MIC2774 datasheet(7/18 Pages) MICROCHIP

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2022 Microchip Technology Inc.

DS20006527A-page 7

MIC2774

4.0

APPLICATION INFORMATION

4.1

Programming the Voltage

Threshold

Referring to the Typical Application Circuit, the voltage

threshold on the IN pin is calculated as follows:

EQUATION 4-1:

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 provides two equations

that can be solved for the two unknown resistor values.

A value such as 1 MΩ for RTOTAL is a reasonable

choice because 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 3 MΩ.

Applying this criteria and rearranging the VIH

expression to solve for the resistor values gives:

EQUATION 4-2:

4.2

Application Example

Figure 4-1 illustrates a hypothetical MIC2774L-23

application 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 ±5%. The main power

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

Figure 4-1, the MIC2774 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

device’s 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 ensure that the

core supply voltage is adequate to ensure 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 device’s

voltage reference, etc., the threshold will be set slightly

below this value. The potential variation in the

MIC2774’s voltage reference (VREF) is specified as

±1.5%. The resistors chosen will have their own

tolerance specifications. This example assumes 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 MIC2774’s very low input bias

current.

To summarize, the various potential error sources are:

• 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 slight below the

minimum VCORE specification 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]

= ±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

EQUATION 4-3:

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

equations above yields:

R1 = 676.3 kΩ ≈ 673 kΩ

R2 = 323.7 kΩ ≈ 324 kΩ

The resulting circuit is shown in Figure 4-1.

VIH VREF

R1 R2

--------------------



Where:

VREF = 0.300V

RTOTAL VREF



VIH

--------------------------------------

RTOTAL R2

–

VTH

VCORE MIN





1.025

------------------------------

0.950

1.025

-------------

0.9268V

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