• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 477 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

CSD96371 -- using this device to drive a small motor -- application advise

Rick Bogdan
Intellectual 930 points
Other Parts Discussed in Thread: CSD96371Q5M

We looking for deep-dive help for customer interested in using the CSD96371 used as a motor driver ….   See below ..

 


The ½ H-Bridge that I currently have in my schematic is the TI: CSD96371Q5M, but I’m not completely comfortable with using that part in my motor driver application.  This part’s best attribute is its small size.  My biggest concern is I’m using this part in a manner not consistent with its intended use and the data sheet doesn’t provide a lot of detailed information about how it may operate in an application other than a buck regulator.  

My concerns and questions with the CSD96371Q5M are:

  •      I’ll be using it at an operating frequency of 40 kHz, 20% of the minimum recommended operating frequency.  To compensate for this I’ll increase the boost cap by 5 times to 0.47uF.  I’m assuming that this will reduce the droop of the voltage on the boost cap to compensate for the lower rate at which I’ll be charging the boost cap.  Is this a good assumption?  How else should I compensate for using a lower operating frequency?
  •      As the motor commutates at slow speed (3000 RPM using a 4 pole BLDC motor) there will be a 1.67ms period where one of the three output drivers is disabled (EN = low) and its boost cap will not be recharged because I’m not switching the output to ground.  What happens to the voltage on the boost cap during this disabled time?  Does the top driver continue to draw current and discharge the boost cap even though I’m not switching the top MOSFET?  I’d simulate this in spice if I could find a model.  Can you find a spice model for this part?  I couldn’t.
  •      Lifecycle of part, will it be available in 5 years?  Are there alternates that I could use if this part becomes unavailable?
  •      Is this part RoHS compliant by exemption?  If so, is a version available that doesn’t require an exemption to be RoHS compliant?  I’m concerned that the exemption will expire and we’ll need to redesign in a few years.
  •      When calculating the Safe Operating Area (SOA) per the datasheet, I’m led to figure 14 where I’m not sure the output voltage with a max of 5.5V applies to my situation.  When I extrapolate the curve and use 10V, which is my source voltage, I get a max temp of 60°C to be within the SOA of the part.  But if I assume that the output voltage is 5V, which would be my average voltage on the far side of my stator winding (similar to the inductor in the buck regulator) I get 90°C to be within the SOA.  Can you provide some guidance to calculate the SOA of the part in a motor driver application?
  •      What is the Vin quiescent current with both Vdd=0V and EN=low?  The Vin looks like an open drain in Figure 3 of the data sheet so I would expect it to be nearly zero, but the 100uA on page 3 of the datasheet makes me think it could be otherwise.

My requirements for an output driver are:

  •      Qty=3 per motor ½ H-Bridge MOSFETs and associated driver to drive a BLDC motor.
  •      An integrated driver and two MOSFETs would be great, but a discrete driver plus integrated high and low side N-channel MOSFETs in a small footprint would be acceptable.
  •      Each driver requires 2 control inputs:  One for high/low output driven from a PWM signal and another to disable the output drive such as an enable signal.  Both inputs are from a controller with a 3.3 V supply, therefore input high =~2.2V & input low =~1V
  •      Bridge Rail Voltage = 10V and I have a 5V supply available if needed.
  •      As small as possible footprint on the PCB.
  •      Peak current for any MOSFET is 8A
  •      RMS current for any top MOSFET is 4.6A
  •      RMS current for any bottom MOSFET is 5.7A
  •      Switching frequency is 40kHz

  • 0
    TI__Mastermind 18200 points

    Rick, 

    Some good questions. Give me a few days to talk to our applications team and get back to you. 

  • 0 in reply to Brett Barr1
    TI__Mastermind 18200 points

    Rick, 

    Sorry for the delayed response. Let me address your questions one at a time. 

    "I’ll be using it at an operating frequency of 40 kHz, 20% of the minimum recommended operating frequency.  To compensate for this I’ll increase the boost cap by 5 times to 0.47uF.  I’m assuming that this will reduce the droop of the voltage on the boost cap to compensate for the lower rate at which I’ll be charging the boost cap.  Is this a good assumption?  How else should I compensate for using a lower operating frequency? "

    This should be fine. Apps guys don't think you will have a problem. 

    "As the motor commutates at slow speed (3000 RPM using a 4 pole BLDC motor) there will be a 1.67ms period where one of the three output drivers is disabled (EN = low) and its boost cap will not be recharged because I’m not switching the output to ground.  What happens to the voltage on the boost cap during this disabled time?  Does the top driver continue to draw current and discharge the boost cap even though I’m not switching the top MOSFET?  I’d simulate this in spice if I could find a model.  Can you find a spice model for this part?  I couldn’t."

    When the driver is disabled the bootstrap pin (BOOT) is high impedance.  The leakage of this pin is specified in the datasheet (IRBOOT) as 0.15uA typical or 1uA max. Depending on circuit conditions, if the SW pin is low enough the boot cap will be charged from VDD through the internal bootstrap diode.  Otherwise, the leakage on the pin combined with the capacitor size will determine the capacitor voltage during the disabled state.

    I have attached a model for the part. 

    0435.csd96371q5m_pspice_encr.lib

    "Lifecycle of part, will it be available in 5 years?  Are there alternates that I could use if this part becomes unavailable?"

    TI will not end of life a product, so yes, it should be available. 

    "Is this part RoHS compliant by exemption?  If so, is a version available that doesn’t require an exemption to be RoHS compliant?  I’m concerned that the exemption will expire and we’ll need to redesign in a few years."

    Yes, it is RoHS by exemption, but no plan to do an entirely green version at this time. 

    "When calculating the Safe Operating Area (SOA) per the datasheet, I’m led to figure 14 where I’m not sure the output voltage with a max of 5.5V applies to my situation.  When I extrapolate the curve and use 10V, which is my source voltage, I get a max temp of 60°C to be within the SOA of the part.  But if I assume that the output voltage is 5V, which would be my average voltage on the far side of my stator winding (similar to the inductor in the buck regulator) I get 90°C to be within the SOA.  Can you provide some guidance to calculate the SOA of the part in a motor driver application?"

    Looking at your conditions, you will be well within the SOA and shouldn't have any problems. 

    "What is the Vin quiescent current with both Vdd=0V and EN=low?  The Vin looks like an open drain in Figure 3 of the data sheet so I would expect it to be nearly zero, but the 100uA on page 3 of the datasheet makes me think it could be otherwise."

    Is 100uA a problem? Its what we test to but in actuality, the leakage is much less.