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DRV8701: Quiescent current and heat Issue

Janos Peter Toth
Prodigy 10 points
Part Number: DRV8701

Hi team,
I made a motor driver which is almost identical to the reference design. (see schematics) The circuit works fine except it's quiescent current is much more than expected. Quiescent current was determined by subtracting  the current consumption in sleep mode from the active mode. No load, EN and PH pins driven to low. The calculated current is 16mA. (VM=24V) Obviously 24V*0.016A=384mW generates some heat. The catalog data shows 5mA(TYP) 9.5mA(MAX) quiescent current consumption. VM is a 70um inner layer plane, GNDPWR is a 70um bottom layer plane.
Q1: What can cause the abnormal current consumption?
Q2: Using R507, R508, R511, R512 resistors is a good practice, useless, or bad?
 

  • 0
    TI__Mastermind 19010 points

    Hey Janos,

    Janos Peter Toth said:
    Q1: What can cause the abnormal current consumption?

    It could be various small loads such as pull-up resistors, and maybe the resistors you mention across the FETs.  

    Janos Peter Toth said:
    Q2: Using R507, R508, R511, R512 resistors is a good practice, useless, or bad?

    Should not be necessary here if you chose your MOSFET's QGD Miller charge and IDRIVE settings appropriately.  I would remove them and see if it helps your current consumption. 

    Just curious, is there a reason not to put the device in to low power sleep mode to use a miniscule 9uA instead? 

    Regards,

    Jacob

  • 0 in reply to Jacob Thompson
    Prodigy 10 points

    Hey Jakob,
    thank you very much for your posting.
    I am afraid that the current drawn by 3-4 pull-up resistors is far below 1-2mA. And we are talking about 16mA. I had removed the gate-source resistors before I asked the forum for help. Nothing changed. It was expected as the gates were not driven.
    Putting the device into sleep mode is not an option for me. Whenever the motor works - and it works nearly continuously at different speeds- the driver must be in active mode. However, the heat sink  can handle the generated heat during operation, even the heat of the higher quiescent current.
    I just want to understand what is the reason of the higher current.
    Yesterday I tried to measure the VM current flowing into the VM pin of the chip by cutting the track between Pin 1 of the chip and the VM pin of C505. Putting the amper meter between the two points yielded more than 50mA! This means for me that the length of the track and the capacitor between VM and GNDPWR has significant effect on the quiescent current. Since the track is very short, I will try different capacitors, whether using a lower ESR capacitor results in a lower quiescent current. 
     Best regards,

    Janos

  • 0 in reply to Janos Peter Toth
    TI__Mastermind 19010 points

    Hey James,

    After further investigation on our side, I have found that you are correct.  Our testing for this chip did not have any FETs installed on the board when power consumption is measured.   Our measurements agree with your findings for a board with FETs installed:

    • nSLEEP is 1: Enable=0%: 14.2mA

    • nSLEEP is 1: Enable=50%: 17.6mA

    We are working on updating the datasheet to add a row for current consumption WITH the FETs installed.  This is the spec that should be stated, since you can't use a gate driver without FETs.  

    If you need lower current consumption you can move to the DRV8705

    Regards,

    Jacob