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DRV2700: Powering the SW pin from 12V

Robert Karpinski
Prodigy 20 points
Part Number: DRV2700


Hi,

I've recently gotten back a PCB with a DRV2700 on it and I'm not getting the expected voltage from the boost converter.

A diagram of a circuit boardAI-generated content may be incorrect.

The plan is to get ~90V from the BST pin, to do this we figured that powering SW from +12V would make the DRV2700 have an easier time of it. L1 is a Wurth Elektronik's 74438357056 5.6uH WE-MAPI inductor.

As per the datasheet for the DRV2700 the Ilim was set to 2.3A (inductor saturates at 3.9A so we're good) so Rext was chosen to be 6.04k. I'm expecting a duty cycle of 91.1% and switching frequency of around 918kHz as recommended by the datasheet.

However I get a voltage of 10.92V at the BST pin. That suggests to me that the MOSFET is not switching. Do you have any suggestions on what I could try to fix this and get ~90V on the BST pin?

Here's my PCB Layout, this is a 4 layer, 1oz on all layers, 0.8mm thick PCB. The top layer has most of the signals/traces, inner 1 is GND/0V, inner 2 is a continuous +5V power plane and the bottom layer is mostly GND/0V but it has trace for BST back to the feedback resistors R1 & R4 placed next to the FB pin and the input +12V supply.

A close up of a circuit boardAI-generated content may be incorrect.

Thanks!

  • 0
    Prodigy 20 points

    Today I tried to fix it by cutting the +12V input and connecting the inductor to +5V. It now seems to switch and I get 92.9V on BST. It's 3.57% higher than the 89.7V I was expecting but it's close enough. I think the output voltage error is due to something in the chip as I'm getting 1.409V on the FB pin when it should be regulating to 1.300V.

    I also found out that I've rotated the symbol for U2, the output BST from the DRV2700 is entering the output pin of the LR12N3-G voltage regulator. Not ideal, but I don't see how that makes the DRV2700 not work at 12V but work at 5V.

  • 0 in reply to Robert Karpinski
    TI__Mastermind 20090 points

    Hello,

    Your original configuration (12 V on SW inductor) is not supported by this device. The supported range is from 3.3 - 5.5 V. This is why the 5 V configuration worked.

    Referring to the difference, it's possible there was some damage due to supplying 12 V on the SW pin. I recommend swapping the part. 

    Regards,
    Sydney Northcutt

  • 0 in reply to Sydney Northcutt
    Prodigy 20 points

    Hi,

    I don't think that's correct at all.


    This datasheet (https://www.ti.com/lit/ds/symlink/drv2700.pdf) has the inductor powered from VDD to 12V on the very first page.

    The Absolute Maximum Rating for the SW pin is listed as 120V on page 4.

    The VDD to 12V on L1/SW is shown again on page 10.

    The VDD pin is limited to 5.5V and it's when the inductor is connected to the VDD pin that the 5.5V limit applies.

  • 0 in reply to Robert Karpinski
    TI__Mastermind 20090 points

    Hi Robert,

    Thanks for pointing this out. You can disregard my comment about damage to the device damage (I removed it from the post). The SW pin will be able to take this voltage without damage. We do not have this spec though listed in out recommending operating spec. It is not the typical implementation of this device and I have not tested this before. 

    You can refer to this thread:  DRV2700: Powering boost with external voltage  . It seems this does work up to 7.2 V. Is this true on your end as well?

    Regards,
    Sydney Northcutt 

  • 0 in reply to Sydney Northcutt
    Prodigy 20 points

    Hi Sydney,

    The +12V rail is pretty buried on this board, so adjusting it kinda tricky. I'll see what I can do.

    Here are scope traces of BST when L1/SW is connected to +12V:


    And BST when L1/SW is connected to +5V:


    The scope is set to AC coupled, 2.00V/div, 2.00ms/div.

    It doesn't look like the DRV2700 is even attempting to switch at +12V input. As Gaël asked in the linked thread, are there any internal protections/transient limits that could explain why it works at 5V (or in their case 7.2V) and not 12V?

  • 0 in reply to Robert Karpinski
    TI__Mastermind 20090 points

    Hello,

    I believe this boils down to the limits of the internal boost. It may be that ~7.2 V is the limit this internal boost can take and this was an error in the diagrams. Considering it works with 5 V, it proves there is not an issue with the hardware configuration. 

    Regards,
    Sydney Northcutt

  • 0 in reply to Sydney Northcutt
    Prodigy 20 points

    To close this thread out for future 'victims' we've just decided to stick with +5V supply for the SW pin. We just chose +12V as we thought it'd be more efficient than +5V and we have +12V available. I don't want to add a regulator for +7V just for the SW pin.

    I've cut the +12V track and wired it to +5V and it works just fine, future iterations of this board will be directly connected to +5V.