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LMZM23600: Long Input Cables

Warren Massey
Prodigy 50 points
Part Number: LMZM23600

I am working on the design of a small board that uses the LMZM23600V5SILR (fixed 5V output) DC/DC regulator. The input voltage is going to be 24VDC ±1VDC. The expected output current may be as low as 10mA or as high as 300mA. The load is predominantly a collection of optoisolators and their resistors. The regulator portion of the design calls for a circuit identical to that shown in the datasheet in Figure 34. (Fixed 5-V or 3.3-V Typical Application Circuit). As per datasheet table 4, the input capacitor is a ceramic 10uF 50V X5R 1206 part. As per table 5 of the datasheet, I had selected a 22uF 10V X5R part but now I see that WeBench prefers that it be a 47uF capacitor. Why the difference?

Section 10.3.2 (Stability) of the datasheet talks about stability problems when the regulator is powered from long cables. The boards using this circuit will indeed be powered over a variety of long cables that in some cases will exceed 300 feet in length (others may only be 10’s of feet). While they are in general multi-twisted-pair 20AWG cables with an overall shield (most are Alpha 5299C), the power and ground conductors going to this circuit are not members of the same pair in the cable. The stability section goes on to say that an electrolytic capacitor with a high ESR should be used in cases like this to help dampen any oscillations that might occur in conjunction with the inductance of the cable. It goes on to suggest a 47uF 0.35-ohm ESR part which would likely need a 35VDC working voltage at a minimum. That type of capacitor is just too big for the space that we have available. We are struggling to fit in a 22uF 1210 50V ceramic capacitor with a 2-ohm resistor in series with it in place of the lossy electrolytic.

 But after thinking about this for a while, it seems to me that if the high-ESR capacitor is in there to provide a lossy RC element to go with the L of the cable, why can’t a low-value resistor be used in series with the input voltage and C1 to do the same thing? Yes it does hurt the overall efficiency of the board because there is a constant DC loss of power across the resistor while the high-ESR cap only dissipates AC power associated with an oscillation if it occurs, but if the resistor-only solution only drives up the total board dissipation by a few 10’s of milliwatts, I don’t think we care.  A single resistor will certainly be easier to fit in. An inline (series) input diode might work as well too.

 Would either a series input diode or resistor work as well for dealing with the possible oscillation caused by a long input cable?

  • 0
    TI__Genius 10695 points

    Hi Warren,

    The Webench recommendation and the recommendation in Table 5 are both valid, it just comes down to whether you want to optimize components sizing or if you want the best performance. Using the 47uF output capacitor recommended by Webench will ensure that your output voltage ripple is minimized, while using the 22uF output capacitor recommended in Table 5 will ensure that your design as enough output capacitance to remain stable.

    Note that capacitor de-rating will make the actual capacitance of the component used less based on the DC bias voltage across the capacitor. So using the 47uF capacitance recommended by Webench is slightly more capacitance than necessary to ensure proper the minimum output capacitance requirement is still met even with de-rating.

    Using a series resistor would work to dampen the oscillations due to the long input cables, the series diode would help protect your power supply from reverse currents but might actually make the oscillations worse on the input due to its junction capacitance resonating with the inductance of the lines.

    Hope this helps.

    Regards,

    Harrison Overturf

  • 0 in reply to Harrison Overturf
    Prodigy 50 points

    Thank you for the reply. I'll see about increasing the output capacitor so that the delivered voltage is better than just good enough to not oscillate. As for the input I'm glad to hear that a small resistor (~3 ohms) will work because it's going to be considerably easier to fit in. Thank you for reminding me of the diode capacitance. For protection against possible line transients, I was planning on including a TVS diode (PTVS24VU1UPAZ) after the resistor (in parallel with the 10uF C1) and I suppose this in-line diode could go after the resistor as well if I want to keep the reverse voltage protection.