MIC2774_05 Datasheet, PDF(6/8 Page) Micrel Semiconductor – Dual Micro-Power Low Voltage Supervisor

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MIC2774_05 Datasheet, PDF (6/8 Pages) Micrel Semiconductor – Dual Micro-Power Low Voltage Supervisor

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MIC2774

Application Information

Programming the Voltage Threshold

Referring to the âTypical Application Circuitâ, the voltage

threshold on the IN pin is calculated as follows:

VTH = VREF

(R1 + R2 )

where VREF = 0.300V

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 = RTOTAL. Imposing this condition on the resistor val-

ues provides two equations that can be solved for the two

unknown resistor values. A value such as 1MÎ© for RTOTAL

is a 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 (IIN). The

maximum recommended value of RTOTAL is 3MÎ©.

Applying this criteria and rearranging the VTH expression to

solve for the resistor values gives:

( )( ) R2 = R TOTAL VREF

VTH

R1 = R TOTAL â R2

Application Example

Figure 1 below illustrates a hypothetical MIC2774L-23 ap-

plication in which the MIC2774L-23 is used to monitor the

core and I/O supplies of a high-performance CPU or DSP.

The core supply, VCORE, in the example is 1.0V Â±5%. The

main power rail and I/O voltage, VI/O, is 2.5V Â±5%. As shown

in Figure 1, the MIC2774 is powered by VI/O. The minimum

value of VI/O is 2.5V â5% = 2.375V; the maximum is 2.5V

+5% = 2.625V. This is well within the deviceâs supply range

of 1.5V to 5.5V.

Resistors R1 and R2 must be selected to correspond to the

VCORE supply of 1.0V. The goal is to insure that the core

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

VCORE â¥ (1.0V â5%) = 0.950V. Because there is always

a small degree of uncertainty due to the accuracy of the

resistors, variations in the devicesâ voltage reference, etc.,

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

tential variation in the MIC2774âs voltage reference (VREF)

is speciï¬ed as Â±1.5%. The resistors chosen will have their

own tolerance speciï¬cation. This example will assume the

use of 1% accurate resistors. The potential worst-case er-

ror 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 contribution will be very small thanks to the MIC2774âs

very low input bias current.

Micrel

To summarize, the various potential error sources are:

â¢ Variation in VREF: speciï¬ed at Â±1.5%

â¢ Resistor tolerance:

chosen by designer (typically â¤ Â±1%)

â¢ Input bias current, IIN:

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 VCORE

speciï¬cation of 0.950V so that when the actual threshold

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

operating range of VCORE. The target threshold voltage will

be set as follows:

Given that the total tolerance on VTH for the IN pin is [VREF

tolerance] + [resistor tolerance]

= Â±1.5% + Â±1% = Â±2.5%,

and VTH(max) = VCORE(min),

then VCORE(min) = VTH + 2.5% VTH = 1.025 VTH,

therefore, solving for VTH results in

VTH

VCORE(min)

1.025

= 0.950 = 0.9268V

1.025

Solving for R1 and R2 using this value for VTH and the equa-

tions above yields:

The resulting circuit is shown in Figure 1.

Input Bias Current Effects

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

culate the maximum potential error due to input bias current,

IIN. As shown in the âElectrical Characteristicsâ table, the

maximum value of IIN is 10nA. (Note that the typical value

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

by IIN is given by:

VERROR = IIN(max) Ã (R1 || R2 ) =

VERROR = Â±1Ã10 â8 A Ã 2.189 Ã10 5 â¦ =

VERROR = Â±2.189 Ã10 â3 V =

VERROR = Â±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 RTOTAL to minimize

offset errors.

M9999-102605

October 2005

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