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TPS76633 Doesn't seem to regulate at low Iout

JULIA TRUCHSESS
Intellectual 310 points


The TPS7633 data sheet states that the part has no minimum output current requirement, however in my board it doesn't regulate unless I throw a dummy load of about 7KΩ or less from the output to ground. Obviously, pulling 470uA just to get it to regulate negates the reason I designed the part in to begin with, namely its very low quiescent current.

My board is currently unpopulated except for the regulator and a few discretes. I have a 10UF tantalum (3Ω ESR) right at the regulator output and four or five 100NF 0603 ceramics scattered around the board. I see no signs of oscillation from the regulator, but if I have more than 7K ohms across the 3.,3V output, the output voltage pretty much just follows the input: 9V in= 9V out. I'm glad I checked before putting in the 3.3V IC's :-)

What am I missing?

Thanks,

Julia

  • 0
    TI__Mastermind 45250 points

    Julia,

    It sounds like this chip may be damaged. Is this a single board or are you seeing this on multiple boards?

    Is this the fixed or the adjustable version? Can you send me the schematic that you are using?

    Richard Elmquist

  • 0 in reply to Richard Elmquist
    Intellectual 310 points

    Richard Elmquist said:

    Julia,

    It sounds like this chip may be damaged. Is this a single board or are you seeing this on multiple boards?

    Is this the fixed or the adjustable version? Can you send me the schematic that you are using?

    Richard Elmquist

    Hi Richard,

    That's what I first thought - bad part. But I bought three parts from Mouser last week and tried two; both have the problem.

    Here's the schematic for the whole system but as I mentioned, I'm seeing the issue with a mostly-unpopulated board - just the TPS76633 and decoupling caps.

    http://www.pragmaticdesigns.com/TPS76633%20problem.pdf

    The TPS76633 is a 3.3V fixed regulator.

    Thanks,

    Julia

  • 0 in reply to JULIA TRUCHSESS
    TI__Mastermind 45250 points

    Julia,

    You said that it will not regulate unless you put a 7K load or less to GND. With the 7K or less load, what is the value of the input current? Are you seeing much more that the 470uA (or calculated amount) that you mentioned? Does it continue to regulate as you decrease the load (<7K)? At what point does it stop regulating? What is the input current at this point?

    Can you isolate the TPS76633 from the circuit totally and see if the part regulates? We need to totally isolate the part to verify if the TPS76633 itself is operating.

    There has to be some interaction happening with some other part of the circuit. Are Q1-Q3 connected at all? This is the only way I could see that 9V could be seen on the output of the LDO without the part being damaged.

    If you can isolate the LDO and it works properly, you can then connect the other parts of the circuit to see which portion is causing the issue. If isolating the part shows that it is not regulating independent of the rest of the circuit, then we will need to try and understand how the part is being damaged.

    Richard Elmquist 

  • 0 in reply to Richard Elmquist
    Intellectual 310 points

    Hi Richard,

    I see it now. When power control transistor Q3 is off, the input and output of the regulator are connected to each other through R1-R3-R4-R2. I need two separate NPN's, one for each PNP power control switch.

    Thanks for your help,

    Julia

     

    Richard Elmquist said:

    Julia,

    You said that it will not regulate unless you put a 7K load or less to GND. With the 7K or less load, what is the value of the input current? Are you seeing much more that the 470uA (or calculated amount) that you mentioned? Does it continue to regulate as you decrease the load (<7K)? At what point does it stop regulating? What is the input current at this point?

    Can you isolate the TPS76633 from the circuit totally and see if the part regulates? We need to totally isolate the part to verify if the TPS76633 itself is operating.

    There has to be some interaction happening with some other part of the circuit. Are Q1-Q3 connected at all? This is the only way I could see that 9V could be seen on the output of the LDO without the part being damaged.

    If you can isolate the LDO and it works properly, you can then connect the other parts of the circuit to see which portion is causing the issue. If isolating the part shows that it is not regulating independent of the rest of the circuit, then we will need to try and understand how the part is being damaged.

    Richard Elmquist