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, irre-
spective 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 inter-
val, RESET goes high (Figure 2). If a power-fail or
brownout 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
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.
Backup-Battery Switchover
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
BATT
, the MAX703/
MAX704 automatically 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 high-
er) switches to V
OUT
. Note that V
BATT
switches to V
OUT
through an 80Ω switch only if V
CC
is below the reset-
threshold voltage 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
connects to V
OUT
. In this mode, the substrate connects
to V
BATT
and internal circuitry is powered from the bat-
tery (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
MAX703/MAX704
Low-Cost Microprocessor Supervisory
Circuits with Battery Backup
_______________________________________________________________________________________ 5
.
p-channel MOSFET switch.
. Supply current is < 1µA when
- 1V).
