MAX6740XKL datasheet(13/16 Pages) MAXIM

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Low-Power Dual-/Triple-Voltage SC70 µP

Supervisory Circuits

______________________________________________________________________________________

Functional Diagram

VCC1

VCC2

RSTIN/PFI

VCC1

RESET

TIMEOUT

PERIOD

VCC2

POK1

RESET

OUTPUT

0.488V

1.23V

PULLUP

MAX6736–

MAX6745

PFO

DELAY

The MAX6741/MAX6744 offer a VCC1 POK (POK1) ideal

for VCC1-to-VCC2 sequencing. POK1 remains low as

long as VCC1 is below its VTH1 threshold. When VCC1

exceeds VTH1 for the POK1 timeout period (tPOKP), the

open-drain POK1 output is deasserted. The POK1 output

can then enable the VCC2 power supply (use an external

POK1 pullup resistor).

RESET is deasserted when both

VCC1 and VCC2 remain above their selected thresholds

for the reset timeout period (tRP). The POK1 output can

be used for I/O before core or core before I/O sequenc-

ing, depending on the selected VCC1/VCC2 thresholds.

See the Typical Application Circuit and Figure 1.

Monitoring a Negative Voltage

The power-fail comparator can be used to monitor a

negative supply voltage using the circuit shown in

Figure 4. 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.

Transient Immunity

The MAX6736–MAX6745 supervisors are relatively

immune to short-duration falling VCC transients (glitch-

es). It is usually undesirable to reset the µP when VCC

experiences only small glitches. The Typical Operating

Characteristics show Maximum VCC1/VCC2 Transient

Duration vs. Reset Threshold Overdrive, for which reset

pulses are not generated. The graph shows the maxi-

mum pulse width that a falling VCC transient might typi-

cally have without causing a reset pulse to be issued.

As the amplitude of the transient increases, the maxi-

mum allowable pulse width decreases. A 0.1µF bypass

capacitor mounted close to the VCC pin provides addi-

tional transient immunity.

Chip Information

TRANSISTOR COUNT: 249

PROCESS: BiCMOS

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