TPS65216: Fails to power on with AC_DET tied high and PB pulsed low

Hi Jay,

I assigned this thread to the responsible engineer late in the day and you should expect to receive a response from them by the end of the day tomorrow.

Best regards,

Layne J

First of all, I really like the color scheme of your schematic symbol in Altium :-)

When AC_DET = IN_BIAS = VSYS, the PMIC is waiting for a push-button press to turn on. A push-button press is defined as a Hi-Lo-Hi event on the PB pin where the low pulse lasts for >50ms (deglitch timer). You do not need to hold PB pin low, nor is it advisable. After 8 seconds, the PMIC will reset if PB is stuck low.

My concern is that PWR_EN might get set high by the processor (>10ms deglitch) but then return to a low voltage, due to the RTC domain of the processor not receiving power correctly.

If you refer to Figure 5. Power Rails for RTC Domain on page 10 of the Powering AMIC110, AMIC120, AM335x, and AM437x with TPS65216 User's Guide, could you please confirm that this portion of your schematic is wired correctly?

Thanks! I hate the default "Altium Yellow" color.

Other than a missing the RC delay capacitor on the reset pin, I think my connections match. If you want to double-check, I rearranged my schematic to show the connections with the AM335x:

I used a Saleae to capture the 5V input, the 4 DC/DC + 1 LDO rails, along with the PB and PMIC_PWR_EN signal. It doesn't look like PWR_EN is ever going high:

It looks like the supply is attempting to start? There's small glitches on the rails. Is that anything? By the way, DCDC4 (3V3) floats up to ~1.1V, but it's not actively being driven there — if I put a 10-ohm load on it, it immediately drops.

Any ideas of next steps?

First of all, I discovered the 1.8V rail on the board I was testing is now shorted (that explains the short spike you were seeing). It wasn't initially this way, so I must have grazed it with the 5V rail while capturing data.

Anyway, I built up another board, so we're back to the initial symptoms. But I've got some better-looking data now:

Both the LDO1 (1.8V) and DCDC3 (1.35V) DDR rails come up in the correct order, but the sequencing stops there --- the DCDC4 (3.3V) supply never comes up. I probed both sides of the inductor and they're both flapping around in the breeze until DCDC4 tries to come up 4 ms later. The inductors are both pulled to GND for 5.45 ms, then it appears that DCDC4 goes into a fault condition and starts sequencing down the other rails. Please note the Y-scale of the L4A and L4B signals in the above image is greatly exaggerated — the peak-to-peak voltage is actually only 50 mV

So why is DCDC4 faulting?

There's no short circuit on that rail. This was the same rail that was floating around up to 1V. You wondered if there was another regulator powering it: definitely no. This is a simple break-out board with no active components other than the TPS65216 and the AM335x. The only 5V connections on the board are to the PMIC and the USB_VBUS pins of the AM335x (P15 and T18). Since it floats up to 1V whenever 5V is applied, it's got to be leaking either through the TPS65216 or through these two AM335x pins.

Here's the other stuff you requested

When I was building up this board, I soldered a 1 uF cap from RTC_PWRONRSTn to GND, just to be sure. That obviously didn't fix anything, unfortunately.

Power supply connections on the AM335x illustrating CAP_VDD_RTC and VDDS_RTC:

Layout illustrating the thermal pad is connected to GND:

Brilliant! Excellent debug work. Thanks for your patience and understanding with the long list of questions, and many thanks for following up with the great photo showing the true root cause of the issue. Nice to have a happy ending and a clear-cut root cause every now and then :-)