MIC2777-XXBM5 MICREL [Micrel Semiconductor], MIC2777-XXBM5 Datasheet - Page 6

• Manufacturer Part Number: MIC2777-XXBM5
• Description: Dual Micro-Power Low Voltage Supervisor
• Manufacturer: MICREL [Micrel Semiconductor]

Application Information
Programming the Voltage Threshold
Referring to the “Typical Application Circuit”, the voltage
threshold on the IN pin is calculated as follows:
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
ues provides two equations that can be solved for the two
unknown resistor values. A value such as 1MΩ for R
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
maximum recommended value of R
Applying this criteria and rearranging the V
solve for the resistor values gives:
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
rail and I/O voltage, V
the MIC2777 is powered by V
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
supply voltage is adequate to insure proper operation, i.e.,
V
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
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.
MIC2777
CORE
CORE
V
where V
R2
R1 = R
TH
≥ (1.0V –5%) = 0.950V. Because there is always a
supply of 1.0V. The goal is to insure that the core
CORE
=
TOTAL
=
(
R
V
TOTAL
REF
TOTAL
REF
, in this example is 1.0V ±5%. The main power
. Imposing this condition on the resistor val-
V
×
TH
= 0.300V
– R2
(
)
R1 R2
(
I/O
V
R2
REF
+
, is 2.5V ±5%. As shown in Figure 1,
)
)
I/O
. The minimum value of V
TOTAL
is 3MΩ.
TH
expression to
IN
REF
). The
TOTAL
) is
I/O
6
To summarize, the various potential error sources are:
Taking the various potential error sources into account, the
threshold voltage will be set slightly below the minimum V
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
be set as follows:
Given that the total tolerance on V
tolerance] + [resistor tolerance]
therefore, solving for V
Solving for R1 and R2 using this value for V
tions above yields:
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
maximum value of I
is a much smaller 5pA) The magnitude of the offset caused
by I
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
offset errors.
IN
• Variation in V
• Resistor tolerance: chosen by designer (typically ≤
• Input bias current, I
. As shown in the “Electrical Characteristics” table, the
IN
±1%)
are known, typically very small
= ±1.5% + ±1% = ±2.5%,
and V
then V
V
R1 = 676.3kΩ ≈ 673kΩ
R2 = 323.7kΩ ≈ 324kΩ
V
V
V
V
is given by:
TH
ERROR
ERROR
ERROR
ERROR
=
TH(max)
V
CORE(min)
CORE(min)
= I
= ±1 × 10
= ±2.189 × 10
= ±2.189mV
1.025
IN(max)
REF
= V
IN
CORE
: specified at ±1.5%
= V
is 10nA. (Note that the typical value
CORE(min)
TH
=
-8
× (R1||R2) =
IN
1.025
A × 2.189 ×10
. The target threshold voltage will
TH
0.950
results in
: calculated once resistor values
-3
+ 2.5% V
V =
= 0.9268V
,
TH
for the IN pin is [V
TH
TOTAL
5
Ω =
= 1.025 V
TH
November 2005
to minimize
and the equa-
TH
,
CORE
REF