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LM5088: LM5088 Efficieny for Low Current

Gokhan Sahin
Prodigy 210 points
Part Number: LM5088
Other Parts Discussed in Thread: LM5008, LMR36006, LMR36506

Hi everyone,

I have designed a circuit with LM5088 because I have it but the circuit needn't high current.

And, I want to test this circuit according to the BCI test. To do this, first, I need to design an input filter consisting of a CMC and capacitors.

And, to design it, I have to calculate the input impedance of SMPS. I have an equation about calculating input impedance;

Zin = ((Vin^2)xN)/Pout
N -> efficiency of SPMS

I have connected a load resistor around 470R for 5V output of SMPS. (Pout=53mW)

I'm confused at this point, while 10mA current flows at the output from 5V, approximately 30mA current flows at the input from 27V.

Is this SMPS unstable for low currents?

  • 0
    TI__Expert 3835 points

    Hi Gokhan,

    Please provide schematic and BOM of your design. There are possible components that could be causing extra leakage current (such as resistor dividers). 

    The LM5008 is a buck controller not really design for low load efficiency I would look into a converter for low-load efficiency which has special modes for these specific conditions. Please also provide power specs for any device recommendations.

    Regards,

    Ethan 

  • 0 in reply to Ethan T
    Prodigy 210 points

    Hi Ethan,

    The circuit needs around 400mA at 5V and the input range is 6-50V.

  • 0 in reply to Gokhan Sahin
    TI__Expert 3835 points

    Hi Gokhan,

    The resolution is small and hard to determine some of the values, but the design itself is fine. Calculate the current through the resistor divider on EN and FB pins. The FET also causes some losses due to the gate charge of the device. Lowering the switching frequency will reduce these losses.

    Overall, I do not recommend using this device for such a low load application. I would look at the LMR36506, our newest device for low iq and low-load effeciency. The LMR36006 is similar device that is in production.

    Regards,

    Ethan