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DRV8402 - Difference between Half anf Fullbridge Mode

wolfgang weinmann
Intellectual 390 points

Hello,

the datasheet-section of the "Differet Mode Operations"  contains the confusing statement:

In full bridge and half bridge modes, PWM_A controls half bridge A, PWM_B controls both half bridge B, etc.

 

Figure 6 shows an application example for full bridge mode operation.

What does this mean?

I interprete this sentence in the way, that at full bridge mode AND half bridge mode each Terminal Out (Out_A, Out_B, Out_C, Out_D) is fully independly controlled by its dedicated input-control-pin (PWM_A ... PWMD) in the manner

PWM_A = L --> Out_A = Gnd

PWM_A = H --> Out_A = Vdd

PWM_B = L --> Out_B = Gnd

and so on

But: If this is true, then where is the difference of half and fullbrigde control. It seems, that the controling logic of both modes is identical?!?!?!?!

Regards Wolf 

  • 0
    Intellectual 390 points

    Hi Wolf,

     

     This is just my take on the documentation as I have not used the chip yet though we are planning on it.  There seem to be typos and possibly some errors in the diagrams as well but I understand the funcitonality to be that

    Full bridge mode :

    PWM A - controls outputs A and B with output A being V+ and B being V-.  PWM B would be at ground in this direction.

    PWM B - control outputs A and B with output B being V+ and A being V-.  PWM A would be ground in this state.

    Same logic for PWM C and D relative to outputs C and D. 

     

    Parallel bridge mode

    PWM A  : controls both output A and B but with same polarity. V+ when high and V- when low

    PWM B : controls both output C and D with same polarity

     

    Half Bridge mode

    Individual control over the outputs A, B, C and D using the individual PWM inputs A, B, C and D.

     

    If this is wrong, could someone from TI please say so.

     

    Cheers

    Peter

     

  • 0
    Prodigy 185 points

    Hi Wolfgang.

    I've used a similar lower current device, the L6205, to drive two motors.

    The documentation for the L6205 is a bit confusing too.

    The L6205 has variations in the way the current sensing is done for full and half bridge, but this isn't relevant for the DRV8402.

    One thing that can be handled differently between full and half bridge modes is what to do when the half bridges are floating, particularly with regard to the boot strap capacitor(s).

    (The L6205 only has one bootstrap capacitor)

    I'm thinking the answer to your question is in the paragraph  "device reset" on page 11.  (And also "powering up" on page 9)

    I'm thinking that the ONLY  thing that mode pin M3 does is to activate the pull down resistors, these are shown on page 5 as "FB/PFB configuration pull down resistor"

    These resistors allow you to precharge the bootstrap capacitors when the device is in the reset state.  Sounds like a neat idea when you say it quickly.

    But in reality you would always expect to have all the outputs at 0v when the chip was reset, which gaurantees a precharge on the bootstrap capacitors.

    And even if you forget to do this, the "automatic bootstrap undervoltage circuit" would cut in and momentarily blip the output to ground.

    Hope this helps.

    Cheers, BobT