DRV8825: Charge Pump Capacitor Selection

Part Number: DRV8825

Hello,

I am a consultant engaged in a detailed design review for a client who is using this part in their design. They are presently stuffing a 0.1uF capacitor for the CP1/CP2 charge pump component instead of the recommended datasheet value of 0.01uF. While I understand the fundamental importance of following datasheet recommendations, "does not follow datasheet recommendation" is not sufficient rationale for presenting a need for change to the client for a medical device in current production. I need to present if their part selection would change the behavior/performance of the charge pump in a way that would create a specific problem, and what that problem would be, as the rationale for design change.

I initially opened a support ticket (CS0502962) for this question and was pointed to the following thread for general performance rationale:https://e2e.ti.com/support/motor-drivers/f/motor-drivers-forum/716344/drv8818-capacitor

If you dig into this thread, you will see that the commenter said to keep the ratio of VCP/CP between 1:2 and 2:1 for things to be OK, but the DRV8825 datasheet is recommending VCP cap=0.1uF and the CP1/2 cap=0.01uF. That's a ratio of 10:1. So the DRV8825 datasheet recommendations do not align with the behavior/performance rationale presented in the thread.

Please advise and thank you for your help in advance.

Chris

  • Hi Chris,

    Thank you for posting to the forum.

    Please give me a day to research your question and I'll get back to you tomorrow with an answer.

  • Hi Chris,

    Let me first answer your question about how different CPL/CPH and VCP capacitors affect the performance of the charge pump. 

    There is a min (Vcp_min) and max (Vcp_max) values for the charge pump. The charge pump will only charge when needed (Vcp getting closer to minimum value) and then discharge when Vcp hits the maximum value (This is to prevent damaged to the device from overcharging the Vcp). The time it takes to discharge depends on many factors such as the Vcp capacitance, leakage current, and more. There is an internal comparator which constantly checks that the Vcp is within bounds. 

    Usually, the difference between Vcp_min and Vcp is around 400mV but can vary. The device is designed such that the difference does not affect its performance. 

     

    The CPL will dump its charge to VCP only for brief moments (enough to charge the Vcp to the max value) and stop dumping charge until Vcp hits min value) This repeats in a periodic manner and its frequency will define the frequency of the ripple. (As shown in the second figure below).

    So having different VCP and CPL-CPH capacitor values than the recommended values will cause the charge pump frequency to differ from what the device was originally designed for. It may cause the device to operate abnormally.

    Is there a reason why the customer wants to use .1µF instead of the recommended 0.01µF?

  • Pablo, thank you for this detailed behavioral explanation. It is very helpful!

    The person who designed this circuit is no longer with the (customer) company and I suspect there isn't any rationale documented for the present design choice. I haven't specifically asked yet, but my MO here is to lead with the technical reason as to why an existing design choice may be problematic. This is a medical device in production and so strong reason for concern, if warranted, is needed to push through a design change.

    Can you speak any more specifically to "It may cause to device to operate abnormally"? In this particular case, it seems like the 10x larger than recommended CP1/CP2 capacitance will result in a 1/10th effective charging/dumping frequency for the charge pump. Without any further insight into the chip design, I can't come up with a reason why this would be problematic for the chip.

    In absence of any more specific information, if this capacitor value selection corresponds to how the part was production qualified/verified/validated then that would probably be sufficient rationale for design change in itself since it speaks to TI having specifically qualified the chip with a capacitor of the specified value, 0.1uF in this case. Thank you.

  • Hi Chris,

    The recommended capacitors values given on the datasheet give the best charge pump performance. During the design phase of this device, simulations were done to determine the best capacitor values that will result is more stable and reliable driver performance. During the validation/testing phase, these capacitor values were used. This does not mean that the driver will not perform well if other capacitor values are used. Just the behavior of the charge pump (as I explained in my previous reply) will change. It is up to the user to determine if the behavior will affect their systems performance.

    I understand this is a medical device and the concern for redesign. Let me do some more research internally to determine if there are any risks in using 0.1µF CPL/CPH capacitor instead of the recommended 0.01µF value. I will get back to you by 5/11.

  • Hi Chris,

    My apologies for late response. I'm still researching your question. Please give me two two more days for a reply.

  • Hi Chris,

    The design is verified in simulations and bench validation/production-test etc for the recommended caps. The below scenarios are not possible to be extensively tested across all variables on a production device.

    1. Higher cap on CP2-CP1 might add stress on the charge pump FETs for a bit longer time during its initial charging after power up. Especially if their VM is close to operating max.
    2. During charge dumping phase from fly-cap to storage-cap, there is VCP voltage monitoring circuit that ensures VCP doesn't overshoot. But this comparator & charge-dumping turn-off circuit have some delay. Higher fly-cap dumps more charge and it might cause higher overshoot on VCP to cause electrical stress.

    We recommend to lower the fly-back cap (CP1-CP2) to prevent potential electrical over-stress issues.