MAX704CSA+ Maxim Integrated Products, MAX704CSA+ Datasheet - Page 5

• 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

A µP’s reset input starts the µP in a known state.
Whenever the µP is in an unknown state, it should be
held in reset. The MAX703/MAX704 assert reset when
V
power-up, power-down, or brownout conditions.
When V
spective of V
remains low. When V
an internal timer holds RESET low for a time equal to
the reset pulse width (typically 200ms); after this inter-
val, RESET goes high (Figure 2). If a power-fail or
brownout condition occurs (i.e., V
reset threshold), RESET is asserted. As long as V
remains below the reset threshold, the internal timer is
continually restarted, causing the RESET output to
remain low. Thus, a brownout condition that interrupts a
previously initiated reset pulse causes an additional
200ms delay from the end of the last interruption.
The PFI input is compared to an internal reference. If
PFI is less than 1.25V, PFO goes low. The power-fail
comparator can be used as an undervoltage detector
to signal a failing power supply. In the Typical
Operating Circuit , an external voltage-divider at PFI is
used to monitor the unregulated DC voltage from which
the regulated +5V supply is derived.
The voltage-divider can be chosen so the voltage at
PFI falls below 1.25V just before the +5V regulator
drops out. PFO is then used as an interrupt to prepare
the µP for power-down.
To conserve power, the power-fail comparator is turned
off and PFO is forced low when the MAX703/MAX704
enter battery-backup mode.
In the event of a brownout or power failure, it may be
necessary to preserve the contents of RAM. With a
backup battery installed at V
MAX704 automatically switch RAM to backup power
when V
As long as V
nects to V
switch. Once V
RESET goes low and V
er) switches to V
through an 80Ω switch only if V
threshold voltage and V
V
MAX703/MAX704 substrate, regardless of the voltage
CC
CC
exceeds the reset threshold, it is connected to the
is low, preventing code-execution errors during
CC
BATT
OUT
fails.
CC
is 2V or more, RESET is always valid, irre-
CC
through a 5Ω p-channel MOSFET power
exceeds the reset threshold, V
OUT
. On power-up, as V
CC
_______________________________________________________________________________________
Backup-Battery Switchover
. Note that V
falls below the reset threshold,
CC
Detailed Description
CC
Low-Cost Microprocessor Supervisory
BATT
exceeds the reset threshold,
Power-Fail Comparator
or V
is greater than V
BATT
CC
BATT
BATT
CC
(whichever is high-
is below the reset-
RESET Output
CC
switches to V
drops below the
, the MAX703/
rises, RESET
Circuits with Battery Backup
CC
CC
. When
con-
OUT
CC
applied to V
(between V
from V
When the battery-backup mode is activated, V
connects to V
to V
tery (Figure 3). Table 1 shows the status of the MAX703/
MAX704 inputs and outputs in battery-backup mode.
When V
switchover comparator draws about 30µA. Once V
Table 1. Input and Output Status in
Battery-Backup Mode
Figure 3. Battery-Switchover Block Diagram
SIGNAL
RESET
V
V
V
PFO
BATT
MR
V
V
V
V
V
PFI
OUT
BATT
CC
CC
CC
CC
CC
CC
V
BATT
BATT
> Reset Threshold
< Reset Threshold and
> V
< Reset Threshold and
< V
CC
and internal circuitry is powered from the bat-
S1
BATT
BATT
Disconnected from V
Connected to V
p-channel MOSFET switch.
Connected to V
V
Logic-low.
Power-fail comparator is disabled.
Logic-low.
Disabled.
is below, but within, 1V of V
CONDITION
BATT
to V
CC
BATT
OUT
< (V
CC
S2
and the substrate) conducts current
. In this mode, the substrate connects
(Figure 3). During this time, diode D1
BATT
if V
BATT
V
D1
- 1V).
OUT
SUBSTRATE
BATT
OUT
MAX703
MAX704
≥ (V
. Supply current is < 1µA when
STATUS
through an internal 80Ω
OUT
D3
D2
CC
.
Closed
S1/S2
Open
Open
+ 0.6V).
S3
BATT
S4
, the internal
Closed
Closed
V
S3/S4
Open
CC
BATT
CC
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