PTV08040WAD datasheet(13/23 Pages) TI

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Hysterisis Adjust

)

H Y S

0 .3 6 5

H Y S

2 6 .1

H Y S

(2)

Threshold Adjust

0.0027]

/2.5

THD

0.014)(V

HYS

39.2[(1/R

39.2

THD

(3)

Calculated Values

PTV08040W

SLTS257A – SEPTEMBER 2005 – REVISED JANUARY 2007

The hysterisis voltage, VHYS, 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 RHYS and RTHD resistors be in place. Adding RHYS alone does

not have the desired effect. The value for RHYS must first be calculated using Equation 2, and then be used to

determine a value for RTHD, using Equation 3.

Equation 3 determines the value of RTHD required to adjust VTHD 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 RTHD must

also be calculated. (This is irrespective of whether VTHD is being adjusted.) If there has been no adjustment for

the hystersis voltage, the term 1/RHYS in Equation 3, may be assigned the value, 0.

Table 4 shows a matrix of standard resistor values for RHYS and RTHD, for different options of the on-threshold

(VTHD) and hysterisis (VHYS) voltages. For most applications, only the on-threshold voltage should need to be

adjusted. In this case select only a value for RTHD 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 RHYS and RTHD must be selected from

Table 4. This is irrespective of whether the on-threshold voltage is being adjusted.

Table 4. Calculated Values of RHYS and RTHD, for Various Values of VHYS and

VTHD

1 V

VHYS

0.5 V

0.6 V

0.7 V

0.8 V

0.9 V

(default)

VTHD

RHYS

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

RTHD

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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