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MIC2774_15 Datasheet, PDF (7/11 Pages) Micrel Semiconductor – Dual Micro-Power Low Voltage Supervisor
◁
Micrel, Inc.
Application Information
Programming the Voltage Threshold
Referring to the Typical Application circuit, the voltage
threshold on the IN pin is calculated as follows:
VIH
=
VREF
×
(𝑅1+𝑅2)
𝑅2
where VREF = 0.300V
In order to provide the additional criteria needed to solve
for the resistor values, the resistorscan 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 1MΩ 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 3MΩ.
Applying this criteria and rearranging the VIH expression
to solve for the resistor values gives:
R2 = (𝑅𝑇𝑇𝑇𝑇𝑇)(𝑉𝑅𝑅𝑅)
𝑉𝐼𝐼
R1 = RTOTAL – R2
Application Example
Figure 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 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
MIC2774
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
VTH
=
𝑉𝐶(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 = ±1×10-8A × 2.189×105Ω =
VERROR = ±2.189×10-3V =
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
March 16, 2015
7
Revision 2.0
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