.
Resetting the µP when V CC experiences only small
glitches is not usually desired.
The Typical Operating Characteristics section contains
a Maximum Transient Duration vs. Reset Threshold
Overdrive graph. The graph shows the maximum pulse
width of a negative-going V
CC
transient that would not
trigger a reset pulse. As the amplitude of the transient
increases (i.e., goes further below the reset threshold
voltage), the maximum allowable pulse width decreas-
es. Typically, a V
CC transient that goes 100mV below
the reset threshold and lasts for 25µs does not trigger a
reset pulse.
A 0.1µF bypass capacitor mounted close to V CC pro-
vides additional transient immunity.
MAX16033–MAX16040
Low-Power Battery Backup
Circuits in Small µDFN Packages
______________________________________________________________________________________ 15
GND
PFI
R2
R1
V-
MAX16033–
MAX16040
3.0V OR 3.3V
V CC
PFO
PFO
0V
V L
V-
V TRIP
Figure 9. Monitoring a Negative Voltage
V R V V
R R
V
R
V R V
R R
V
R
V V
V mV
TRIP PFT PFH
CC
L PFT
CC
PFT
PFH
.
= + ( ) +
⎛
⎝
⎜
⎞
⎠
⎟ −
⎡
⎣
⎢
⎤
⎦
⎥
=
( )
+
⎛
⎝
⎜
⎞
⎠
⎟ −
⎡
⎣
⎢
⎤
⎦
⎥
=
=
2
1
1
1
2 1
2
1
1
1
2 1
1 235
12