This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS7A4001: TPS7A4001 dropout voltage

Part Number: TPS7A4001
At the input of the LDO I have 70V.
I put an NPN (PHPT61006NY) at the output of the tps7a4001 regulator to have a stronger current.
Idle LDO with no load works as it should.
once I put a 1kohm load at the output. the output voltage crashes; it goes from 61v to 50v. even the FB goes from 1.18v to 0.96V.
Is this behavior normal?
thank you in advance.
  • Hi Nasri,

    I put an NPN (PHPT61006NY) at the output of the tps7a4001 regulator to have a stronger current.

    When you say at the output, do you mean the LDO is tied to the collector or the base of the NPN?

    If it is tied to the base of the NPN, then the LDO is acting almost as a base driver circuit using the small current it produces to create a large current for the output. However this configuration does not allow the load to see the advantages of an LDO. VOUT regulation, PSRR, low noise, etc.

    If it is tied to the collector of the NPN, then you are just adding an additional voltage drop across from input to output, not allowing more current to flow as the LDO has internal current limit and will provide current up to that limit to regulate VOUT. Now you are using a load that should be 61mA, which is below Ilim for this device. However it is outside recommended operating conditions for this part. If you look at current limit, the min current limit spec is 51mA. This means that it is possible the part is entering a current limit condition and therefore the output is collapsing as a result.

    The FB going down to 0.96V is a good sign as that lines up with a VOUT setting of 50V, so the device internally is regulating to the correct value

    If you decrease the load down to ~30mA, does the device regulate at 61V?



  • Thank you for your reply.
    in fact the output of LDO is linked to the base of the NPN.
    LDO regulates perfectly at 61V for a load less than 50mA.
    For a load of 200mA the output voltage begins to crush.
    If I replace the NPN transistor with an N channel Mos.
    Can this solve the problem?
    thank you.
  • Hi Nasri,

    This device has an Iout limit of 50mA, once you go beyond 50mA of load current, then the output is no longer following the datasheet conditions, it will collapse. In order to have the LDO regulate it's output voltage, you need to have the load that it has to provide be less than or equal to the 50mA limit that it internally contains. (Or follow the below series resistance metrics in order to maintain LDO regulation) The LDO needs to be tied only to the BJT base and the COUT, not the load directly in order for the BJT to provide additionally current on top of what the LDO is providing. In this case the load is also on the LDO forcing IOUT to exceed the current limit which the device then throttles back by decreasing VOUT.

    This app note also details how to properly set up an NPN on the input side. In your use case, The series resistance that is utilized would be very important for your circuit to try and limit the total current that the LDO has to provide. By placing the correct series resistance you will be able to limit the current through the LDO while still obtaining the regulation properties of the LDO.