Datasheet

LT1719
13
1719fa
–
+
LT1719S8
1719 F05
R2ʹ
V
REF
V
TH
R3
+V
S
2
V
AVERAGE
=
R1
Figure 5. Model for Additional Hysteresis Calculations
With this in mind, calculation of the resistor values needed
is a two-step process. First, calculate the value of R3 based
on the additional hysteresis desired, the output voltage
swing and the impedance of the primary bias string:
R3 = (R1
⏐⏐ R2)(+V
S
– 0.6V)/(additional hysteresis)
Additional hysteresis is the desired overall hysteresis less
the internal 4mV hysteresis.
The second step is to recalculate R2 to set the same av-
erage threshold as before. The average threshold before
was set at V
TH
= (V
REF
)(R1)/(R1 + R2). The new R2 is
calculated based on the average output voltage (+V
S
/2)
and the simplifi ed circuit model in Figure 5. To assure
that the comparator’s noninverting input is, on average,
the same V
TH
as before:
R2ʹ = (V
REF
– V
TH
)/(V
TH
/R1 + [V
TH
– (+V
S
)/2]/R3)
For additional hysteresis of 10mV or less, it is not un-
common for R2ʹ to be the same as R2 within 1% resistor
tolerances.
This method will work for additional hysteresis of up to a
few hundred millivolts. Beyond that, the impedance of R3 is
low enough to effect the bias string, and adjustment of R1
may also be required. Note that the currents through the
R1/R2 bias string should be many times the input currents
of the LT1719. For 5% accuracy, the current must be at
least 20 times the input current, more for higher accuracy.
This illustration used an LT1719S8; with an LT1719S6 the
same procedure is used with V
+
substituted for +V
S
.
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