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UVLO Adjustment

Adjustment Method

PTH12040W

2

4

6

7

Inhibit

GND

GND

13

5

V

I

C

I

1000 F

m

V

I

R

HYS

R

THD

Hysterisis Adjust

PTH12040W

SLTS237A–DECEMBER 2004–REVISED OCTOBER 2005

The UVLO feature of the PTH12040W module allows for limited adjustment of both the on threshold and

hysterisis voltages. The adjustment is made via the UVLO Prog control pin. When the UVLO Prog pin is left open

circuit, the on threshold and hysterisis voltages are internally set to their default values. The on threshold has a

nominal voltage of 7.5 V, and the hysterisis 1 V. This ensures that the module produces a regulated output when

the minimum input voltage is applied (see specifications). The combination correlates to an off threshold of

approximately 6.5 V. The adjustments are limited. The on threshold can only be adjusted higher, and the

hysterisis voltage can only be reduced in magnitude.

The on threshold might need to be raised if the module is powered from a tightly regulated 12-V bus. This would

prevent it from operating if the input bus failed to completely rise to its specified regulation voltage. The hysterisis

should not be changed unless absolutely necessary. A generous amount of hysterisis ensures that the module

exhibits a clean startup. Therefore, adjustment of the hysterisis should only be considered if there is a system

requirement to specifically set the off threshold voltage (in addition to the on threshold). Depending on the load

regulation of the input source, the hysterisis should not be adjusted below 0.5 V without careful consideration.

The recommended adjustment method requires that any change to the hysterisis be determined first. If the

Figure 8. UVLO Program Resistor Placement

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 induce

start-up oscillations.

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