NCP346
Design Steps for Adjusting the Overvoltage Threshold
1. Use Typical R
in
, and V
th
Values from the Electrical
Specifications
2. Minimize R
in
Effect by Selecting R
1
<< R
in
since:
VOV
+
Vth(1
)
R1 R2
)
R1 Rin).
(eq. 6)
3. Let X = R
in
/ R
1
= 100.
4. Identify Required Nominal Overvoltage Threshold.
5. Calculate nominal R
1
and R
2
from Nominal Values:
R1
+
Rin X
(eq. 7)
R1
R2
+
(VOV Vth
*
R1 Rin
*
1)
(eq. 8)
The specification takes into account the hysteresis of the
comparator, so the minimum input threshold voltage (V
th
)
is the falling voltage detection point and the maximum is the
rising voltage detection point. One should design the input
supply such that its maximum supply voltage in normal
operation is less than the minimum desired overvoltage
threshold.
8. Use worst case resistor tolerances to determine the
maximum V(V
CC
,IN)
V(VCC, IN) min
+
VCCmax * (R1min (R1min
)
R2max))
(eq. 12)
V(VCC, IN)typ
+
VCCmax * (R1typ (R1typ
)
R2typ)) (eq. 13)
V(VCC, IN) max
+
VCCmax * (R1max (R1max
)
R2min))
(eq. 14)
6. Pick Standard Resistor Values as Close as Possible to
these Values
7. Use min/max Data and Resistor Tolerances to
Determine Overvoltage Detection Tolerance:
VOVmin
+
Vthmin(1
)
R1min R2max
)
R1min Rinmax)
(eq. 9)
VOVtyp
+
Vthtyp(1
)
R1typ R2typ
)
R1typ Rintyp)
(eq. 10)
VOVmax
+
Vthmax(1
)
R1min R2max
)
R1max Rinmin)
(eq. 11)
This is shown empirically in Tables 2 through 4.
The following tables show an example of obtaining a 6 V
detection voltage from the NCP346SN2T2, which has a
typical V
th
of 5.5 V.
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