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BOOST-DRV8711 Peak Current Rating

Victoria Rodriguez
Prodigy 30 points
Other Parts Discussed in Thread: BOOST-DRV8711, CSD88537ND

Hello from Argentina!.
I have a question about the BOOST-DRV8711 , when study the schematics i note that the Rsense is 0.05ohm in series with each motor winding, so in the datasheet of DRV7711 we have 

ICHOP = 2.75V * TORQUE / 256 * ISGAIN * Rsense , if ISGAIN=5 and TORQUE=255 the total chopper current was 10.95AMP so why the BOOST-DRV8711 is rated for 4.5AMP peak only??.

Thanks!

  • 0
    TI__Mastermind 21320 points

    Hi Victoria,

    I split your post into a saperate thread so others can find it more easily.

    The current rating for the BOOST-DRV8711 is limited by thermal dissipation not the sense resistor. At 4.5A out of each H-bridge you will see that the power MOSFETs are close to their peak temperature rating. If you can add heatsinking or active cooling, you could drive more current.

    We chose a smaller sense resistor in order to use a smaller footprint. A larger sense resistor will dissipate more heat and generally comes in a larger package.

    Let me know if you have any other questions.

  • 0 in reply to Nicholas Oborny
    Prodigy 30 points

    Yes, i have another question. in the pair of mosfet (csd88537nd) only one is turn On at the time (is a half-bridge, no posibility of short circuit) if the termal resistance is 75ºC/W and Ron=12.5mohm (the worst number in the datasheet) the power dissipation was 4.5A^2*12.5mOhm = 0.25Watt so for a ambient temperture (say 25ºC) the final Temp it would be rougtly about 45ºC, my question is.. why you say that are close to their peak temperature rating?

    Thanks!.

  • 0 in reply to Victoria Rodriguez
    TI__Mastermind 21320 points

    Hi Victoria,

    In an ideal world this would hold true. There are several other factors in an actual design.

    1. The Junction to Ambient number is determined from a JEDEC standard and generally used for apple to apple comparisons of devices. System level testing is required for more accurate results. 
    2. There are multiple MOSFETs producing heat.
    3. There are also two power resistors producing heat.
    4. Switching loses and diode loses.
    5. Rds(on) increases with temperature

    http://www.ti.com/lit/an/spra953b/spra953b.pdf

    You can find more information on thermal metrics here.