MIC841/2

Micrel

MIC841/2

6

January 2005

Once the desired trip points are determined, set the V

IN(HI)

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

3.6V

1.24

1M

R3

Ω













R3

344k

=

Ω

Once R3 is determined, the equation for V

to determine R2. A single lithium-ion cell, for example, should

not be discharged below 2.5V. Many applications limit the

drain to 3.1V. Using 3.1V for the V

calculation of the two remaining resistor values.

V

3.1V

1.24

1M

R2

344k

Ω

+













R2

56k

=

Ω

1M

R2 R3

R1

Ω−

−

R1 600k

=

Ω

The accuracy of the resistors can be chosen based upon the

accuracy required by the system.

Programming the MIC842 Thresholds

The voltage threshold is calculated using:

VV

R1 R2

R2

IN(LO)

REF

=

+













where:

V

1.240V

INP

OUT

VDD

GND

MIC842N

470k

R1

R2

4

2

1

5

Figure 2. MIC842 Example Circuit

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 = R

it draws minimum current but has no significant effect on

accuracy.

Applications Information

Output

The MIC841N and MIC842N outputs are an open-drain

MOSFET, so most applications will require a pull-up resistor.

The value of the resistor should not be too large or leakage

effects may dominate. 470kΩ is the maximum recommended

value. Note that the output of “N” version may be pulled up as

high as 6V regardless of the ICs supply voltage. The “H” and

“L” versions of the MIC841 and MIC842 have a push-pull

output stage, with a diode clamped to V

mum output voltage of the “H” and “L” versions is V

“Electrical Characteristics.”

When working with large resistors on the input to the devices,

a small amount of leakage current can cause voltage offsets

that degrade system accuracy. The maximum recommended

total resistance from V

the resistors can be chosen based upon the accuracy re-

quired by the system. The inputs may be subjected to

voltages as high as 6V steady-state without adverse effects

of any kind regardless of the ICs supply voltage. This applies

even if the supply voltage is zero. This permits the situation

in which the IC’s supply is turned off, but voltage is still present

on the inputs. See “Electrical Characteristics.”

Programming the MIC841 Thresholds

The low-voltage threshold is calculated using:

VV

R1 R2 R3

R2 R3

IN(LO)

REF

=

++

+













The high-voltage threshold is calculated using:

VV

R1 R2 R3

R3

IN(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 = R

because it draws minimum current but has no significant

effect on accuracy.

HTH

OUT

VDD

LTH

GND

MIC841N

604k

1%

56k

1%

340k

1%

470k

R1

R2

R3

5

3

1

4

2

Figure 1. MIC841 Example Circuit