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TPS54531 (Vin = 24v Vout = 7.4v) Load is a servo motor HS-7950TH

Eric Gu
Prodigy 30 points
Other Parts Discussed in Thread: TPS54531

Duro_Main_Power_2.0.pdf

We built the circuit as shown on Page 5 of the Attached PDF. The circuit uses the TPS-54531 to allow us to have a constant output of 7.4v. We applied 21.7v and with no load we get an output of 7.9v. However when we attach our servo (HS-7950TH) the voltage drops to 2.06v and the servo doesn't react. The servo is being controlled by an arduino for testing purposes, we programmed the basic sweep from 0 to 180 sketch.

Additionally we attach a Strobe LED and two other servo motors and both had the same results. We also attached a 2 M-ohm resistor to see if high impedance was the issue but the output was a constant 7.9v. When ever we attach a Variable Load like a strobe led or servo motor the output simply drops to 2.06v and the load doesn't react.

We simply want to provide a constant 7.4v supply to our servo motor (HS-7950TH) from a 24v battery. How can we achieve that with our current setup? 

  • 0
    TI__Intellectual 2795 points

    Did you tried larger pure resistor like 4.7ohm or E-load with different load? Please hlep capture some wave include PH and VOUT.

  • 0 in reply to Milo Zhu
    Prodigy 10 points

    We have tried attaching a 100 K-ohm and a 4.7 ohm. With both resistors we get 7.9v as the output.

    Also after leaving the servo attached, after about 3-5mins the TPS54531 shorts out internally between the pins VIN and PH. After replacing the chip with a new TPS54531 we let the circuit sit for about an hour without any loads attached. The circuit remained fully functional without any damage, only when we attach the Servo motor does the chip short out.

    Additionally we have tested the servo by it self using a bench dc-supply and it functions perfectly fine.

    Attached are images of the waves at PH (blue wave) and VOUT (yellow wave).

  • 0 in reply to Mohammed Akram
    TI__Intellectual 2795 points

     After replacing the chip with a new TPS54531 with servo, the damage " shorts out internally between the pins VIN and PH" can be repeated, right? With servo, how about the inductor, very hot or not becaues have some concern the power on inrush current of servo should be very large, any possbile touch the current limit or inductor saturation to make it short or you can check the inductor current?