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PTH08T221W - capacitor, transistor, and idle current drain

CYNTHIA HOWDER
Prodigy 40 points
Other Parts Discussed in Thread: PTH08T221W

Re: PTH08T221W

Need assistance in choosing a 100uF 16V MLCC ceramic capacitor.  Will be using 3x capacitors on input and 3x capacitors on output.  Am not using TurboTrans.  Are these suitable or is there a better choice?  C1210C107M4PAC7800 or EMK325ABJ107MM-T

Also, which transistor is more suitable (or does it matter) to use with inhibit: 2N3904 or ZVNL110A

One more thing, I will be powering the converter with high drain li-polymer batteries.  Is there anything available (internal or externally added in non smt size) that can turn off inhibit's idle current drain when the UVLO threshold is reached other than adding a power kill switch?  My concern is continued current drain on the batteries after the shutoff voltage has been reached.

Thank you!

  • 0
    TI__Mastermind 38230 points

    PTH08T221W input current is typical <1ma when the inhibit pin is grounded.(pin 11) Thres is no other module  method for reducing input current when inhibitted.

    The data sheet on both ceramic capacitors C1210C107M4PAC7800 or EMK325ABJ107MM-T states their 16V and X5R ratings.Both appear to be acceptable.

    Both 2N3904 or ZVNL110A can be used on the inhibit pin. A dual FET control will insure PTH08X2 is inhibited  until gate drive is high on Q1.

    Tom

  • 0 in reply to Tom Guerin
    Prodigy 40 points

    Thank you for you fast reply and confirmation that the capacitors and transistors are ok.

    I have inhibit wired as shown below.  I need to use an N.O. push button switch to turn on and turn off inhibit.

    Everything is working as it should, but I am having a problem with UVLO.  I used a 15K for RUVLO.  According to the datasheet, the output should be disabled at less than 8.5V, but input voltage went down to 5.8V before output was disabled.  Then I tried a 28.7K for RUVLO and according to the datasheet, the output should be disabled at less than 6.5V, but input voltage went down to 4.4V before output was disabled.  I need to set UVLO precisely at the voltage I need to disable output.  How do I do this?

    http://www.cssolutions.biz/images/221-inhibit.jpg

  • 0 in reply to CYNTHIA HOWDER
    Prodigy 40 points

    CYNTHIA HOWDER said:

    Thank you for you fast reply and confirmation that the capacitors and transistors are ok.

    I have inhibit wired as shown below.  I need to use an N.O. push button switch to turn on and turn off inhibit.

    Everything is working as it should, but I am having a problem with UVLO.  I used a 15K for RUVLO.  According to the datasheet, the output should be disabled at less than 8.5V, but input voltage went down to 5.8V before output was disabled.  Then I tried a 28.7K for RUVLO and according to the datasheet, the output should be disabled at less than 6.5V, but input voltage went down to 4.4V before output was disabled.  I need to set UVLO precisely at the voltage I need to disable output.  How do I do this?

  • 0 in reply to CYNTHIA HOWDER
    TI__Mastermind 38230 points

    Hi Cynthia:

    I reviewed your schematic on inhibit pin and UVLO .The UVLO  on controls only the voltage on-threshold  for  selected resistors. The UVLO power down with hysteresis is not controllable.

    Send me an  e-mail  tguerin@ti.com if you want to discuss this in more detail.

    Regrds

    Tom

  • 0 in reply to Tom Guerin
    Prodigy 40 points

    Thanks for your fast reply.

    Yes, I understand hysteresis is not controllable, but is it changing, or variable, to cause these unexpected results?  I had taken into account the 0.5V hysteresis difference between the ON and OFF threshold voltages when I calculated the resistor to use for the ON threshold voltage I needed  Once the UVLO resistor is set, I need reliability that the converter will always shut off output at the same voltage and not be inconsistent.

    I will contact you through email then as right now I don't know what resistor to use for my desired cutoff voltage other than by trial and error by choosing various resistor values and testing to see what the output off voltage is .

    Thank you again.