MIC2776_05 Datasheet, PDF (6/8 Pages) Micrel Semiconductor – Micro-Power Low Voltage Supervisor

MIC2776
Application Information
Programming the Voltage Threshold
Referring to the “Typical Application Circuit”, the voltage
threshold is calculated as follows:
V_TH
=
V_REF
×
(R1+ R2)
R2
where V_REF = 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 = R_TOTAL. 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 1MΩ for R_TOTAL
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 (I_IN). The
maximum recommended value of R_TOTAL is 3MΩ.
Applying this criteria and rearranging the V_TH expression to
solve for the resistor values gives:
( )( ) R2 = RTOTAL VREF
V_TH
R1 = R_TOTAL – R2
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_CORE, in
this example is 1.0V ±5%. The main power rail and I/O volt-
age, V_I/O, is 2.5V ±5%. As shown in Figure 1, the MIC2776
is powered by V_I/O. The minimum value of V_I/O is 2.5V –5%
= 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_CORE supply of 1.0V. The goal is to insure that the core supply
voltage is adequate to insure proper operation, i.e., V_CORE
≥ (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 potential 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% 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 MIC2776’s very low input bias current.
Micrel, Inc
To summarize, the various potential error sources are:
• Variation in V_REF: specified at ±1.5%
• Resistor tolerance:
chosen by designer (typically ≤ ±1%)
• Input bias current, I_IN:
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_CORE
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 V_CORE. The target threshold voltage will
be set as follows:
Given that the total tolerance on V_TH is [V_REF tolerance] +
[resistor tolerance]
= ±1.5% + ±1% = ±2.5%,
and V_TH(max) = V_CORE(min),
then V_CORE(min) = V_TH + 2.5% V_TH = 1.025 V_TH,
therefore, solving for V_TH results in
V_TH
=
V_CORE(min)
1.025
=
0.950
1.025
= 0.9268V
Solving for R1 and R2 using this value for V_TH and the equa-
tions 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 cal-
culate the maximum potential error due to input bias current,
I_IN. As shown in the “Electrical Characteristics” table, the
maximum value of I_IN is 10nA. (Note that the typical value
is a much smaller 5pA!) The magnitude of the offset caused
by I_IN is given by:
V_ERROR = I_IN(max) × (R1||R2) =
V_ERROR = ±1 × 10^-8A × 2.189 ×10⁵Ω =
V_ERROR = ±2.189 × 10^-3V =
V_ERROR = ±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_TOTAL to minimize
offset errors.
MIC2776
6
November 2005