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Is it possible to cascade PTH08T220W modules?

fabu
Prodigy 230 points
Other Parts Discussed in Thread: PTH08T220W

I have a 12v source as an input to PTH08T220W to create a 5v supply.

I am planning on using the 5v as an input to another PTH08T220W to create a 3.3V supply. Also, I am planning on using the 5V as an input to another PTH08T220W to create a 1V supply.

Is it OK to do this? If yes, what are the consequences.

I am doing this because the 12V has a maximum load capacitance of 800uF. And Each PTH08T220W must have at least a 680uF C_IN as recommended by the datasheet.

See attached:

  • 0
    TI__Mastermind 38230 points

    Fabu:

    This application schematic  as drawn  with  (3) PTH08T220W  has many questions concerning output stability( output capacitance),turbo trans function and input capacitance.

    The power supply cascading is not recommended for a number of reasons including stability  and circuit   impedance. The  PTH08T220Wx3 input capacitors when  sharing a common input bus copper plane can be reduced to 220uf polarized +22UF for each regulator.Then the 12V input bus can b used as the input capacitance is less than  800uF.

    a. The total amount of input capacitance is about 744uF.

    Questions on the output bus with turbo trans resistors are:

    1. What is the type( vendor) and ESR on each output 470uF / 10V  polarized capacitor?

    a. The 5V and 3.3V Rtt(turbotrans resistor) must be open with this 470UF capacitance.

    b. If the RTT on the 1.0V module has only one (470UF) ESR < 15mΩ on the output bus and it is a polymer  tantaulum type an d not aluminum type, the  RTT turbo trans resistor is 20.5kΩ.

    2. The Smart sync. if not used must be grounded

    3. The Vout voltage adjustment pin 8 selected resistor for output voltage is recommended to be connected to the (-) remote sense pin 7.The (-) remote must be connected very close  to ground pin 4  within 10cm or less.

    Tom

    You can contact me direct  for more discussion on this application

    My direct E-mail address is tguerin@ti.com

  • 0 in reply to Tom Guerin
    Prodigy 230 points

    Hello Tom,

    Thanks for your prompt reply. I now understand that the stability of the regulator depends on the impedance looking back from the output. That's why it is not recommended to cascade. I will use the 12v with 220uF pol as you recommended.

    To reply to your questions:

    1- AVX TAJE477K010RNJ; ESR is 0.5ohm

    a- I haven't decided if I wanted to use the TT or not. That's why I put a 0ohm jumper.

    b- ok

    2- I am using smartsync. the circuit is not show here. the last signal is mode sync 1v not 3.3v

    3- ok

    Thanks again