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DRV8353: What is the lowest value sense resistor, before Sense Amp bottoms out.

Nick Marsh50
Prodigy 130 points
Part Number: DRV8353

Trying to design a new BLDC drive, but trying to reduce heat.

Current a sense resistor of 0.001ohms is on the schematic, but will it work ?

eg.

A small motor without any loading uses 0.5A ,

V across 0.001R is  0.5 x 0.001 = 0.0005V  0.5mV

Selecting the gain of 40V/V

@0.5A     0.5mV x 40 = 20mV     <---- this is the problem, Does the embedded Current Sense Amplifier output go down this close to the rail ?

@ 5A       5mV x 40 = 200mV

@50A      50mV x 40 = 2V

  • 0
    TI__Guru 65330 points
    Nick,

    Sorry for the delay here due to the holiday. 20mV should be no problem. If accuracy down low is more important then the resistance can be slightly increased or the gain increased but understand that the top end measurement may be effected. I'd recommend example in the datasheet, section 9.2.2.2.1.

    Regards,

    -Adam
  • 0 in reply to Adam Sidelsky
    Prodigy 130 points

    Excellent, the details are here 9.2.2.2

    this is the answer i was after:

    " The linear range of the SOx output for the DRV8353x device is 0.25 V to VVREF – 0.25 V (from the VLINEAR specification)."
    " The differential range of the sense-amplifier input is –0.3 to +0.3 V (VDIFF)."

    See the linear output cannot go below 0.25V  as I thought...

    Either I run the slightly larger motors above 0.5A  or, as suggested by Adam, increase the value of Rsense.

    but this leads me to issues of heat.

    minimum Current 7.5A with 0.001 Resistor dissipating 0.056W Vsense is  0.0075V   using Gain 40    Analog Output = 0.3V only just inside working range.

    maximum Current 100A with 0.001 dissipates 10W  Vsense is 0.1V selecting the lowest gain setting 5V/V   0.5V over range of the input

    this goes to show, that you cannot make a board with this chip that would suit all motors.

    the sense resistor is critical and will dissipate much heat at higher currents.

    Thanks