LP3982 datasheet(12/23 Pages) TI

12 / 23 page

R3 =

VLT

VREF (GT)

VUT1

R1 =

GT =

VUT = R1 (VREF) GT

VLT = R1 // R2 (VREF) GT

LM385A-1.2V

LMC7215

VREF

2.74M

180k

768k

301k

2.22F

4 Cells

NiMH

SHDN

GND

OUT

LP3982

FAULT

100k

2.22F

CERAMIC

LP3982

SNVS185D – FEBRUARY 2002 – REVISED APRIL 2013

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Figure 21. Minimum Battery Detector that Disconnects the Load Via the SHDN Pin of the LP3982

Resistor value for VUT and VLT are determined as follows:

(8)

(The application of Figure 21 used a GT of 5μ mho)

(9)

(10)

(11)

The above procedure assumes a rail-to-rail output comparator. Essentially, R2 is in parallel with R1 prior to

reaching the lower threshold, then R2 becomes parallel with R3 for the upper threshold. Note that the application

requires rail-to-rail input as well.

The resistor values shown in Figure 21 are the closest practical to calculated values.

Fast Start-Up

The LP3982 provides fast start-up time for better system efficiency. The start-up speed is maintained when using

the optional noise bypass capacitor. An internal 500

μA current source charges the capacitor until it reaches

about 90% of its final value.

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