Low-Power, Single/Dual-Level Battery Monitors

with Hysteresis

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7

Detailed Description

The MAX6427–MAX6438 family is available with several

monitoring options. The MAX6427/MAX6428/MAX6429

offer factory-trimmed battery-monitor thresholds, with a

single low-battery output (see Figure 1). The

MAX6430/MAX6431/MAX6432 also feature factory-

trimmed thresholds, but have two low-battery outputs

(see Figure 2). The factory-trimmed thresholds of the

MAX6427–MAX6432 eliminate the requirement for

external components.

The user sets the threshold voltages for the

MAX6433–MAX6438, which permit the user to select

the hysteresis range, and consequently the sensitivity

of the system to noise. A wide hysteresis range pre-

vents chattering that can result during battery recovery

or load removal. The MAX6433/MAX6434/ MAX6435

have single low-battery outputs and the MAX6436/

MAX6437/MAX6438 have dual low-battery outputs (see

Figures 3 and 4).

The MAX6427–MAX6438 combine a 615mV reference

with two comparators, logic, and timing circuitry to pro-

vide the user with information about the charge state of

the power-supply batteries. The MAX6427–MAX6438

monitor separate high-voltage and low-voltage thresh-

olds to determine battery status. The output(s) can be

used to signal when the battery is charged, when the

battery is low, and when the battery is empty. Factory-

trimmed thresholds are ideal for monitoring single-cell

Li+ or multicell alkaline/NiCd/NiMH power supplies.

User-programmable versions are also available with

thresholds determined using an external resistor-

divider (see Resistor-Value Selection).The adjustable

versions can monitor down to 0.62V, making them suit-

able for monitoring single-cell alkaline, NiMH, and NiCd

≥ 1.6V).

When the power-supply voltage drops below the speci-

fied low threshold, the low-battery output asserts (see

Low-Battery Output). When the voltage rises above the

specified high threshold following a 140ms (min) time-

out period, the low-battery output is deasserted (see

Threshold Monitoring). This ensures the supply voltage

has stabilized before power-converter or microproces-

sor activity is enabled.

Low-Battery Output

The low-battery outputs are available in active-low

(LBO, LBOH, LBOL, push-pull and open-drain) and

active-high (LBO, LBOL, LBOH, open-drain) configura-

tions. Push-pull outputs are referenced to BATT (for

old versions).

The open-drain devices can be pulled to a voltage inde-

R

S

Q

LBO TIMEOUT

PERIOD

( ) ARE FOR THE MAX6429

615

mV

+

-

Q

LBO

(LBO)

Figure 1. MAX6427/MAX6428/MAX6429 Functional Diagram