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LMZ21700: Issues with regulator EN pin - regulator no longer responds to EN signals

ND3904
Prodigy 60 points
Part Number: LMZ21700

Hello,

I am working on a design that uses a LMZ21700 to produce a 3.3V supply voltage from an 11.1V li/po battery pack. The previous revision of the design had the EN pin of the LMZ21700 connected directly to VIN at all times, so the regulator would be powered as long as a valid voltage input was supplied. This worked fine throughout all of the testing. The latest revision of the design, however, uses a JS102011SAQN SPDT slide switch as an on/off switch, where VIN is shorted to EN to turn the regulator on and EN is open-circuited (and pulled to GND through the EN pin's internal pull-down resistor).

Here is the schematic for the latest revision of the 3.3V supply:

This design has not been working as expected when testing the new PCB with this circuit.

Testing procedure with PCB 1:

  • I started with the on/off switch SW1 in its default OFF position, to have EN internally pulled to GND.
  • I supply power to the VBAT connection, and there is no voltage output as expected since the regulator is off.
  • I then remove the VBAT power supply, switch the SW1 to the ON position, reconnect the power supply and expect to see 3.3V, but there is no 3.3V; instead, all I see is ~172mV DC output from the regulator. I've powered the board up again many times but the regulator does not seem to respond to either switch position now.

On a second PCB (one that was never used before), and I tested again using the following steps:

  • Before powering the VBAT connection, I instead switched SW1 to its ON position (pins 2/3 shorted) to verify that the regulator worked initially. I powered the VBAT connection, and the 3.3V supply worked perfectly. While the board was still powered, I switched SW1 to its OFF position to check that the regulator would transition to the OFF state and nothing happened, the 3.3V continued to be outputted.
  • I then removed the VBAT power source, connected a 1k pull-down resistor to pin-1 of SW1, just in case the internal pull-down resistor on the EN pin was not strong enough
  • I then switched SW1 to its ON position again, powered the board, and 3.3V outputted fine, then switched SW1 it to the OFF position, and the regulator finally turned off.
  • To see if the switch would consistently turn the regulator on/off, I switched SW1 ON, then OFF, then back ON, with ~5 seconds in between each switch position change. After ~2 seconds of running ON, the regulator's 3.3V output suddenly turned off. Similarly to the first PCB, this PCB now only outputs ~172mV DC when turned on.

I would like to know if anyone has had a similar issue with the EN pin on the LMZ21700 before, or if anyone has suggestions on why this issue is occurring or how to fix it.

Thank you,

Nicholas

  • 0
    TI__Guru 50120 points

    Hi Nicholas,

    I've review your schematic and for the most part it follows the typical application for Vout=3.3V referenced in the datasheet Section 9.2.3.4. One thing that caught my attention was 100nF capacitor you have on EN pin. What is the reason for this? This capacitor would store voltage from the battery input and still keep EN pin high even though you manually turn SW1 off to keep EN pin floating.

    Also, since you are manually turning SW1 on/off, this is like a hot plug event. With your battery input application, this could result in high input spike that is generally twice the input supply. In this case, the voltage spike could be anywhere up to 22V voltage spike which is outside the 20V maximum VIN spec of the device. I'd recommend adding a 47uF aluminum electrolytic capacitor on the input line to dampen the input supply spike.


    Regards,
    Jimmy

  • 0 in reply to Jimmy Hua
    Prodigy 60 points

    Hi Jimmy,

    Thank you for your reply; I very much appreciate the assistance. The 100nF capacitor was added to account for any switch bouncing that could occur on SW1; I assumed that switch bouncing would cause the regulator to unnecessarily start/stop while the switch state settles. Would switch bouncing on the EN pin cause any wear or damage to the regulator? If not, I can try removing capacitor C4.

    That's a great point you made concerning the input spike; I had never considered that until now. I'm not familiar with input spiking with battery-powered applications. Would this be caused by the inductor used in the switching regulator, or would it be caused by the the battery itself in most cases?

    Concerning your suggestion for increasing the capacitance on the input, there is another section of the circuit for the switch-mode battery charger (BQ24105RHLR) that I didn't picture in my previous post; I've attached part of that circuit below. Would the 10uF capacitor C7 (used for the switch-mode section of the circuit) be sufficient for the spike suppression, or would you suggest adding more capacitance to the VBAT net? The PCB design is quite height-constrained, so if a ~47uF electrolytic capacitor would most-likely not mechanically fit; I would probably have to resort to using a tantalum or ceramic capacitor instead to keep the board a lower profile. Also, would adding a choke or TVS diode be suitable for this form of voltage surge protection?

    Thanks,

    Nicholas

  • 0 in reply to ND3904
    TI__Guru 50120 points
    Hi Nicholas,

    To summarize, I think the main issue you had was with your EN signal not working. This is caused by the capacitor that is storing enough charge and preventing you from pulling EN pin low. I'd recommend removing this 100nF capacitor. Regarding mitigating the switch bouncing, you can place a voltage divider between SW1 and EN pin. Perhaps a 1k resistive divider will clean up the EN pin.

    In most battery powered application, the voltage spike is caused by the battery itself since it is not well regulated. I initially thought the input to the LMZ21700 was connected directly to a 11.1V Li/Po battery which now I know is not the case. Since you are using a battery charger the output voltage should be regulated enough to not cause voltage spikes.

    If your layout permits it, putting a TVS diode will surely suppress any voltage spikes that happen on the input path to the module.

    Regards,
    Jimmy