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LM3478 design

Joao Figueiredo
Intellectual 335 points
Other Parts Discussed in Thread: LM3478, INA168

I'm developing a SMPS and I need the following features:

- Vin: 20~30Vdc;

-Vout: 32V;

- Ioutmax: 7A;

Using the Wbench tool, I got a circuit using the LM3478 chip. As I studied its datasheet, I realized that the overcurrent protection cuts off the output switching signal until the next cycle.

I guess this is supposed to work like a hiccup protection (when the output voltage is pulsed, because the output switching sinal turns off, and then try to turn on again, and so it goes...)

So I would like to know if it's possible to implement a timer between these pulses, is there some way I can do this with this chip?

If it's not possible to do this timer with this device, do you recommend me some other design?

Regards,

João P. B. Figueiredo

  • 0
    TI__Expert 6245 points

    Hello Mr. Figueiredo,

    Current is limited in two steps:

    1)      The power device should be turned off whenever maximum current sense voltage exceeds ~180 mV. Since there is a blanking time at the beginning of each cycle, the power device will always turn on for ~325 ns during each clock cycle unless the short circuit current sense threshold is exceeded. See Figure 30 in the datasheet.

    2)      If current sense voltage exceeds ~340 mV, short circuit mode is activated. Once activated, 4 clock cycles are skipped entirely, reducing the minimum duty factor. (Also shown in Figure 30 in the datasheet.)

    Is the second step what you are looking for or do you want to modify this scheme in your circuit.

    Regards, Robert Blattner.

  • 0 in reply to Robert Blattner
    Intellectual 335 points

    Hi, Mr. Blattner.

    Thank you for your support with precise information.

    1) Still related to the current limit value, the design I got from the Webench tool is shown below:

    It's not clear to me how to calculate the Rsense resistor (3.7mOhms @ 10A). Can you help me to come to this value according to the situation above?

    2) Unfortunately, even in the second step you described in your answer (in which 4 clocks cycles are skipped) the output voltage would oscillate in a frequency considered too high for the device I'm powering.

    The approach I intend to use is a high-side measurement current shunt monitor chip (eg INA168) which provides a voltage to a comparator that triggers a timer chip (eg 555). Then, by applying some voltage (555 output) at the FA/SD pin, I would have a over current protection associated with a programable cutt-off time (that is what I need in this case).

    Can you see any impediment or malfunction aspect with the approach I described you above?

    Regards,

    João P. B. Figueiredo

  • 0 in reply to Joao Figueiredo
    TI__Expert 6245 points

    Hello Mr. Figueiredo,

    Concerning Webench’s calculation for RSEN, RSEN is chosen to guarantee that maximum current can be delivered under worst case conditions.

    Step 1)  Calculate peak current under worst case operation:

    Step 1a) Average current is Iout/(1-D) where D is 1-Vin/Vout. Worst case is at lowest input voltage. Also, I added 0.5 V to the output voltage to account for diode drop and subtracted 0.2 V from input voltage to account for drop across the FET. I get 16.4 A as maximum average inductor current.

    Step 1b) Current ripple is Vin*D/L/frequency. Again 0.5 V is added to output and 0.2 V is subtracted from input. Again, lowest input voltage is used for this calculation. I used 100kHz as your frequency. With 20 V input, peak to peak ripple is approximately 5 A.

    Step 1c) Peak FET current is Iav + Ipkpk/2, with 20 V input, peak current will reach 18.9 A.

    Step 2) Calculate RSEN

    Step 2a) Using Equation 19 from the most recent datasheet – available at TI.com, RSEN = VSENSE/Ipk*(1-D*VSLRATIO). Using minimum VSENSE and maximum VSLRATIO from the EC table, (w.c. VSENSE = 0.125 V and w.c. VSLRATIO is 0.7) maximum RSEN to allow 10A operation is 4.81 mOhm.

    Step 2b) Allowing 30% margin for dynamic operation and variation in passive components, the maximum RSEN is 3.7 mOhm.

    I have checked to startup from shutdown and found the parts starts up normally with soft start.

  • 0 in reply to Robert Blattner
    Intellectual 335 points
    Hi, Mr. Blattner

    I would like to thank you for your nice explanation.
    Now it's clear the choice of this value for this shunt.

    Regards,
    João P. B. Figueiredo