MIC2776

Micrel

MIC2776

6

September 29, 2000

Application Information

Programming the Voltage Threshold

Referring to the “Typical Application Circuit”, the voltage

threshold is calculated as follows:

VV

R1 R2

R2

TH

REF

=×

+

()

where V

In order to provide the additional criteria needed to solve for

the resistor values, the resistors can be selected such that the

two resistors have a given total value, that is, R1 + R2 =

R

vides two equations that can be solved for the two unknown

resistor values. A value such as 1M

Ω for R

reasonable choice since it keeps quiescent current to a

generally acceptable level while not causing any measurable

errors due to input bias currents. The larger the resistors, the

larger the potential errors due to input bias current (I

maximum recommended value of R

Applying this criteria and rearranging the V

solve for the resistor values gives:

R2

RV

V

TOTAL

REF

TH

R1

R

R2

TOTAL

=−

Application Example

Figure 1 below illustrates a hypothetical MIC2776 application

in which the MIC2776 is used to monitor the core supply of a

high-performance CPU or DSP. The core supply, V

this example is 1.0V

±5%. The main power rail and I/O

voltage, V

is powered by V

= 2.375V; the maximum is 2.5V +5% = 2.625V. This is well

within the MIC2776’s power supply range of 1.5V to 5.5V.

Resistors R1 and R2 must be selected to correspond to the

V

supply voltage is adequate to insure proper operation, i.e.,

V

small degree of uncertainty due to the accuracy of the

resistors, variations in the devices’ voltage reference, etc.,

the threshold will be set slightly below this value. The poten-

tial variation in the MIC2776’s voltage reference is specified

as

±1.5%. The resistors chosen will have their own tolerance

specification. This example will assume the use of 1% accu-

rate resistors. The potential worst-case error contribution due

to input bias current can be calculated once the resistor

values are chosen. If the guidelines above regarding the

maximum total value of R1+R2 are followed, this error contri-

bution will be very small thanks to the MIC2776’s very low

input bias current.

To summarize, the various potential error sources are:

• Variation in V

specified at

±1.5%

• Resistor tolerance:

chosen by designer (typically

≤ ±1%)

• Input bias current, I

calculated once resistor values are known, typically

very small

Taking the various potential error sources into account, the

threshold voltage will be set slightly below the minimum

V

old voltage is at its maximum, it will not intrude into the normal

operating range of V

be set as follows:

Given that the total tolerance on V

[resistor tolerance]

=

±1.5% + ±1% = ±2.5%,

and V

then V

therefore, solving for V

V=

V

1.025

=

0.950

1.025

= 0.9268V

TH

CORE(min)

Solving for R1 and R2 using this value for V

equations above yields:

R1 = 676.3k

Ω ≈ 673kΩ

R2 = 323.7k

Ω ≈ 324kΩ

The resulting circuit is shown in Figure 1.

Input Bias Current Effects

Now that the resistor values are known, it is possible to

calculate the maximum potential error due to input bias

current, I

the maximum value of I

is a much smaller 5pA!) The magnitude of the offset caused

by I

V

I

R1|| R2

ERROR

IN(max)

=×

V

1 10

A

2.189 10

=

ERROR

5

=± ×

×

×

−8

Ω

V

2.189 10

V =

ERROR

3

=±

V

2.189mV

The typical error is about three orders of magnitude lower

than this - close to one

microvolt! Generally, the error due

to input bias can be discounted. If it is to be taken into

account, simply adjust the target threshold voltage

downward by this amount and recalculate R1 and R2. The

resulting value will be very close to optimum. If accuracy

is more important than the quiescent current in the

resistors, simply reduce the value of R

offset errors.