Datasheet

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Hysterisis Adjust
kΩ
)
H Y S
V(10.365
H Y S
V26.1
H Y S
R
-´
´
=
(2)
Threshold Adjust
kΩ
10.0027]1)/2.5
THD
0.014)(V
HYS
39.2[(1/R
39.2
THD
R
---+
=
(3)
Calculated Values
PTV08040W
SLTS257B – SEPTEMBER 2005 – REVISED JANUARY 2008
The hysterisis voltage, V
HYS,
is the difference between the ON and OFF threshold values. The default value is 1
V and it can only be adjusted to a lower value.
Caution should be used when changing the hysterisis voltage to a lower value, as it could permit start-up
oscillations.
Any change in the hysterisis voltage requires both R
HYS
and R
THD
resistors be in place. Adding R
HYS
alone does
not have the desired effect. The value for R
HYS
must first be calculated using Equation 2 , and then be used to
determine a value for R
THD
, using Equation 3 .
Equation 3 determines the value of R
THD
required to adjust V
THD
to a new value. The default value is 7.5 V, and it
may only be adjusted to a higher value. If the hysterisis value has been adjusted, then a value for R
THD
must also
be calculated. (This is irrespective of whether V
THD
is being adjusted.) If there has been no adjustment for the
hystersis voltage, the term 1/R
HYS
in Equation 3 , may be assigned the value, 0.
Table 4 shows a matrix of standard resistor values for R
HYS
and R
THD
, for different options of the on-threshold
(V
THD
) and hysterisis (V
HYS
) voltages. For most applications, only the on-threshold voltage should need to be
adjusted. In this case select only a value for R
THD
from far right-hand column.
The hysterisis should only be adjusted if there is a specific requirement to independently adjust the off-threshold,
separately from the on-threshold voltage. In this case, a value for both R
HYS
and R
THD
must be selected from
Table 4 . This is irrespective of whether the on-threshold voltage is being adjusted.
Table 4. Calculated Values of R
HYS
and R
THD
, for Various Values of V
HYS
and
V
THD
1 V
V
HYS
0.5 V 0.6 V 0.7 V 0.8 V 0.9 V
(default)
V
THD
R
HYS
71.5 k Ω 107 k Ω 165 k Ω 287 k Ω 649 k Ω N/A
8 V 30.1 k Ω 43.2 k Ω 63.4 k Ω 97.6 k Ω 169 k Ω 402 k Ω
8.5 V 25.5 k Ω 36.5 k Ω 51.1 k Ω 73.2 k Ω 110 k Ω 187 k Ω
9 V 23.2 k Ω 30.9 k Ω 42.2 k Ω 57.6 k Ω 82.5 k Ω 124 k Ω
9.5 V 20 k Ω 27.4 k Ω 36.5 k Ω 48.7 k Ω 64.9 k Ω 90.9 k Ω
10 V R
THD
18.2 k Ω 24.3 k Ω 31.6 k Ω 41.2 k Ω 54.9 k Ω 73.2 k Ω
10.5 V 16.2 k Ω 21.5 k Ω 28 k Ω 36.5 k Ω 46.4 k Ω 60.4 k Ω
11 V 15 k Ω 19.6 k Ω 25.5 k Ω 32.4 k Ω 41.2 k Ω 52.3 k Ω
11.5 V 14 k Ω 18.2 k Ω 23.2 k Ω 28 k Ω 36.5 k Ω 45.3 k Ω
12 V 12.7 k Ω 16.5 k Ω 21 k Ω 26.1 k Ω 32.4 k Ω 40.2 k Ω
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