Detailed Description

Supply Voltages

The MAX6715A–MAX6729A/MAX6797A µP supervisory

circuits maintain system integrity by alerting the µP to

fault conditions. These ICs are optimized for systems

that monitor two or three supply voltages. The output-

reset state is guaranteed to remain valid while either

Threshold Levels

Input-voltage threshold level combinations are indicat-

ed by a two-letter code in the

Reset Voltage Threshold

Suffix Guide (Table 1). Contact factory for availability of

other voltage threshold combinations.

Reset Outputs

The MAX6715A–MAX6729A/MAX6797A provide an

active-low reset output (RST) and the MAX6725A/

MAX6726A also provide an active-high (RST) output.

RST, RST, RST1, and RST2 are asserted when the volt-

threshold level, RSTIN drops below threshold, or MR is

pulled low. Once reset is asserted, it stays low for the

RSTIN goes below the reset threshold before the reset

timeout period is completed, the internal timer restarts.

The MAX6715A/MAX6717A/MAX6719A/MAX6721A/

MAX6723A/MAX6725A/MAX6727A/MAX6728A contain

open-drain reset outputs, while the MAX6716A/

MAX6718A/MAX6720A/MAX6722A/MAX6724A/

MAX6726A/MAX6729A/MAX6797A contain push-pull

reset outputs. The MAX6727A provides two separate

open-drain RST outputs driven by the same internal logic.

Manual-Reset Input

Many µP-based products require manual-reset capabil-

ity, allowing the operator, a test technician, or external

logic circuitry to initiate a reset. A logic-low on MR

asserts the reset output. Reset remains asserted while

returns high. This input has an internal 50k

Ω pullup

used. MR can be driven with CMOS logic levels, or with

open-drain/collector outputs. Connect a normally open

momentary switch from MR to GND to create a manual-

reset function; external debounce circuitry is not

required. If MR is driven from long cables or if the

device is used in a noisy environment, connect a 0.1µF

capacitor from MR to GND to provide additional noise

immunity.

Adjustable Input Voltage

The MAX6719A/MAX6720A and MAX6723A–MAX6727A

provide an additional input to monitor a third system volt-

age. The threshold voltage at RSTIN is typically 626mV.

Connect a resistor-divider network to the circuit as shown

in Figure 1 to establish an externally controlled threshold

Low-leakage current at RSTIN allows the use of large-

valued resistors resulting in reduced power consump-

tion of the system.

Watchdog Input

The watchdog monitors µP activity through the watch-

dog input (WDI). To use the watchdog function, con-

nect WDI to a bus line or µP I/O line. When WDI

remains high or low for longer than the watchdog time-

out period, the reset output asserts.

The MAX6721A–MAX6729A/MAX6797A include a dual-

mode watchdog timer to monitor µP activity. The flexi-

ble timeout architecture provides a long period initial

watchdog mode, allowing complicated systems to

complete lengthy boots, and a short period normal

watchdog mode, allowing the supervisor to provide

quick alerts when processor activity fails. After each

watchdog reset), there is a long initial watchdog period

of 35s minimum. The long watchdog period mode pro-

vides an extended time for the system to power-up and

fully initialize all µP and system components before

assuming responsibility for routine watchdog updates.

Dual/Triple, Ultra-Low-Voltage, SOT23 µP

Supervisory Circuits

10

______________________________________________________________________________________

MAX6719A/

MAX6720A/

MAX6723A–

MAX6727A

R1

R2

RSTIN

GND

Figure 1. Monitoring a Third Voltage