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OPA548: Unity Gain AMP / 0.25 ohm load

user5163865
Genius 4120 points

Part Number: OPA548
Other Parts Discussed in Thread: DRV8412

Hi,

Does OPA548 operate stably with Unity Gain, 0.25 ohm load, and DC input?
The customer's application is 3A constant current output.

In the evaluation by the customer, when 0V is input, about 250mA is measured for the load. I don't know the cause.

When I check with PSpice for TI, it looks fine.

If you know the cause, please let me know.

Best regard,

Hiroshi

  • 0
    Guru 140200 points

    Hi Hiroshi,

    10V times 3A means that the OPA548 has to dissipate 30W ! Unless you provide sufficient cooling, the OPA548 will go into thermal shutdown.

    I would massively decrease the supply voltage of OPA548. And installing a dummy resistor within the feedback loop (in combination with a suited phase lead capacitance to restore the phase margin) could furtherly help to decrease the heat dissipation of OPA548.

    Kai

  • 0 in reply to kai klaas69
    TI__Guru* 93105 points

    Hiroshi-san,

    Yes, the OPA548 is unity gain stable and should be stable driving a resistive load Kai's assessment of the power dissipation is the issue of concern because as he concludes it is very high and thermal shutdown is certainly to occur. 

    The OPA548 Safe Operating Area (SOA) information is covered in the datasheet section 10.1.1. Figure 40 illustrates the SOA performance. For example, the Tc = 125°C curve shows that for a 3 Amp output that the maximum difference between the supply voltage (Vs) and output voltage (Vo) should be kept to approximately 3 V. So if the intended output voltage is Vo = Io x RL = (3 A) (0.25 Ohm) = 0.75 V, then applying a Vs - Vo difference of 3 V the maximum positive supply would be +3.75 V. That falls short of the OPA548 +/-4 V minimum supply requirement. If the Vs is made higher, then the power dissipation increases and the likelihood of the OPA548 going into thermal shutdown increases.

    If the Tc can be kept to 85°C or lower the Vs - Vo difference can be increased as the SOA curves indicate. Therefore, good thermal planning and management are necessary if you want to use the OPA548 to provide a continuous 3 Amp output to a 0.25 Ohm load.

    Regards, Thomas

    Precision 

  • 0 in reply to Thomas Kuehl
    Genius 4120 points

    Hi,

    Thank you for your answer. I will give them feedback on your answers.
    The TEC will be driven by the DRV8412 and the temperature will be controlled.

    Best regards,
    Hiroshi