MAX16033 View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX16033

Low-Power Battery Backup Circuits in Small µDFN Packages

Maxim Integrated 

Low-Power Battery Backup

Circuits in Small µDFN Packages

START

SET

WDI

LOW

SUBROUTINE

OR PROGRAM LOOP

SET

WDI HIGH

RETURN

END

VCC

VCC

RESET

TO µP

V+

R1

R2

MAX16033–

MAX16040

MR

PFI

PFO

GND

Figure 6. Watchdog Flow Diagram

Figure 7. Monitoring an Additional Power Supply

Watchdog Software Considerations

One way to help the watchdog timer to monitor soft-

ware execution more closely is to set and reset the

watchdog at different points in the program, rather than

pulsing the watchdog input periodically. Figure 6

shows a flow diagram where the I/O driving the watch-

dog is set low in the beginning of the program, set high

at the beginning of every subroutine or loop, and set

low again when the program returns to the beginning. If

the program should hang in any subroutine, the watch-

dog would timeout and reset the µP.

Replacing the Backup Battery

Decouple BATT to GND with a 0.1µF capacitor. The

backup power source may be removed while VCC

remains valid without the danger of triggering a reset

pulse. The device does not enter battery-backup mode

when VCC stays above the reset threshold voltage.

Power-Fail Comparator

Monitoring an Additional Power Supply

Monitor another voltage by connecting a resistive divider

to PFI as shown in Figure 7. The threshold voltage is:

VTH(PFI) = 1.235 (R1 / R2 + 1)

where VTH(PFI) is the threshold at which the monitored

voltage will trip PFO.

To simplify the resistor selection, choose a value for R2

and calculate R1.

R1 = R2 [(VTH(PFI) / 1.235) - 1]

Connect PFO to MR in applications that require RESET to

assert when the second voltage falls below its threshold.

RESET remains asserted as long as PFO holds MR low,

and for 140ms (min) after PFO goes high.

Adding Hysteresis to the Power-Fail Comparator

The power-fail comparator provides a typical hysteresis

of 12mV, which is sufficient for most applications where

a power-supply line is being monitored through an

external voltage-divider. Connect a voltage-divider

between PFI and PFO as shown in Figure 8a to provide

additional noise immunity. Select the ratio of R1 and R2

such that VPFI falls to 1.235V when VIN drops to its trip

point, VTRIP. R3 adds hysteresis and is typically more

than 10 times the value of R1 or R2. The hysteresis win-

dow extends above (VH) and below (VL) the original trip

point, VTRIP. Connecting an ordinary signal diode in

series with R3 as shown in Figure 8b causes the lower

trip point (VL) to coincide with the trip point without hys-

teresis (VTRIP). This method provides additional noise

margin without compromising the accuracy of the

power-fail threshold when the monitored voltage is

falling. Set the current through R1 and R2 to be at least

10µA to ensure that the 100nA (max) PFI input current

does not shift the trip point. Set R3 to be higher than

10kΩ to reduce the load at PFO. Capacitor C1 adds

additional noise rejection.

______________________________________________________________________________________ 13

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