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LM3478MM DRAWS EXCESSIVE CURRENT WHEN Rfa RESISTOR IS CONNECTED

Chandrasekhar Dutt
Prodigy 20 points
Other Parts Discussed in Thread: LM3478

I HAVE SEPIC DESIGN BASED ON LM3478MM. MY CIRCUIT HAS BEEN DESIGNED AS PER THE TEXAS INSTRUMENTS WEB BENCH (DESIGN 3533301/5 LM3478MM). 

MY REQUIREMENT IS TO GENERATE 12V, 3A OUTPUT OVER AN INPUT RANGE OF 10V - 16 VOLTS. MY PROBLEM IS THAT WHEN THE FREQUENCY ADJUST RESISTOR IS CONNECTED BETWEEN THE 7TH PIN OF LM3478 AND GROUND, THE CIRCUIT DRAWS VERY EXCESSIVE CURRENT (EVEN UPTO 1.5A, AS MY POWER SUPPLY IS RESTRICTED TO 1.5A). WHEN I REMOVE THE SAME RESISTOR THE CURRENT DRAWN IS OK, BUT THERE IS NO OUTPUT VOLTAGE. 

CAN ANYBODY PLEASE HELP IN SOLVING THIS ISSUE AND GETTING THE REQUIRED OUTPUT VOLTAGE. 

  • 0
    TI__Intellectual 2930 points

    Hi Chandrasekhar,

    Are you starting with a 3A load? If you want 3A out at 12V you are going to need more than 1.5A input current.

    Power_Out = Power_In *Efficiency     ==>   Current_In = (Voltage_Out*Current_Out/(Voltage_In*Efficiency)

    Assuming around 85% Efficiency you will need an average input current of 2.6-4.25A.

    If you are starting with a load current of less than 700mA and are still seeing more than 1.5A input current draw then I might need to look further into your design.

    Best regards,
    Tommy 

  • 0 in reply to Tommy Jewell - WEBENCH Design Center
    Prodigy 20 points

    Dear Tommy,

    Thanks for the reply. I am very sorry that there was some mistake in my schematic and after correcting the same I could be able to get the proper output. Sorry for the inconvenience. 

    Now I came across 2 new problems. They are explained below

    1) My output voltage 12V is not stable upto 1A load current. It is swinging between 12.03V and 12.36V. Above 1A load current it is stable.

    2) I am generating 12V output and the load current is 2A. When I increase the load current gradully from 0.1A to 2A in steps of 0.05A, the output is OK. But if I apply the load current of 2A suddenly, then the IC is drawing very excessive current and the output voltage drops down to 6.8V and increasing gradually further.

    Please advise me a solution

  • 0 in reply to Chandrasekhar Dutt
    TI__Intellectual 2930 points

    Hi Chandrasekhar,

    Sounds like stability issues. Do you have a finalized schematic and layout I could see?
    There might be layout improvements that can be made, or we might have to tweak the compensation values some.


    -Tommy