Datasheet

Detailed Description
RESET Output
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
CC
is
low, preventing code-execution errors during power-up,
power-down, or brownout conditions.
When V
BATT
is 2V or more, RESET is always valid,
irrespective of V
CC
. On power-up, as V
CC
rises, RESET
remains low. When V
CC
exceeds the reset threshold, an
internal timer holds RESET low for a time equal to the
reset pulse width (typically 200ms); after this interval,
RESET goes high (Figure 2). If a power-fail or brown-
out condition occurs (i.e., V
CC
drops below the reset
threshold), RESET is asserted. As long as V
CC
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.
Power-Fail Comparator
The PFI input is compared to an internal reference. If PFI
is less than 1.25V, PFO goes low. The power-fail com-
parator 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.
Backup-Battery Switchover
In the event of a brownout or power failure, it may be nec-
essary to preserve the contents of RAM. With a backup
battery installed at V
BATT
, the MAX703/MAX704 auto-
matically switch RAM to backup power when V
CC
fails.
As long as V
CC
exceeds the reset threshold, V
CC
con-
nects to V
OUT
through a 5Ω p-channel MOSFET power
switch. Once V
CC
falls below the reset threshold, RESET
goes low and V
CC
or V
BATT
(whichever is higher) switch-
es to V
OUT
. Note that V
BATT
switches to V
OUT
through
an 80Ω switch only if V
CC
is below the resetthreshold volt-
age and V
BATT
is greater than V
CC
. When V
CC
exceeds
the reset threshold, it is connected to the MAX703/
MAX704 substrate, regardless of the voltage applied to
V
BATT
(Figure 3). During this time, diode D1 (between
V
BATT
and the substrate) conducts current from V
BATT
to
V
CC
if V
BATT
≥ (V
CC
+ 0.6V).
When the battery-backup mode is activated, V
BATT
con-
nects to V
OUT
. In this mode, the substrate connects to
V
BATT
and internal circuitry is powered from the battery
(Figure 3). Table 1 shows the status of the MAX703/
MAX704 inputs and outputs in battery-backup mode.
When V
CC
is below, but within, 1V of V
BATT
, the internal
switchover comparator draws about 30μA. Once V
CC
Figure 3. Battery-Switchover Block Diagram
Table 1. Input and Output Status in
Battery-Backup Mode
SIGNAL STATUS
V
CC
Disconnected from V
OUT
.
V
OUT
Connected to V
BATT
through an internal 80Ω
p-channel MOSFET switch.
V
BATT
Connected to V
OUT
. Supply current is < 1µA
when V
CC
< (V
BATT
- 1V).
RESET Logic-low.
PFI Power-fail comparator is disabled.
PFO Logic-low.
MR Disabled.
S1 S2
D1
D3
D2
S3 S4
V
BATT
SUBSTRATE
V
OUT
V
CC
MAX703
MAX704
CONDITION S1/S2 S1/S2
V
CC
> Reset Threshold Open Closed
V
CC
< Reset Threshold and
V
CC
> V
BATT
Open Closed
V
CC
< Reset Threshold and
V
CC
< V
BATT
Closed Open
MAX703/MAX704 Low-Cost Microprocessor Supervisory
Circuits with Battery Backup
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