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DRV8305: DRV8305 Why is the charge pump output pin VCPH connected to PVDD via capacitor？

Xiaotian Chen
Prodigy 41 points
Part Number: DRV8305
Other Parts Discussed in Thread: DRV8301

I just found the charge pump output VCPH pin should be connected to the PVDD pin using  a 2.2 μf capacitor,  rather than to the SHA pin (Source pin of high side MOSFET).

And I have two questions about it:

1) The reference gound of charge pump output (VCPH)  is PVDD？

2）I also found the charge pump was usually connected to the source pin of high side MOSFET from other high side gate driver datasheet shown as FIG.2. What's the differences between two patterns?

Fig.1

Fig.2

  • 0
    TI__Mastermind 18981 points

    Hi Xiaotian,

    FYI - forwarding the same answer below that we sent over to the other E2E thread

    - link: https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/1019669/drv8305-why-is-the-capacitor-of-the-charge-pump-output-pin-vcph-connected-to-pvdd

     

    Please see responses below to your questions. 

    1. Question1: The manual recommends connecting a 2.2μf capacitor between VCPH and PVDD.
      Customer would like to know is the function to make the charge pump output refer to PVDD? 
      A: Yes, that is correct. In our DRV8305 device, we drive the HS gate by having VCPH = PVDD + 10V for an NMOS
      We need this 10V differential between the gate and source of the HS gate so that it can turn on and drive the FET.

    1. Question2: The customer's understanding is that most of the charge pump output capacitors of the high-side driver chip boosted by the charge pump are connected to the S pole of the high-side MOSFET. Does DRV8305 have the same effect on PVDD connecting capacitors? What are the benefits of doing this?
      A: In the second diagram from your post, the capacitor C_b is actually a 'bootstrap capacitor' like you mentioned
      This means that it is a different gate-drive architecture being implemented.
      The charge pump architecture implemented in the DRV8305 creates a constant PVDD+10V supply to use for the gate drivers.
       

    Thanks and Best Regards,

    Andrew

  • 0 in reply to Andrew Liu
    TI__Mastermind 21805 points

    Hi Xiaotian,

    Please let us know if this helps, or if you have any follow-up questions

    Thanks,

    Matt

  • 0 in reply to Andrew Liu
    Prodigy 41 points

    Hi Andrew,

    Thanks for your reply. It really helps me a lot.
    From your responses to Question1, I can understand the charge pump VCPH refers to PVDD as the power supply for high side gate driver HS. Does it mean the gate of the high side MOSFET will withstand PVDD+VCPH while turning on? If we use +24V for PVDD, the maximum Vgs might be up to 34V and exceeds the safe MOSFET gate voltage level ±20V. How about the risk of gate-source breakdown though there is an internal gate-source clamping circuit?

    For Question2, the Cb between BST(charge pump output) and TS(source pin of the high side MOSFET), is charged by the internal charge pump in Fig2, which supplies +12V voltage for high side gate driver referring TS pin. It's not a traditional boost capacitor circuit in my mind due to the diode D1 and VCC could be removed. Can we convert the usage of the capacitor between VCPH and PVDD in Fig1 to that in Fig2?

    Thanks,
    Xiaotian

  • 0 in reply to Xiaotian Chen
    TI__Mastermind 18981 points

    Hi Xiaotian,

    We will look into these latest questions and will get back to you shortly.

    Thanks and Best Regards,

    Andrew

  • 0 in reply to Xiaotian Chen
    TI__Mastermind 18981 points

    Hi Xiaotian, 

    Please see below response to your questions 

    1. Gate-source voltage of high-side FET
      1. The short answer is: No, the gate-source voltage of the high-side MOSFET should not ever have to endure the full VCPH+PVDD voltage (34V in your example)
        1. The gate driver acts as a current source or resistor to slowly charge the MOSFET gate from 0V VGS to 10V VGS. When the MOSFET enters the miller plateau, the VDS will reduce and the remainder of the MOSFET enhancement occurs when the MOSFET source (SHx) is sitting close to PVDD
      2. The gate driver is designed such that the HS FET's gate-source voltage is limited to <15V by the clamping diode circuit
        1. Refer to section '7.3.5 GHx/GLx: Half-Bridge Gate Drivers' (page 21 in the datasheet) 

    2. "Can we convert the usage of the capacitor between VCPH and PVDD in Fig1 to that in Fig2?"
      1. Answer: No, it is not possible to use Fig2’s implementation in our DRV8305 device. The DRV8305 capacitor is designed as a charge pump capacitor and not as a bootstrap capacitor.
      2. In the fig2 design, it looks like you have a bootstrap capacitor with a charge pump to maintain the cap voltage
        1. Most of the time the charge pump is a “trickle charge pump” which only supplies a small current (uA-range) to maintain the existing charge of the capacitor
        2. We are not familiar with this specific part so we cannot speak to its operation. What is the part shown in the fig 2 diagram? It may help us to understand the circuit better 

    Thanks and Best Regards,

    Andrew

  • 0 in reply to Andrew Liu
    Prodigy 41 points

    Hi Andrew,

    Thanks for your response.

    Please see the complete block diagram for gate driver IC in fig2  as shown below. That's a high side N-channel MOSFET gate driver, using a charge pump and a paralleled capacitor to regulate the BST-TS voltage to 12V.

    Why do we think it's a trickle charge pump in the picture below? And what's the essential difference between two charge pump applications in DRV8305 and showed IC?

    Thank you so much,

    Xiaotian 

  • 0 in reply to Xiaotian Chen
    TI__Mastermind 21805 points

    Hi Xiaotan,

    Without knowing that other part number, I cannot say much about its operation. I guessed it was a trickle charge pump because they are normally found alongside a bootstrap to achieve 100% duty cycle.

    That part is a load switch / high-side MOSFET driver while DRV8305 is a motor driver - they are very different parts. It's a bit like like comparing apples and oranges.

    That IC needs to drive a single high-side MOSFET while the DRV8305 needs to drive six MOSFETs (3 high-side and 3 low-side). The DRV8305 charge pump is capable of driving up to 30mA output current.

    Thanks,

    Matt

  • 0 in reply to Matt Hein
    Prodigy 41 points

    Hi Matt

    Thank you so much. I can't find more details in the datasheet of that IC really. I also get some technical notes to help me understand the concept of "trickle charge pump". As you said, a trickle charge pump is normally used alongside a bootstrap capacitor to achieve 100% duty cycle. The picture(from TI E2E FORUM) showed a trickle charge pump used inside an H-bridge IC that keeps charging the external bootstrap capacitor. It could be found in other half-bridge drivers.

    When the high-side MOSFET turns on, the current from the bootstrap capacitor will charge the Cgate in the picture below. But there is no bootstrap capacitor in the circuit of DRV8305, so the large current for driving MOSFET is supplied by the charge pump circuit directly.

    I'm not sure my understanding is right or not. Is this the main difference between two charge pump usages?

    Thanks,

    Xiaotian 

  • +1 in reply to Xiaotian Chen
    TI__Mastermind 21805 points

    Hi Xiaotan,

    You are right.

    • In the bootstrap & trickle charge pump case (for example DRV8301) the bootstrap supplies the energy to charge the bootstrap capacitor (and MOSFET gate). The trickle charge pump then supplies a small amount of charge to keep the bootstrap capacitor charged.
    • The DRV8305 does not have a bootstrap, the charge pump supplies the energy to charge the MOSFET gate, and subsequent current to keep the gate ON.

    Thanks,

    Matt