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Part Number: LM2776
In a prototype PCB I have a linear power supply (120V to 12V then a LDL1117-5V regulator) that power the LM2776. The load are multiple OPA4188 amplifiers with a +5 and -5 supply. At startup, when using AC mains, *sometimes* the output of the LM2776 does not reach -5V but stay oscillating around -1.3V. If I use a battery to power the linear regulator directly, the problem always happens.
This is the schematic, PCB layout and Vin / Vout signals:
I don't know if this a layout issue or if the positive voltage in Vout before the LM2776 is enabled may cause this. The only difference I can see between a failed startup and a successful one is the slope of the linear regulator output voltage (depending at what time of the 60Hz AC cycle is turned on, its output voltage raise faster or slower).
In case it matters, the following are the component parts:
Any suggestion is welcome. Thanks in advance.
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In reply to Ivo:
The CH1 plot is the input voltage, but it was probed at a test point a few millimeters away from the VIN pin of the LM2776. If this is important, I can test again probing directly the VIN pin.
The load contains three OPA4188 op amps (+5 and -5) and a DG408 multiplexer (+5 and -5), also (powered only by +5V) a microcontroller, FRAM memory, USB-UART bridge (unconnected during tests), and two LM4040 references. During the tests all external inputs connected to the amplifiers were connected to ground.
The following are the plots of the device when powered from AC mains (all successful). I also included again a failed one when powered with a battery, using the same scale (for comparison).
Thanks in advance.
In reply to Mario Valdez:
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In reply to Chris Glaser:
Hi. These are zooms of two failed startup:
This is the zoom of a successful one:
If a hold the EN pin low until the 5V input is fully on, the LM2776 always fail to start correctly. Also, the negative rail raises up to 640mV while EN is low. My guess is that there is current flowing from the load. The following are two waveforms, when the positive rail gets to +5V while EN pin is low and what happens when enable the LM2776.
Hi. Yes, if I disconnect the load from the supply the LM2776 starts correctly, at the same time the positive rail raises or later if I delay the EN pin.
So, Is safe to assume the positive voltage in the output of the LM2776 is causing this?
Hi. For that test there was no load on the LM2776 and also there was no load on the positive rail.
Yes, if I apply power to the LDL1117 that feeds the LM2776, it starts almost symmetricaly. But if I delay the EN signal the LM2776 starts at a slower pace. Both tests were done without load. Even if I bypass the positive LDO and apply 5V directly to the LM2776 while delaying the EN signal, it starts slower.
I'll populate another PCB with only the power supply and another with only the LM2776 to continue these tests. I'll let you know what I find. Of course any extra idea you may have is welcome.
Regards and thanks.
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