LTC2966
10
2966fb
For more information www.linear.com/LTC2966
APPLICATIONS INFORMATION
Threshold Configuration
Each LTC2966 channel (A/B) monitors the voltage applied
to the corresponding V
IN
input. A comparator senses the
V
IN
pin on one of its inputs through the internal resistive
divider. The other input is connected to INH/INL that is
in turn biased with external resistive dividers off of the
REF pin as shown in Figure 2a and 2b. The V
IN
rising and
falling thresholds are determined by:
V
IN(RISE)
= RANGE • V
INH
V
IN(FALL)
= RANGE • V
INL
Where RANGE is the configured range of the internal
resistive divider. In order to set the threshold for the
LTC2966, choose an appropriate range setting for the
desired V
IN
voltage threshold such that the INH and INL
voltages are within the specified common mode range,
V
CM
. For example, if a falling threshold of 18V is desired
for monitoring a 24V power supply then a range greater
than 10x is allowed. However, to maximize the accuracy
of the V
IN
threshold the smallest acceptable range is used,
10x in this case. To implement 2V of hysteresis referred
to V
IN
this means:
V
INH
= 2V, V
INL
= 1.8V
With 10x range the V
IN
thresholds are:
V
IN(RISE)
= 20V, V
IN(FALL)
= 18V
One possible way to configure the thresholds is by us-
ing three resistors to set the voltages on INH and INL.
See Figure
2a.
The solution for R1, R2 and R3 provides
three equations and three unknowns. Maximum resistor
size is governed by maximum input leakage current. The
maximum input leakage current below 85°C is 1nA. For
a maximum error of 1% due to both input currents, the
resistive divider current should be at least 100 times the
sum of the leakage currents, or 0.2µA.
If in this example, a leakage current error of 0.1% is desired
then the total divider resistance is 1.2MΩ which results in
a current of 2µA through this network. For R
SUM
= 1.2MΩ
SUM
=
+
+
R1=
V
INL
•R
SUM
( )
V
REF
=
1.8V •1.2MΩ
( )
2.402V
= 899.5kΩ
The closest 1% value is 909kΩ. R2 can be determined from:
R2=
V
INH
•R
SUM
V
REF
–R1
=
2V •1.2MΩ
( )
–909kΩ = 90.2kΩ
The closest 1% value is 90.9kΩ. R3 can be determined
from R
SUM
:
R3 = R
SUM
– R1 – R2 = 1.2MΩ – 909kΩ – 90.9kΩ
= 200.1kΩ
The closest 1% value is 200kΩ. Plugging the standard
values back into the equations yields the design values
for the V
INH
and V
INL
voltages:
V
INH
= 2.001V, V
INL
= 1.819V
The corresponding threshold voltages are:
V
IN(RISE)
= 20.01V, V
IN(FALL)
= 18.19V
Another possible way to configure the thresholds is with
independent dividers using two resistors per threshold to
set the voltages on INH and INL. See Figure 2b. Care must
be taken such that the thresholds are not set too close to
each other, otherwise the mismatch of the resistors may
cause the voltage at INL to be greater than the voltage at
INH which may cause the comparator to oscillate.
As in the previous example, if R
SUM
= 1.2MΩ is chosen
and the target for V
INL
is 1.8V:
SUM
=
+
R1=
V
INL
•R
SUM
( )
V
REF
=
1.8V •1.2MΩ
( )
2.402V
= 899.5kΩ
The closest 1% value is 909kΩ. R2 can be determined by:
R2= V
REF
– V
INL
( )
•
V
INL
= 2.402V –1.8V
( )
•
909kΩ
( )
= 304kΩ
