MAX6848 데이터 시트보기 (PDF) - Maxim Integrated

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MAX6848

Low-Power, Adjustable Battery Monitors with Hysteresis and Integrated µP Reset

Maxim Integrated 

Low-Power, Adjustable Battery Monitors with

Hysteresis and Integrated µP Reset

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 1.5kΩ pullup resistor to VCC and can be left

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

is internally debounced for the tDEB timeout period) to

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

VDD

VDD

LBO*

R1

MAX6846

MAX6847

MAX6848

LTHIN MAX6849

(LBOH)

R2

(LBOL)

HTHIN

GND

R3

* FOR THE MAX6846/MAX6847.

( ) FOR THE MAX6848/MAX6849.

Figure 5. Adjustable Threshold Selection

2) Calculate R3 based on RTOTAL and the desired

upper trip point:

R3 = 615mV × RTOTAL

VTRIPHIGH

3) Calculate R2 based on RTOTAL, R3, and the desired

lower trip point:

R2

=





615mV × RTOTAL

VTRIPLOW





-

R3

4) Calculate R1 based on RTOTAL, R3, and R2:

R1 = RTOTAL - R2 - R3

VLTH = VHTH = 615mV

VTRIPLOW

=

VLTH

×

 R1 + R2 + R3

 R2 + R3 

VTRIPHIGH

=

VHTH

×

 R1 + R2 + R3



R3



RTOTAL = R1 + R2 + R3

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

and R3 of Figure 5.

1) Choose a value for RTOTAL, the sum of R1, R2, and

R3. Because the MAX6846/MAX6847 have very high

input impedance, RTOTAL can be up to 500kΩ.

VLTH- = VHTH- = 582mV

LBOL low-trip level:

MAX6848/MAX6849

VTRIPLOW

=

VLTH-

×

 R1 + R2 + R3

 R2 + R3 

LBOH low-trip level:

VTRIPHIGH

=

VHTH-

×

 R1 + R2 + R3

 R3 

RTOTAL = R1 + R2 + R3

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

and R3 of Figure 5.

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