參數(shù)資料
型號(hào)： PTV08T250WAH
廠(chǎng)商： TEXAS INSTRUMENTS INC
元件分類(lèi)： 電源模塊
英文描述： 1-OUTPUT DC-DC REG PWR SUPPLY MODULE
文件頁(yè)數(shù)： 11/30頁(yè)
文件大?。?/td> 795K
代理商： PTV08T250WAH
ADJUSTING THE UNDERVOLTAGE LOCKOUT (UVLO) OF THE PTV08T250W POWER MODULES
UVLO Adjustment
Adjustment Method
PTV08T250W
6,7
13,14
20,21
4
5
11
16 Inhibit/
UVLOProg
GND
VI
CI
RHYS
RTHD
VI
GND
www.ti.com .......................................................................................................................................... SLTS260E – OCTOBER 2005 – REVISED NOVEMBER 2008
The PTV08T250W 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 overridden, 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 PTV08T250W 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
prevents it from operating if the input bus fails to completely rise to its specified regulation voltage. The hysterisis
should not be changed unless absolutely necessary. The 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 18), 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 18. UVLO Program Resistor Placement
Copyright 2005–2008, Texas Instruments Incorporated
19
Product Folder Link(s): PTV08T250W
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