Low-Power, 1%-Accurate Battery

Monitors in µDFN and SC70 Packages

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The MAX6779, MAX6780, and MAX6781 monitor two

battery levels or two independent voltages. A common

application for this type of dual-battery monitor is to use

one output as an early warning signal and the other as

a dead-battery indicator.

Hysteresis

Input hysteresis defines two thresholds, separated by a

small voltage (the hysteresis voltage), configured so

the output asserts when the input falls below the falling

threshold, and deasserts only when the input rises above

the rising threshold. Figure 4 shows this graphically.

Hysteresis removes, or greatly reduces, the possibility of

the output changing state in response to noise or battery

terminal voltage recovery after load removal.

Fixed Hysteresis

The MAX6775/MAX6776/MAX6779/MAX6780/MAX6781

have factory-set hysteresis for ease of use, and reduce

component count. For these devices, the absolute hys-

teresis voltage is a percentage of the internally generat-

ed reference. The amount depends on the device

option. “A” devices have 0.5% hysteresis, “B” devices

have 5% hysteresis, and “C” devices have 10% hystere-

sis. Table 2 presents the threshold voltages for devices

with internally fixed hysteresis.

Adjustable Hysteresis

The MAX6777/MAX6778 offer external hysteresis con-

trol through the resistive divider that monitors battery

voltage. Figure 3 shows the connections for external

hysteresis. See the

Calculating an External Hysteresis

Resistive Divider section for more information.

Applications Information

Resistor-Value Selection

Choosing the proper external resistors is a balance

between accuracy and power use. The input to the volt-

age monitor, while high impedance, draws a small cur-

rent, and that current travels through the resistive

divider, introducing error. If extremely high resistor val-

ues are used, this current introduces significant error.

With extremely low resistor values, the error becomes

negligible, but the resistive divider draws more power

from the battery than necessary and shortens battery

case LBI leakage current. For example, with 0.5%

accuracy, a 2.8V battery minimum, and 5nA leakage,

R

Vx R

VV

L

LBIF

H

LBIF

BATT

−

R

ex V

I

H

A

BATT

L

=

LBO

MAX6775

MAX6776

Figure 4. Hysteresis

Table 2. Typical Falling and Rising Thresholds for

MAX6775/MAX6776/MAX6779/MAX6780/MAX6781

DEVICE OPTION

PERCENT

HYSTERESIS (%)

FALLING THRESHOLD

RISING THRESHOLD

HYSTERESIS VOLTAGE

A

0.5

1.2159

1.222

6.11

B

5

1.1609

1.222

61.1

C

10

1.0998

1.222

122