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LM2574HV: Voltage regulator very hot

She Wai Foo
Prodigy 40 points
Part Number: LM2574HV

I have a thermal issue with a design circuit which I would like some advice.

The input is 24VDC and will step down to 5VDC via voltage regulator and then to 3.3VDC.  I used a DMM and measured that the regulator's input current to be 100mA and the output current is 90mA.

My problem is that the voltage regulator gets very hot, to the point that it will trigger thermal shutdown.

Would this circuit work without a heat sink?

Attached is the circuit for reference.  Any help is appreciated, thanks in advance.

  • 0
    TI__Guru**** 191445 points
    That does not sound right. For 90 mA output, the input current should be about 26 mA. The IC junction temperature should be about 36 C.
  • 0 in reply to JohnTucker
    Prodigy 40 points
    Hi, thanks for the reply.
    Looking back at the example circuits in the data sheet and the webench simulation, I noticed the catch diode is placed before the inductor while in my circuit I placed the diode after the inductor.
    Would this cause some sort of problem? perhaps some sort of short circuit?
    Thanks
  • 0 in reply to She Wai Foo
    TI__Guru**** 191445 points

    The catch diode needs to be placed with the cathode at the SW pin and the anode to GND.  I doubt that the circuit will work properly other wise.

  • 0 in reply to JohnTucker
    Prodigy 40 points
    Thank you very much! After placing the catch diode as you mentioned, the input current dropped and the regulator is no longer burning hot!

    I want to make sure I understand this correctly...
    In my previous circuit, when the switching regulator is in ON mode, the diode will be in forward mode. Therefore shorting the inductor to the ground which causes the regulator's input current to increase?
    When the switching regulator is in OFF mode, no current is being delivered to the circuit because the inductor is not connected to anything?
    Thanks
  • 0 in reply to She Wai Foo
    TI__Guru**** 191445 points
    LM2574HV is a nonsynchronous converter. There is an internal high side switch and the external catch diode. As a buck regulator, the output at the SW pin is a pulse train whose average value equals the output voltage, so it operates at a duty cycle equal to Vout / Vin. During the high side on time, the SW node is approximately equal to Vin and the voltage across the inductor is Vin - Vout and the inductor current is ramping up at a rate (Vin - Vout) / L, supplied by the input source. The catch diode is reverse biased. When the high side FET is turned off, the inductor current must remain continuous and now flows thru the catch diode which clamps the SW node pin at one diode drop below ground.