ADP7142(ARDZ-5.0-R7) output out of range, fixed in -0.786v

Hi all, I hope you can help me with the following:

 

I'm testing a dual power supply with the adp7142 for +10v (positive 200mA) and the tps7a30 for -10v (negative 200mA). The source is a pair of 12v batteries in series configuration so the adp7142 is applied with +12v on its input and the tps7a30 is applied with -12v. When both regulators are started through a common switch (nearly at the same time) the output voltages are within range -10.02v and +9.99v respectively. The problem arises when only the positive input is disconnected and then connected back, after this the output of the adp7142 is -0.786v and the sense/adj pin is -0.401v. Also repetitive disconnections and connections won’t fix the problem, the adp7142 output stays at -0.786v. Only by restarting both regulators through the common switch returns the output voltages to the correct values  

 

Here is the schematic for the adp7142:

 

 

Thank you very much

David

  • 0
    •  Analog Employees 
    on Feb 26, 2018 2:17 PM over 2 years ago

    Hi, David.

    I tried to replicate this setup using ADP7142 and ADP7182 as positive and negative LDOs (input voltage (VIN) of ±12V and output voltage (VOUT) of ±10V) and I am NOT experiencing the problem that you've encountered as stated above. ADP7142 VOUT still reaches 10V after VIN is connected and disconnected several times. 

    Having scopeshot of VOUT and board schematics would be helpful too in debugging this issue. Check to see if there is any external pull down current path on VOUT or sense/adj pin to a negative potential when VIN is disconnected that can turn on the protection diodes. I recommend also investigating other paths external to the LDO that could potentially creating this problem.

    Regards,

    Errgy

  • 0
    •  Analog Employees 
    on Aug 2, 2018 4:02 PM over 2 years ago
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin
  • Hi all,
    I have the same problem with this LDO, actually sometimes it does not turn on and gives this -0.7V voltage on its output rail. My setup is the following: I use a bipolar supply from HMP4040 to create a +/-15V that supplies a ADP7142 and a ADP7182 LDO pair which in turn on the output give +/-10.3V. These LDOs supply 4 amplifiers: AD8429,AD8512, and two AD8421ARZs. So they are all connected to these bipolar rails on their output. The enable pins are all fixed to VIN as it is suggested in the datasheet. On their input I have 7u-7u and also on their outputs 4.4u-4.4u.  On the feedback I have R1=75K and R2=10K that create these +/-10.3V.  I use a 5 pin (SOT-23-5) configuration without SS pin.
    When I switch on the supply of the LDOs at the same time, what I observed (see my attached scope shots in the link) is that sometimes the positive LDO (ADP7142) does not turn on and on its output it gives -0.7V, instead the negative LDO (ADP7182) always turn on. When this happen the positive LDO still consumes 14mA and the other negative one that is OK consumes 20.3mA. This happens, when the negative rail comes before the positive rail (see the scope shots). Instead, when the positive rail comes first both LDOs works well (still on the attached scope shots). Additionally I have to say that I have 10K bleeding resistor on the inputs but I think it does not change anything.
    We suspect that since we see -0.7V on its output of the positive LDO, it could be the case that some internal protection turns on suddenly which takes control over the LDO and does not turn it on finally.

    Here is my link to the flies: https://drive.google.com/drive/folders/1Enxx-wdaLROM370Ypi-GBE_udlAhpOAw?usp=sharing

    Thanks in advance for your reply,

    Andras

  • 0
    •  Analog Employees 
    on Sep 20, 2018 8:22 AM over 2 years ago in reply to andras.magyar37

    Hi, Andras.

    We have tried to test a demo board and we don’t see this issue. NEG output comes first before POS output.

    Based from your scopeshots, the POS output is pulled negative when the NEG output turns on first which unintentionally turns on the protection diodes of ADP7142. I suggest you should follow the power supply sequencing requirement of the amplifiers you used to avoid experiencing this issue.

    Regards,
    Errgy

  • Hi Errgy,

    So basically the when the negative supply turns on, throughout the opamps it creates immediately some negative voltage on the output which in turn activates the protection inside the pos LDO and since it was first it remains latched and the pos cannot be activated. Am I right?

    In this case the enable pins can be used with some delays respect to each other to make sure the order is correct.

    Andras