MAX691A/MAX693A/MAX800L/MAX800M
Microprocessor Supervisory Circuits
_______________________________________________________________________________________ 7
______________________________________________________________Pin Description
NAME FUNCTION
1 VBATT
Battery-Backup Input. Connect to external battery or capacitor and charging circuit. If backup battery is not
used, connect to GND.
2 V
OUT
Output Supply Voltage. When V
CC
is greater than VBATT and above the reset threshold, V
OUT
connects to
V
CC
. When V
CC
falls below VBATT and is below the reset threshold, V
OUT
connects to VBATT. Connect a 0.1µF
capacitor from V
OUT
to GND. Connect V
OUT
to V
CC
if no backup battery is used.
PIN
3 V
CC Input Supply Voltage, 5V Input.
4 GND
Ground. 0V reference for all signals.
8 OSC SEL
Oscillator Select. When OSC SEL is unconnected or driven high, the internal oscillator sets the reset delay and
watchdog timeout period. When OSC SEL is low, the external oscillator input (OSC IN) is enabled (Table 1).
OSC SEL has a 10µA internal pull-up.
7 OSC IN
External Oscillator Input. When OSC SEL is unconnected or driven high, a 10µA pull-up connects from V
OUT
to
OSC IN, the internal oscillator sets the reset and watchdog timeout periods, and OSC IN selects between fast
and slow watchdog timeout periods. When OSC SEL is driven low, the reset and watchdog timeout periods may
be set either by a capacitor from OSC IN to ground or by an external clock at OSC IN (Figure 3).
6
LOW LINE
LOW LINE output goes low when V
CC
falls below the reset threshold. It returns high as soon as V
CC
rises above
the reset threshold.
5 BATT ON
Battery-On Output. When V
OUT
switches to VBATT, BATT ON goes high. When V
OUT
switches to V
CC,
BATT ON
goes low. Connect the base of a PNP through a current-limiting resistor to BATT ON for V
OUT
current require-
ments greater than 250mA.
13
CE IN
Chip-Enable Input. The input to chip-enable gating circuit. If CE IN is not used, connect CE IN to GND or V
OUT.
12
CE OUT
Chip-Enable Output. CE OUT goes low only when CE IN is low and V
CC
is above the reset threshold. If CE IN is
low when reset is asserted, CE OUT will stay low for 15µs or until CE IN goes high, whichever occurs first.
11 WDI
Watchdog Input. WDI is a three-level input. If WDI remains either high or low for longer than the watchdog time-
out period, WDO goes low and reset is asserted for the reset timeout period. WDO remains low until the next tran-
sition at WDI. Leaving WDI unconnected disables the watchdog function. WDI connects to an internal voltage
divider between V
OUT
and GND, which sets it to mid-supply when left unconnected.
10
PFO
Power-Fail Output. This is the output of the power-fail comparator. PFO goes low when PFI is less than 1.25V.
This is an uncommitted comparator, and has no effect on any other internal circuitry.
9 PFI
Power-Fail Input. This is the noninverting input to the power-fail comparator. When PFI is less than 1.25V, PFO
goes low. When PFI is not used, connect PFI to GND or V
OUT
.
16 RESET
RESET is an active-high output. It is open drain, and the inverse of RESET.
15
RESET
RESET Output goes low whenever V
CC
falls below the reset threshold. RESET will remain low typically for
200ms after V
CC
crosses the reset threshold on power-up.
14
WDO
Watchdog Output. If WDI remains high or low longer than the watchdog timeout period, WDO goes low and reset
is asserted for the reset timeout period. WDO returns high on the next transition at WDI. WDO remains high if
WDI is unconnected.
_______________Detailed Description
–
R
—
E
—
S
—
E
—
T
–
and RESET Outputs
The MAX691A/MAX693A/MAX800L/MAX800M’s RESET
and RESET outputs ensure that the µP (with reset
inputs asserted either high or low) powers up in a
known state, and prevents code-execution errors dur-
ing power-down or brownout conditions.
The RESET output is active low, and typically sinks
3.2mA at 0.1V saturation voltage in its active state.
When deasserted,
–
RESET sources 1.6mA at typically
V
OUT
- 0.5V. RESET output is open drain, active high,
and typically sinks 3.2mA with a saturation voltage of
0.1V. When no backup battery is used, RESET output is
guaranteed to be valid down to V
CC
= 1V, and an
external 10kΩ pulldown resistor on RESET insures that
it will be valid with V
CC
down to GND (Figure 1). As
V
CC
goes below 1V, the gate drive to the RESET output
switch reduces accordingly, increasing the R
DS(ON)
and the saturation voltage. The 10kΩ pulldown resistor
insures the parallel combination of switch plus resistor
is around 10kΩ and the output saturation voltage is
below 0.4V while sinking 40µA. When using a 10kΩ
external pulldown resistor, the high state for
RESET output with V
CC
= 4.75V will be 4.5V typical.
For battery voltages ≥ 2V connected to VBATT, RESET
and RESET remain valid for V
CC
from 0V to 5.5V.
