MAX791CUE+ Maxim Integrated, MAX791CUE+ Datasheet - Page 15

• Manufacturer Part Number: MAX791CUE+
• Description: Supervisory Circuits
• Manufacturer: Maxim Integrated
• Series: MAX791r

Specifications of MAX791CUE+

• Number Of Voltages Monitored: 1
• Monitored Voltage: 4.65 V
• Undervoltage Threshold: 4.5 V
• Overvoltage Threshold: 4.75 V
• Output Type: Active Low, Open Drain, Push-Pull
• Manual Reset: Resettable
• Watchdog: Watchdog
• Battery Backup Switching: Backup
• Power-up Reset Delay (typ): 280 ms
• Supply Voltage - Max: 5.5 V
• Maximum Operating Temperature: + 70 C
• Mounting Style: SMD/SMT
• Package / Case: TSSOP-16
• Chip Enable Signals: Yes
• Maximum Power Dissipation: 533 mW
• Minimum Operating Temperature: 0 C
• Power Fail Detection: Yes
• Supply Current (typ): 150 uA
• Supply Voltage - Min: 0 V

The power-fail comparator can be used to monitor a
negative supply voltage using Figure 15’s circuit. When
the negative supply is valid, PFO is low. When the neg-
ative supply voltage drops, PFO goes high. This cir-
cuit’s accuracy is affected by the PFI threshold toler-
ance, the V
The backup battery may be disconnected while V
above the reset threshold. No precautions are neces-
sary to avoid spurious reset pulses.
While issuing resets to the µP during power-up, power-
down, and brownout conditions, these supervisors are
relatively immune to short-duration negative-going V
transients (glitches). It is usually undesirable to reset
the µP when V
Figure 16 shows maximum transient duration vs. reset
comparator overdrive, for which reset pulses are not
generated. The graph was produced using negative-
Figure 13. Alternate CE Gating
*MAXIMUM Rp VALUE DEPENDS ON
THE NUMBER OF RAMs.
MINIMUM Rp VALUE IS 1kΩ
CC
CE IN
CC
voltage, and resistors R1 and R2.
Negative-Going V
MAX791
Monitoring a Negative Voltage
V
GND
Backup-Battery Replacement
experiences only small glitches.
OUT
______________________________________________________________________________________
CE OUT
Rp*
ACTIVE-HIGH CE
LINES FROM LOGIC
Microprocessor Supervisory Circuit
CC
Transients
CE
CE
CE
CE
CE
CE
CE
CE
RAM 1
RAM 2
RAM 3
RAM 4
CC
CC
is
going V
reset threshold by the magnitude indicated (reset com-
parator overdrive). The graph shows the maximum
pulse width that a negative-going V
typically have without causing a reset pulse to be
issued. As the amplitude of the transient increases (i.e.,
goes farther below the reset threshold), the maximum
allowable pulse width decreases. Typically, a V
sient that goes 100mV below the reset threshold and
lasts for 40µs or less will not cause a reset pulse to be
issued.
A 100nF bypass capacitor mounted close to the V
pin provides additional transient immunity.
SWT is internally connected to a ±100nA current
source. When a capacitor is connected from SWT to
ground (to select an alternative watchdog-timeout peri-
od), the current source charges and discharges the
timing capacitor to create the oscillator that controls the
watchdog-timeout period. To prevent timing errors or
oscillator start-up problems, minimize external current
leakage sources at this pin, and locate the capacitor as
Figure 14. Adding Hysteresis to the Power-Fail Comparator
PFO
+5V
R2
R1
0V
Connecting a Timing Capacitor to SWT
V IN
0V
CC
V
V
TRIP
H
pulses, starting at 5V and ending below the
= 1.25 /
R3
= 1.25 R1 + R2
TO µP
R1 + R2
C1*
R2
R2
||
R3
||
R3
PFI
PFO
VL - 1.25 + 5 - 1.25 = 1.25
V
MAX791
L
R1
GND
+5V
V
V
CC
TRIP
V
IN
CC
V
H
R3
transient may
*OPTIONAL
R2
CC
tran-
CC
15