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DRV8908-Q1: DRV8908 to drive 4 Peltiers

Takuchi
Prodigy 185 points
Part Number: DRV8908-Q1
Other Parts Discussed in Thread: DRV8830

Hello,

I want to use this motor driver to drive my 4 Peltiers (rated 5V @ 1,5A) 

Can I use the DRV8908 in Parallel mode (i.e 2OUT to drive 1 Peltier) to drive my Peltier at 75% continuous without the device going into thermal shutdown?

Or should I use the DRV8912 (i.e 3OUT to drive 1 Peltier)?

Or, can you recommend some appropriate part please. We are really flexible, for example, 4 pieces of DRV8830 can also work?

We don't have PWM output from our MCU so something with SPI controlled and Internal PWM is required.

Thank you very much in advance.

  • +1
    TI__Expert 8535 points

    Hello Takuchi,

    The formula to estimate the device junction temperature is the following:

    If we consider the DRV8908 then we have to first estimate the total power dissipation, P_TOT. If we consider the case where your main supply is 13.2 V (V_VM in the datasheet), the PWM frequency is 2 kHz. If you are driving the Peltier’s at 75% capacity continuously, then the switching losses can be calculated as follows:

    N_F is the number of half bridges, so if we are using two outputs to support a current of up to 1.5 A, N_F would be 4. Since there are only 12 half bridges on on drv8908 IC, two IC’s are required to drive four peltiers. If we are driving 2 peltiers then we need 8 half bridges. Plugging everything in gives. The rise and fall times are 9 us:

    P_SW = P_SW_RISE + P_SW_FALL = 0.268 W

    The other source of dissipation is from the On-state resistance.

    Plugging in values

    P_FB_CONT = (.5625)^2 *(0.75/2 + 0.75/2) = 0.237 W

    P_DRV = 8*0.475 = 1.90 W

    The last source of dissipation is due to the quiescent current, which can be calculated using the following expression:

    If we assume the V_VM and V_VDD are 13.2 V and 3.3 V respectively and the associated currents I_VM and I_VDD are both 3 mA, then

    P_Q = 49.5 mW

     This results in

    P_TOT = 2.218 W

    If we assume the ambient temperature, T_A to be 25 °C and from the datasheet we know that for a four-layer PCB with 2 oz copper with 16 cm^2 copper area, the R_ΘJA to be 22 °C/W

    T_J = 25 + (2.218 * 22)

    T_J = 73.8 °C

    The typical temperature for shutdown is 175 °C. Consult the graphs on page 138 to find a R_ΘJA that is closest to your board design.

    Here are some considerations:

    1. If you use the drv8908 or the drv8912, you have to use two IC’s since you have to parallel outputs to drive the peltiers and there aren’t enough outputs for four peltiers
    2. The calculation above is only an estimate for two peltiers on the drv8908 and is really dependent on-board design
    3. The drv8908 and drv8912 both have a maximum PWM frequency 2 kHz. The drv8830 can go up to 44 kHz. Is 2 kHz enough for your design ?

     

    Best,

    Keerthi Kumanan

  • 0 in reply to Keerthivasan Kumanan
    Prodigy 185 points

    Hello Keerthi,

    I think for Peltier Element, the higher the frequency, the better.

    Do you think 1 DRV8830 can drive 1 Peltier?

    I also have 5V input so the losses can be a bit less?

    Best,

  • 0 in reply to Takuchi
    TI__Expert 8535 points

    Hello Takuchi,

    I don't think the DRV8830 can drive one Peltier as the abs max DC current is 1 A, and the min on the overcurrent protection setting is 1.3 A. You could try lowering the current to be less than 1A, but other than that if an internal PWM is required then the only available options are the 2 kHz. You can't really use two drv8830 to drive one Peltier either as you would have synchronization issues because of the internal PWM. For 5V, the losses would be a little lower.

    Best, 

    Keerthi Kumanan

  • 0 in reply to Keerthivasan Kumanan
    Prodigy 185 points

    Hello Keerthivasan,

    Thank you very much for your help.

    We decided to use several PWM generators and changed to the DRV8873S.

    Thank you very much.

    Best Regards,