Adjusting the Undervoltage Lockout (UVLO) of the PTH12040W Power Modules

UVLO Adjustment

Adjustment Method

PTH12040W

2

4

6

7

8 UVLOProg

Inhibit

GND

GND

1

3

5

V

I

C

I

1000 F

m

V

I

R

HYS

R

THD

PTH12040W

www.ti.com............................................................................................................................................... SLTS237G – DECEMBER 2004 – REVISED MARCH 2009

The PTH12040W power modules incorporate an input undervoltage lockout (UVLO). The UVLO feature prevents

the operation of the module until there is sufficient input voltage to produce a valid output voltage. This enables

the module to provide a clean, monotonic powerup for the load circuit, and also limits the magnitude of current

drawn from the regulator’s input source during the power-up sequence.

The UVLO characteristic is defined by the on-threshold (VTHD) and hysterisis (VHYS) voltages. Below the on

threshold, the Inhibit control is overriden, and the module does not produce an output. The hysterisis voltage is

the difference between the on and off threshold voltages. It ensures a clean power-up, even when the input

voltage is rising slowly. The hysterisis prevents start-up oscillations, which can occur if the input voltage droops

slightly when the module begins drawing current from the input source.

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 resistors, RTHD and RHYS (see Figure 9), provide the adjustment of the on-threshold and hysterisis voltages.

RTHD connects between the UVLO Prog control pin and GND, and RHYS is connected between the UVLO Prog

and VI. RTHD alone is used to adjust the on-threshold voltage higher. However, to adjust the hystersis to a lower

value requires both the RHYS and RTHD resistors to be placed in the circuit.

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

hysterisis is changed, then a value for RTHD must also be calculated. This is irrespective of whether a change is

required to the value of VTHD. If there is no change to VHYS, then a resistor should not be placed in the RHYS

location. RHYS should then be assigned an infinite value for calculating the value of RTHD.

Figure 9. UVLO Program Resistor Placement

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