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LMR36015-Q1: LMR36015 light load abnormal power requirement

Part Number: LMR36015-Q1
Other Parts Discussed in Thread: LMR36015

Hi!

I'm using a  LMR36015FSC3RNXRQ1 in classic circuit configuration

Due the shortage of PMIC, we ar try to use a 3V3 regulator with feedback resistors, to obtain a 14Vdc required by our circuit

The output voltage is correct. We have problem with IC power dissipation. Without or with a light load (for example 2K2 on 14Vdc) the power

dissipated by IC is about 1W

We measure, without load, 14Vdc on output, with 30Vdc in input and 30mA current in input! So without load the circuit use about 1W of power

We try to remove the feedback resistors, so we have on output 3V3. But the inrush current is light lower, but still high (about 20mA). So I think the problem

is not on required rising output voltage for the 3V3 device.

We change also the inductor, lowering it to 10uH and 4.7uH... but (about) nothing change in input power used, still without load (and also with lights load)

Rising the load on output, the circuit still work (mantain 14Vdc on output and supply the required current). But the power required in input is still about +1W that output

There is some solution to lower the power requirement (and for this the heat on the IC) ?

Thanks!

  • Hi Paolo,

    I'm a bit confused. Can you please clarify the following: What is your output voltage requirement from LMR36015? 14-V? The feedback resistor divider network shared in schematic does not set the output of device to 14-V.

    Thanks,
    Alejandro

  • Yes, we need 14V on output. The resistor divider is right, because the LMR36015 used LMR36015FSC3RNXRQ1 have a 3V3 fixed feedback. With this divider we have (about) 14V output on prototipe.

    Anyway, to exclude the problem (we want 14V from a device born for 3V3), we try with feedback direct connect to output. The output goes correctly to 3V3, but the input current (PD1) was still high (about 20mA) also without any load.

    This is our problem... the 1W power disspation of device in any configuration

    Thanks!

    Paolo

  • Hi Paolo,

    Understood. We do not recommend to use the fixed variants with a FB resistor network. One of the concerns would be in regards to temperature coefficients between the internal silicon resistors vs the external resistors. The output voltage may drift over temperature. Also, there might be a concern with noise immunity. Our FB node is high impedance so it's noise sensitive. Turning the low-impedance VOUT connection into another noise-sensitive resistor divided node could increase noise immunity issues. Aside from that:

    • A 20~30mA input current at no load is expected. The device still needs to regulate (turn ON) even at no load. For example, see Figure 50 Input Supply Current.
    • The following device variant has a fixed frequency (FPWM) of 2.1MHz.Therefore, at light loads it would still switch at 2.1MHz drawing the same amount of input supply current.
    • To improve efficiency at no loads/light loads, select a PFM variant which reduces switching frequency at light loads.

    Regards,
    Alejandro

  • Thanks Alejandro

    We use the 3V3/FPWM device because was the only device available now. Feedback noise is not a big problem (the output is used to drive relais coil... so we have some voltage range margin)

    Anyway : we tried to attach the regulator to our circuit that draw about 100mA @ 14V

    The chip body goes to 100°C in few seconds (voltage on ouput stable at 14V)

    So seems that the biased 1W required by device in idle, was still present with the 1.4W used by circuit.

    Will be nice to try with PFM device, but they are not available anywhere. And also we have about 80 circuit with this device, ready to solder oven, that will be throw if not soldered in 2..3 days

    I think that this device is unusable (and probaly for this reason was still available on the market)

     

  • Hi Paolo,

    I understand. Unfortunately,  PFM variants are unavailable due to PMIC shortages.

    Regards,
    Alejandro