MAX691A/MAX693A/MAX800L/MAX800M
Microprocessor Supervisory Circuits
_______________________________________________________________________________________ 7
______________________________________________________________Pin Description
NAME FUNCTION
1VBATT Battery-Backup Input. Connect to external battery or capacitor and charging circuit. If backup battery is not
used, connect to GND.
2 VOUT
Output Supply Voltage. When VCC is greater than VBATT and above the reset threshold, VOUT connects to
VCC. When VCC falls below VBATT and is below the reset threshold, VOUT connects to VBATT. Connect a 0.1µF
capacitor from VOUT to GND. Connect VOUT to VCC if no backup battery is used.
PIN
3 VCC Input Supply Voltage, 5V input.
4GND Ground. 0V reference for all signals.
8OSC 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.
7OSC IN
External Oscillator Input. When OSC SEL is unconnected or driven high, a 10µA pull-up connects from VOUT 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).
6LOW LINE LOW LINE output goes low when VCC falls below the reset threshold. It returns high as soon as VCC rises above
the reset threshold.
5BATT ON
Battery On Output. When VOUT switches to VBATT, BATT ON goes high. When VOUT switches to VCC, BATT ON
goes low. Connect the base of a PNP through a current-limiting resistor to BATT ON for VOUT 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 VOUT.
12 CE OUT Chip-Enable Output. CE OUT goes low only when CE IN is low and VCC 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 VOUT 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.
9PFI 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 VOUT .
16 RESET RESET is an active-high output. It is open drain, and the inverse of RESET.
15 RESET RESET Output goes low whenever VCC falls below the reset threshold. RESET will remain low typically for
200ms after VCC 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
VOUT - 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 VCC = 1V, and an
external 10kΩpull-down resistor on RESET insures
that it will be valid with VCC down to GND (Figure 1).
As VCC goes below 1V, the gate drive to the RESET
output switch reduces accordingly, increasing the
RDS(ON) and the saturation voltage. The 10kΩpull-
down resistor insures the parallel combination of switch
plus resistor is around 10kΩand the output saturation
voltage is below 0.4V while sinking 40µA. When using
a 10kΩexternal pull-down resistor, the high state for
RESET output with VCC = 4.75V will be 4.5V typical.
For battery voltages ≥2V connected to VBATT, RESET
and RESET remain valid for VCC from 0V to 5.5V.