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LMR36015: USB Application Queries

Part Number: LMR36015

Hi,

I'm currently prototyping with a 1MHz non-forced part which powers a small MCU and is switched into a USB load via a low side NMOS switch.

The MCU listens to the PG pin and if it receives a fault switches the USB out. On start-up there is a delay before the USB is switched in so that the LMR36015 is in PFM mode (MCU is a few mA max).

The LMR36015 has a typical 93% PG point, therefore to adhere to lower USB boundaries I've set the output to 4.85V nominal. If the voltage drops below 4.5v for the Tpg of 140us then PG will fire.

Specification

Input voltage: 10v, 4.7uF X7R 50v + 1000uF Elec 50v
Output: 4.85v, 2x 22uF 0805 X5R 16V, 220uF tantalum 6.3v
Inductor: Coilcraft XGL4030 5.6uH

Two issues have been observed. 

One

A USB device should have only 10uH capacitance on it's output, but many violate this. When connected the 2x22uH 16v X5R's on the output immediately sag far quicker than the LMR36015 can keep up, which drops the 4.85v probably to half or less, which resets the MCU. To resolve this a 220uF 1ohm Tantalum has been added to the output. 

Two

Most USB devices work well but the iPhone 8 is troublesome. It doesn't drop the voltage enough to reset the MCU but presents a load that is still enough the LMR36015 can't keep up with within the 140uS timeout.

Resolving

Low mA efficiency is not super critical. Will moving to the forced LMR36015 part tighten the timing as it switches from PFM to PWM and remove the issue of the problematic reset?

  • Hi Andrew,

    Could you share your schematic, VOUT wave form, SW waveform, system VIN/VOUT/IOUT requirements?

    Thanks,

    Andrew

  • Hi Andrew,

    I can't share the entirely but the part that matters I can.

    Vin/Vout/Iout = 6-50v, 4.85v, 1A

    I haven't taken waveforms of the various, I could to see what's going on but probably from the PG it's enough. 

    I should mention the PCB is incredibly tight and well designed - I'm aware of all the good practices around switch-mode design and have considered, so no problem there.

    I suspect switching to the forced part will tighten up the regulation as it switches mode and resolve both one and two, but it is a guess and waiting on TI parts to become available to test the theory is troublesome! Another option is extra output capacitance but I'm fresh out of board space. Another option is to use another monostable in the MCU to lengthen the 140us PG signal.

    Regards,

    Andrew

  • Hi Andrew,

    What is causing the device to droop to 4.5V? Is this under steady state condition or a load transient?

    Can you share the 4.85V -> 4.5V VOUT transition waveform with Switch node probed as well?

    Thanks,

    Andrew

  • It can only be one thing, the additional output capacitance (should be 10uF max according to USB spec) takes voltage from the output capacitors and the buck can't correct it before the PG glitch timeout occurs. The tantalum 220uF has high ESR of 1ohm so won't help so much with the insertion shock, though it is enough to prevent the MCU resetting (1.8v).

    The question is whether the FPWM is going to be a better option speed wise than the device switching from PFM to PWM or whether the device should be able to switch from PFM to PWM without triggering PG (140us glitch timeout). For the iPhone 8 this is going from 4mA to about 450mA. The waveforms are possible but tricky as I have to disassemble the device to get to the outputs but if I can get this confirmed first then I'll know whether to start inspecting the circuit.

    Just to add, there is no performance issue once a load is established (if I ramp one up).

  • I'm not entirely convinced as you mentioned you way over specced on Cout, and you still have an issue for the Iphone 8 as an edge case. I know it's a pain, but please obtain the wave forms, so I can discuss with my team. Unfortunately there's not much to go on as of now, and I can't really say if the Forced PWM version will fix your issue.

  • Hi Andrew,

    Will do, tomorrow morning GMT time.

    Regards, Andrew

  • Hi,

    I inspected the ripple in both modes and it's very clean (now with 4x 22uF X5R's).

    I've added a 10ms secondary monostable and it's working so for USB usage the PG is too sensitive.

    Regards,

    Andrew