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LM7705: Vout increases with Load Current

Christian Sacares
Prodigy 10 points
Part Number: LM7705

Hi, we are using the LM7705 in one of our applications board to generate a negative voltage. We noticed that Vout increases with load. Here is what we get. 

0mA Load:  Vout = -0.238V
1mA Load:  Vout =-0.528V 
10mA Load:  Vout =-0.682V 
20mA Load:  Vout =-0.751V 

The board layout is based on the recommendation on page 19 of the datasheet. One difference is that we have a 1206 CRES right across pins 5 and 7 to reduce parasitics. The Vout track goes in between the CRES cap termnals. Vout is measured from the (+) end of one of the Vout capacitor (C5) to the board GND (C6 GND).

We would have expected Vout to be in the -200mV range as indicated in the datasheet. The LM7705 is powered from a 5V regulator. CFLY = 4.7uF, CRES=4.7uF. We have tried to increase CRES = 20uF, to match the datasheet, but we still observe the same increase in Vout vs load as in the original 4.7uF CRES.

Vout is clean and stable, and there are no oscillations. It is just that we do not understand this increase in voltage. Would you be able to help us figure out the possible causes for this issue?

Thank you.

  • 0
    TI__Mastermind 33231 points

    Hello Ian,

    Thanks for reaching out to us.

    The schematic diagram and PCB look fine. Hence, it's not clear to me the cause of the unregulated Vout at a higher load current. To better understand the issue, I need your help to provide waveforms as indicated below.

    1. Vin, VCRES, and VOUT waveforms at 0mA and 10mA load
    2. Temporarily remove FB5. Re-measure Vin, VCRES, and VOUT waveforms at 0mA and 10mA load (connect a temporary Rload across C5)

    By the way, is the issue consistent with several boards? Did you try to change the IC if the unregulated Vout is consistent?

    Best regards,

    Excel R.

  • 0 in reply to Excel Regidor
    Prodigy 10 points

    Hello Excel, 

    I appreciate the quick reply. You can close off this ticket. This isn't an issue with the LM7705, but rather a silly connection mistake on my end. This had previously been verified using multiple ICs and the behaviours were the same. After looking at this further today, I realized that our setup was inadvertently sinking current from Vout, instead of sourcing that current into it. I now see the expected behaviour on our setup after making the necessary corrections and I can confirm the LM7705 is regulating well. 

    Thanks for your help. 

    Best regards, 

    Christian