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
VCC is low, preventing code-execution errors during
power-up, power-down, or brownout conditions.
When VBATT is 2V or more, RESET is always valid, irre-
spective of VCC. On power-up, as VCC rises, RESET
remains low. When VCC 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., VCC drops below the
reset threshold), RESET is asserted. As long as VCC
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 VBATT, the MAX703/
MAX704 automatically switch RAM to backup power
when VCC fails.
As long as VCC exceeds the reset threshold, VCC con-
nects to VOUT through a 5Ωp-channel MOSFET power
switch. Once VCC falls below the reset threshold,
RESET goes low and VCC or VBATT (whichever is high-
er) switches to VOUT. Note that VBATT switches to VOUT
through an 80Ωswitch only if VCC is below the reset-
threshold voltage and VBATT is greater than VCC. When
VCC exceeds the reset threshold, it is connected to the
MAX703/MAX704 substrate, regardless of the voltage
applied to VBATT (Figure 3). During this time, diode D1
(between VBATT and the substrate) conducts current
from VBATT to VCC if VBATT ≥(VCC + 0.6V).
When the battery-backup mode is activated, VBATT
connects to VOUT. In this mode, the substrate connects
to VBATT 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 VCC is below, but within, 1V of VBATT, the internal
switchover comparator draws about 30µA. Once VCC
MAX703/MAX704
Low-Cost Microprocessor Supervisory
Circuits with Battery Backup
_______________________________________________________________________________________ 5
p-channel MOSFET switch.