MAX704CSA+ Maxim Integrated Products, MAX704CSA+ Datasheet - Page 7

• Manufacturer Part Number: MAX704CSA+
• Description: IC MPU SUPERVISORY CIRCUIT 8SOIC
• Manufacturer: Maxim Integrated Products
• Type: Battery Backup Circuitr

Specifications of MAX704CSA+

• Number Of Voltages Monitored: 1
• Output: Push-Pull, Totem Pole
• Reset: Active Low
• Reset Timeout: 140 ms Minimum
• Voltage - Threshold: 4.4V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Monitored Voltage: 4.4 V
• Undervoltage Threshold: 4.25 V
• Overvoltage Threshold: 4.5 V
• Manual Reset: Resettable
• Watchdog: No
• Battery Backup Switching: Yes
• Power-up Reset Delay (typ): 280 ms
• Supply Voltage (max): 5.5 V
• Supply Voltage (min): 0 V
• Supply Current (typ): 350 uA
• Maximum Power Dissipation: 471 mW
• Mounting Style: SMD/SMT
• Maximum Operating Temperature: + 70 C
• Chip Enable Signals: No
• Minimum Operating Temperature: 0 C
• Output Type: Active Low or Push-Pull
• Power Fail Detection: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

When V
output no longer sinks current; it becomes an open cir-
cuit. High-impedance CMOS logic inputs can drift to
undetermined voltages if left as open circuits. If a pull-
down resistor is added to the RESET pin as shown in
Figure 6, any stray charge or leakage currents will flow
to ground, holding RESET low. Resistor value R1 is not
critical. It should be about 100kΩ, which is large
enough not to load RESET and small enough to pull
RESET to ground.
The backup battery can be removed while V
valid without triggering a reset. As long as V
above the reset threshold, battery-backup mode cannot
Figure 6. RESET Valid to Ground Circuit
Figure 7. Adding Hysteresis to the Power-Fail Comparator
PFO
+5V
R1
R2
0V
V
V
V
CC
H
TRIP
IN
0V
=
1 25
falls below 1V, the MAX703/MAX704 RESET
.
=
R3
1 25
TO µP
.
/
⎛
⎜
⎝
Ensuring a Valid RESET Output
R
⎛
⎜
⎝
1
Replacing the Backup Battery
R
R
+
1
2
_______________________________________________________________________________________
C1*
R
R
+
2
| |
2
V
R
V
BATT
CC
2
| |
R
⎞
⎟
⎠
3
R
MAX703
MAX704
Low-Cost Microprocessor Supervisory
3
⎞
⎟
⎠
V
L
PFI
PFO
V
L
RESET
V
V
V
−
TRIP
R
OUT
IN
1 25
1
Down to V
MAX703
MAX704
.
GND
V
+5V
CC
+
V
5 1 25
H
−
R
3
R1
.
=
*OPTIONAL
CC
1 25
R
.
Circuits with Battery Backup
CC
2
CC
remains
= 0V
stays
be entered. This is an improvement on switchover ICs
that initiate a reset when V
the same voltage level (regardless of the reset thresh-
old voltage). If the voltage on the unconnected V
pin floats up toward V
initiate a reset when using the MAX703/MAX704.
Hysteresis adds a noise margin to the power-fail com-
parator and prevents repeated triggering of PFO when
V
shows how to add hysteresis to the power-fail com-
parator. Select the ratio of R1 and R2 so that PFI sees
1.25V when V
Resistor R3 adds hysteresis. It will typically be an order
of magnitude greater than R1 or R2. The current
through R1 and R2 should be at least 1µA to ensure
that the 25nA (max) PFI input current does not shift the
trip point. R3 should be larger than 10kΩ to prevent it
from loading down the PFO pin. Capacitor C1 adds
additional noise rejection.
The power-fail comparator can be used to monitor a
negative supply voltage using Figure 8’s circuit. When
the negative supply is valid, PFO is low. When the neg-
ative supply voltage droops, PFO goes high. This cir-
cuit’s accuracy is affected by the PFI threshold
tolerance, the V
Figure 8. Monitoring a Negative Voltage
IN
PFO
+5V
0V
is near the power-fail comparator trip point. Figure 7
NOTE: V
5 1 25
−
R
1
.
TRIP
=
+5V
1 25
IS NEGATIVE
IN
.
R1
R2
CC
Monitoring a Negative Voltage
R
−
falls to the desired trip point (V
2
V
voltage, and resistors R1 and R2.
TRIP
V-
CC
Adding Hysteresis to the
Power-Fail Comparator
, this condition alone cannot
PFI
V
CC
TRIP
V-
and V
MAX703
MAX704
GND
V
CC
BATT
PFO
are at or near
0V
TRIP
BATT
7
).