MIC2777
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
for the IN pin is [V
REF
tolerance] + [resistor tolerance]
= ±1.5% + ±1% = ±2.5%,
and V
TH(max)
= V
CORE(min)
,
Application Information
Programming the Voltage Threshold
Referring to the “Typical Application Circuit”, the voltage
threshold on the IN pin is calculated as follows:
V
TH
= V
REF
×
(
R1+ R2
)
R2
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 =
where V
REF
= 0.300V
(
R
TOTAL
)
(
V
REF
)
V
TH
therefore, solving for V
TH
results in
V
CORE(min)
0.950
= 0.9268V
V
TH
=
=
1.025
1.025
then V
CORE(min)
= V
TH
+ 2.5% V
TH
= 1.025 V
TH
,
R1 = R
TOTAL
– R2
Application Example
Figure 1 illustrates a hypothetical MIC2777-23 application
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, 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 MIC2777 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 MIC2777’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 resis-
tors, variations in the devices’ voltage reference, etc., the
threshold will be set slightly below this value. The potential
variation in the MIC2777’s input voltage reference (V
REF
) 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 contri-
bution 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.
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
-8
A × 2.189 ×10
5
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.
MIC2777
6
November 2005
MICREL [ MICREL SEMICONDUCTOR ]
