MIC2776 Micrel Semiconductor, MIC2776 Datasheet - Page 6

• Manufacturer Part Number: MIC2776
• Description: Micro-Power Low Voltage Supervisor
• Manufacturer: Micrel Semiconductor

MIC2776
Application Information
Programming the Voltage Threshold
Referring to the “Typical Application Circuit”, the voltage
threshold 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
vides two equations that can be solved for the two unknown
resistor values. A value such as 1MΩ for R
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 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
this example is 1.0V ±5%. The main power rail and I/O
voltage, V
is powered by V
= 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
supply voltage is adequate to insure proper operation, i.e.,
V
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.
MIC2776
CORE
CORE
TOTAL
where V
R2
R1 R
V
≥ (1.0V –5%) = 0.950V. Because there is always a
TH
. Imposing this condition on the resistor values pro-
supply of 1.0V. The goal is to insure that the core
=
=
I/O
=
(
R
, is 2.5V ±5%. As shown in Figure 1, the MIC2776
V
TOTAL
REF
TOTAL
REF
I/O
V
×
. The minimum value of V
TH
= 0.300V
−
(
) (
R1 R2
R2
V
REF
R2
+
)
)
TOTAL
is 3MΩ.
TH
I/O
expression to
is 2.5V –5%
TOTAL
CORE
IN
). The
is a
, in
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
old 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
[resistor tolerance]
therefore, solving for V
Solving for R1 and R2 using this value for V
equations 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
calculate the maximum potential error due to input bias
current, I
the 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.
CORE
• Variation in V
• Resistor tolerance:
• Input bias current, I
IN
chosen by designer (typically ≤ ±1%)
calculated once resistor values are known, typically
very small
= ±1.5% + ±1% = ±2.5%,
and V
then V
R1 = 676.3kΩ ≈ 673kΩ
R2 = 323.7kΩ ≈ 324kΩ
V
is given by:
V
V
V
V
specification of 0.950V so that when the actual thresh-
ERROR
ERROR
ERROR
ERROR
TH
IN
=
. As shown in the “Electrical Characteristics” table,
TH(max)
CORE(min)
V
CORE(min)
=
= ± ×
= ±
= ±
1.025
I
IN(max)
REF
1 10 A 2.189 10
2.189 10 V =
2.189mV
= V
CORE
: specified at ±1.5%
= V
IN
CORE(min)
TH
−8
IN
×
×
=
is 10nA. (Note that the typical value
:
. The target threshold voltage will
(
TH
results in
1.025
0.950
R1|| R2
×
−
+ 2.5% V
3
,
= 0.9268V
)
TH
=
×
is [V
TH
TOTAL
September 29, 2000
5
Ω
= 1.025 V
=
REF
to minimize
tolerance] +
TH
TH
and the
Micrel
,