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

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

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

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

Hi, Andras.

You are correct with regards to the POS LDO protection diode being turned on when the NEG LDO is supplied first. Yes you can use the EN pins for proper power supply sequencing.

Regards,
Errgy