January 2006

5

MIC2779

MIC2779

Micrel, Inc.

Applications Information

Programming the Thresholds

The low-voltage threshold is calculated using:

V

V

R1 R2 R3

R2 R3

BAT(lo)

REF

=

+

+

+









The high-voltage threshold is calculated using:

V

V

R1 R2 R3

R3

BAT(hi)

REF

=

+

+









where, for both equations:

V

1.240V

In order to provide the additional criteria needed to solve

for the resistor values, the resistors can be selected such

that they have a given total value, that is, R1 + R2 + R3 =

value because it draws minimum battery current but has no

measurable effect on accuracy.

When working with large resistors, a small amount of leak-

age current can cause voltage offsets that degrade system

accuracy. The maximum recommended total resistance from

HTH

/ RST

VDD

LTH

GND

MIC2779

604k

1%

56k

1%

340k

1%

R1

R2

R3

R ST

Figure 1. Example Circuit

threshold first.

For example, use a total of 1MΩ = R1 + R2 + R3. For a typical

single-cell lithium ion battery, 3.6V is a good “high threshold”

because at 3.6V the battery is moderately charged. Solving

for R3:

V

1.24 1M

R3

Ω









R3 344k

=

Ω

used to determine R2. A single lithium-ion cell should not be

discharged below 2.5V. Many applications limit the drain to

of the two remaining resistor values.

V

3.1V 1.24

1M

R2 344k

=

Ω

+









R2 56k

=

Ω

R1 1M

R2 R3

=

−

−

Ω

R1 600k

=

Ω

The accuracy of the resistors can be chosen based upon the

accuracy required by the system.

Input Transients

The MIC2779 is inherently immune to very short negative-

threshold without tripping the output.

As shown in Figure 2, the narrower the transient, the deeper

the threshold overdrive that will be ignored by the MIC2779.

The graph represents the typical allowable transient dura-

tion for a given amount of threshold overdrive that will not

generate a reset.

0

20

40

60

80

100

120

140

1

10

100

1000

Input Transient

Response

Figure 2. Input Transient Response