MIC2777-17BM5(2000) データシートの表示（PDF） - Micrel

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MIC2777-17BM5
(Rev.:2000)

Dual Micro-Power Low Voltage Supervisor

Micrel 

MIC2777

Application Information

Programming the Voltage Threshold

Referring to the “Typical Application Circuit”, the voltage

threshold on the IN pin is calculated as follows:

VTH

=

VREF

×

(R1+ R2)

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 values pro-

vides two equations that can be solved for the two unknown

resistor values. A value such as 1MΩ 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 3MΩ.

Applying this criteria and rearranging the VTH expression to

solve for the resistor values gives:

( )( ) R2 = RTOTAL VREF

VTH

R1= RTOTAL − R2

Application Example

Figure 1 below illustrates a hypothetical MIC2777-23 applica-

tion in which the MIC2777-23 is used to monitor the core and

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

supply, VCORE, in this example is 1.0V ±5%. The main power

rail and I/O voltage, VI/O, is 2.5V ±5%. As shown in Figure 1,

the MIC2777 is powered by VI/O. The minimum value of VI/O

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

This is well within the MIC2777’s power 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 ≥ (1.0V –5%) = 0.950V. Because there is always a

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 MIC2777’s input voltage reference (VREF)

is specified as ±1.5%. The resistors chosen will have their

own tolerance specification. This example will assume the

use of 1% accurate 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 contribution will be very small thanks to the MIC2777’s

very low input bias current.

To summarize, the various potential error sources are:

Micrel

• Variation in VREF: specified at ±1.5%

• Resistor tolerance:

chosen by designer (typically ≤ ±1%)

• 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 specification of 0.950V so that when the actual thresh-

old 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]

= ±1.5% + ±1% = ±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

1.025

=

0.9268V

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

equations above yields:

R1 = 676.3kΩ ≈ 673kΩ

R2 = 323.7kΩ ≈ 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, IIN. As shown in the “Electrical Characteristics” 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) × (R1|| R2) =

VERROR = ±1×10−8 A × 2.189 ×105 Ω =

VERROR = ±2.189 ×10−3 V =

VERROR = ±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.

MIC2777

6

September 29, 2000

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