MAX6723AUTTHD2-T View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX6723AUTTHD2-T

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

Maxim Integrated 

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

Supervisory Circuits

Applications Information

Interfacing to µPs with Bidirectional

Reset Pins

Most µPs with bidirectional reset pins can interface

directly to open-drain RST output options. Systems

simultaneously requiring a push-pull RST output and a

bidirectional reset interface can be in logic contention.

To prevent contention, connect a 4.7kΩ resistor

between RST and the µP’s reset I/O port as shown in

Figure 4.

Adding Hysteresis to the Power-Fail

Comparator

The power-fail comparator has a typical input hysteresis

of 3mV. This is sufficient for most applications where a

power-supply line is being monitored through an external

voltage-divider (see the Power-Fail Comparator section).

If additional noise margin is desired, connect a resistor

between PFO and PFI as shown in Figure 5. Select the

values of R1, R2, and R3 so PFI sees VPFI (626mV) when

VEXT falls to its power-fail trip point (VFAIL) and when VIN

rises to its power-good trip point (VGOOD). The hysteresis

window extends between the specified VFAIL and VGOOD

thresholds. R3 adds the additional hysteresis by sinking

current from the R1/R2 divider network when PFO is

logic-low and sourcing current into the network when PFO

is logic-high. R3 is typically an order of magnitude greater

than R1 or R2.

The current through R2 should be at least 2.5µA to ensure

that the 100nA (max) PFI input current does not signifi-

cantly shift the trip points. Therefore, R2 < VPFI/10µA <

62kΩ for most applications. R3 will provide additional hys-

teresis for PFO push-pull (VOH = VCC1) or open-drain

(VOH = VPULLUP) applications.

Monitoring an Additional Power Supply

These µP supervisors can monitor either positive or

negative supplies using a resistor voltage-divider to

PFI. PFO can be used to generate an interrupt to the µP

or cause reset to assert (Figure 3).

Monitoring a Negative Voltage

The power-fail comparator can be used to monitor a

negative supply voltage using the circuit shown in

Figure 3. When the negative supply is valid, PFO is low.

When the negative supply voltage drops, PFO goes

high. The circuit’s accuracy is affected by the PFI

threshold tolerance, VCC, R1, and R2.

Negative-Going VCC Transients

The MAX6715A–MAX6729A/MAX6797A supervisors are

relatively immune to short-duration negative-going VCC

transients (glitches). It is usually undesirable to reset

the µP when VCC experiences only small glitches. The

Typical Operating Characteristics show Maximum

Transient Duration vs. Reset Threshold Overdrive, for

which reset pulses are not generated. The graph was

produced using negative-going VCC pulses, starting

above VTH and ending below the reset threshold by the

VCC1 VCC2

RESET TO OTHER SYSTEM COMPONENTS

MAX6715A–

MAX6729A/

MAX6797A

VCC2

RST

VCC1

4.7kΩ

µP

RESET

GND

GND

R3

A

VIN

VEXT

MAX6729A

PFO

VGOOD

VFAIL

R1

PFI

PFO

R2

VGOOD = DESIRED VEXT GOOD VOLTAGE THRESHOLD

VFAIL = DESIRED VEXT FAIL VOLTAGE THRESHOLD

VOH = VCC1 (FOR PUSH-PULL PFO)

R2 = 50kΩ (FOR > 10µA R2 CURRENT)

GND

R1 = R2 ((VGOOD - VPFI) - (VPFI)(VGOOD - VFAIL)/VOH)/VPFI

R3 = (R1 x VOH)/(VGOOD - VFAIL)

Figure 4. Interfacing to µPs with Bidirectional Reset I/O

Figure 5. Adding Hysteresis to Power-Fail for Push-Pull PFO

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