Manual Reset

Many microprocessor-based products require manual

reset capability, allowing the operator, a test technician,

or external logic circuitry to initiate a reset while the

monitored supplies remain above their reset thresholds.

These devices have a dedicated active-low MR pin.

When MR is pulled low, RESET asserts a one-shot low

pulse for the MR reset timeout period. The MR input has

an internal 1.5k

unconnected if not used. MR can be driven with CMOS-

logic levels, open-drain/open-collector outputs, or a

momentary pushbutton switch to GND (the MR function

create a manual reset function. 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 (see Figure 4).

Hysteresis

Hysteresis increases the comparator’s noise margin by

increasing the upper threshold or decreasing the lower

threshold. The hysteresis prevents the output from

oscillating (chattering) when monitor input is near the

low-battery threshold. This is especially important for

applications where the load on the battery creates sig-

nificant fluctuations in battery voltages (see Figures 2

and 3).

For the MAX6846/MAX6847, hysteresis is set using three

external resistors (see Figure 5). The MAX6848/MAX6849

have dual, low-battery input levels. Each input level has a

5% (typ) hysteresis.

Applications Information

Resistor-Value Selection (Programming

the Adjustable Thresholds)

MAX6846/MAX6847

Use the following steps to determine values for R1, R2,

and R3 of Figure 5.

R3. Because the MAX6846/MAX6847 have very high

Ω.

upper trip point:

lower trip point:

MAX6848/MAX6849

LBOL low-trip level:

LBOH low-trip level:

Use the following steps to determine values for R1, R2,

and R3 of Figure 5.

VV

RR

R

R

RR

R

R

TRIPHIGH

HTH

TOTAL

=×

++









=+

+

-

12

3

3

12

3

VV

RR

R

RR

TRIPLOW

LTH

=×

++

+









-

12

3

23

RR

R

R

TOTAL

12

3

=

--

R

mV

R

V

R

TOTAL

TRIPLOW

2

615

3

=

×









-

R

mV

R

V

TOTAL

TRIPHIGH

3

615

=

×

VV

mV

VV

RR

R

RR

VV

RR

R

R

RR

R

R

LTH

HTH

TRIPLOW

LTH

TRIPHIGH

HTH

TOTAL

==

=×

++

+









=×

++









=+

+

615

12

3

23

123

3

12

3

Low-Power, Adjustable Battery Monitors with

Hysteresis and Integrated µP Reset

_______________________________________________________________________________________

9

MAX6846

MAX6847

MAX6848

MAX6849

LTHIN

HTHIN

GND

* FOR THE MAX6846/MAX6847.

( ) FOR THE MAX6848/MAX6849.

(LBOH)

(LBOL)

R1

R2

R3

LBO*

Figure 5. Adjustable Threshold Selection