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LM350T adjusting multiple regulators with single control

Kevin Marcy
Prodigy 20 points

I am using several LM350T voltage regulators in a power supply circuit as shown on page 11 of the data sheet. The goal was to have 7 independent outputs, all at the same voltage adjusted by 1 pot. What seems to be happening is if one of the outputs encounters a heavy load/short circuit, all of the outputs shut down. It also seems that something is causing the voltage to fluctuate under "normal" loads (maybe noise?).  The loads are small DC motors (HO slot cars), less than 1/2 amp each.  I'm quite sure that I'm not exceeding the capacity of the transformer/filter cap feeding the circuit.

Is there any way to configure the circuit so that a load on one output will not affect the others?  Or is this a noise problem?

  • 0
    TI__Mastermind 22610 points

    On page 11 of the present LM350 data sheet I see "Figure 13. 10A Regulator".

    On page 11 of an older (i.e. pre-TI) LM350 data sheet I see "Adjusting Multiple On-Card Regulators with Single Control"

    I will presume that you are referencing the 'pre-TI' version of the datasheet.

    In this configuration the LM150 on the far left (with R1 from Vout pin to common ADJ connections) would be considered the "Master", and all other regulators are 'slaves'.

    The 'Master' creates a constant current through R1 equal to Vref/R1. This constant current flows through R2 to Gnd and creates an offset voltage between the ADJ pin and Gnd equal to R2*(Vref/R1).

    This offset voltage is applied to all the ADJ pins. Since Vout = Vadj+Vref, all the Vouts track.

    But only the MASTER regulator is in a closed loop condition.

    If any one of the slave regulators output is pulled more than a diode below the ADJ voltage, the diode will conduct and the common ADJ voltage will be affected and ALL of the regulators will track the disruption.

    Elimination of ~noise~ depends on the source of the noise. Adding some capacitance to each of the Vout pins should provide the best improvent as I'm guessing that noise would be from the DC motors. Adding capacitance across R2 might provide some relief under most normal conditions, but not for over-load or shorts. Adding these capacitors will require adding the requisite protection diodes.See "FIGURE 3. Regulator with Protection Diodes"

     Elimination of the output voltage disruption during over-load, or output short, is not possible.

  • 0 in reply to Donald Jones
    Prodigy 20 points

    Thanks for the info.  Looks like my best fix is to go back to fixed regulators.  The adjustable voltage was really just a "luxury item" anyway.