MIC2776 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 values pro-
vides 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
voltage, 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 poten-
tial 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% accu-
rate 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 contri-
bution will be very small thanks to the MIC2776’s very low
input bias current.
Micrel
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 thresh-
old 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
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, 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^−8 A × 2.189 ×10⁵ Ω =
V_ERROR = ±2.189 ×10^−3 V =
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
September 29, 2000