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TPS7A4701EVM-094: Need Expert Advise on LDO Adjustable Operation

Rosalino Jr Tabliago
Prodigy 20 points
Part Number: TPS7A4701EVM-094
Other Parts Discussed in Thread: TPS7A4701-EP,

Hi,

Our application is to use this LDO part TPS7A4701-EP in adjustable opration with switching some values on the R2.

Could you help check if R2 Values and Calculated Vout doesn't violate any specs on this TPS7A4701EVM-094 device like Volatge dropout, etc?

I had evaluted this using the TPS7A4701EVM-094 and here are my questions:

Questions:

1. If I lower the 47uF Cout to 10uF are there any issues (tested in ambient temp seems not much difference in Vout signal),

Might this be a concern at low and max operating temp?

 

2. Is there a chance to minimize the ripple at 12.5 Output voltage? I tried to add decoupling (1uF, 0.1uF on input voltage)   

Thank you in advance.

Kind Regards,

Rosalino

  • 0
    TI__Guru 50025 points

    Hi Rosalino,

    To be clear, what exactly do you mean by switching out R2? Will you be toggling a transistor to short in different resistors or do you mean there will be 3 different devices to supply the 3 fans?

    You listed the max input voltage; what about the minimum? This will be more important in regards to dropout.

    Rosalino Jr Tabliago said:

    1. If I lower the 47uF Cout to 10uF are there any issues (tested in ambient temp seems not much difference in Vout signal),

    Might this be a concern at low and max operating temp?

    There is no issue with this.

    Rosalino Jr Tabliago said:
    2. Is there a chance to minimize the ripple at 12.5 Output voltage? I tried to add decoupling (1uF, 0.1uF on input voltage)   

    Can you tell me what both channels are showing? This device looks unstable. The datasheet recommends using 10uF or larger for both the input and output. See if that helps to stabilize the device.

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 20 points

    This is the simulation circuit that I'm trying to work with this TPS7A4701. Yes I'm planning to toggle 2 transistors to set different values of R2. (Disregard the the R1 and R2 values)

    Min Input Volatge is 11.8V.

    For 

    1. So this is confirmed that single 10uF can support the max load 0.5A?

    2. The blue is Ouput Voltage (12.5V) while the Yellow is the input Voltage (13.8V). Is it normal to have higher ripple on higher voltage output and load current?

    This measurement still have the default 10uF in input and 47uF on output volatge in the TPS7A4701EVM-094.

  • 0 in reply to Rosalino Jr Tabliago
    TI__Guru 50025 points

    Hi Rosalino,

    The datasheet says that the device is stable with at least 10uF on the output, so yes the max load is included.

    It is not normal to have a large ripple like this, regardless of the load current. It looks like the output is dipping because the input dips enough to put it into dropout. Where is the input supply coming from? Is there some sort of transient on it that is causing it to dip so much? It seems very noisy.

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 20 points

    Hello Nick,

    Thank you so much for your support. 

    I do not have control on the 12V Fan Voltage input so I'm just doing the tuning on the LDO circuit.

    Is there a way to compensate the ripple, here is the evaluation measurement I did. The ripple is increasing while fan volatge and current is increasing.

    Adding a large input caps improve this ripple?

    Kind Regards,

    Rosalino

  • 0 in reply to Rosalino Jr Tabliago
    TI__Guru 50025 points

    Hi Rosalino,

    I don't think the issue is with the LDO. The issue is that the input ripple is less than the set point for the LDO output, so it is dipping into dropout with the input. Try AC coupling the input and output and put their zeros on top of each other. You should see that the output is following the input when it dips low enough. 

    If a large input cap reduces the ripple on the input side, it could help with this issue. The LDO has no control over the input side so it is up to you to get the input ripple down enough such that the LDO never enters dropout. 

    Regards,

    Nick