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I want to design a power supply (buck converter) which converts 9 V to 5 V and supplies it to the LED strip. The LED strip is a basic 1 meter long 70 LEDs/meter WS2812B strip. In my project, I want to set the brightness around 5%.

Basic Schematic from my Problem

The problem: I am confused how to size the buck converter. The total current at 100% brightness is around 4.2 A (60mA per LED) but the average is only about 210 mA and the RMS about 900 mA (Imax * sqrt(D)).

The LEDs change brightness by changing the PWM signal. This means, that the ideal current draw would be a 400 Hz square wave with 5% duty cycle. But if you look closer at the strip, every LED has a bypass capacitor. Also, the buck converter has some capacitance (around 40 uF) and I want to place big bulky capacitors parallel to the output, over 1 mF in total.

I want to simulate/calculate how much current gets drawn from the bulk capacitors by the led strip. This is crucial, because I wouldn't need to design my converter for >4.2 A. For now, I have this buck converter from TI: LM60440, Datasheet

Could anyone help with these calculations or help me simulate it in LTspice? Also, does anyone have tips for selecting a buck converter for this application? Low EMI is important for me, frequency spreading is key here.

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  • \$\begingroup\$ Have you tried TI webench? It will basically size everything for you. If you say within LTspice, I would recommend you find a suitable Analog devices/Maxim part and find the test bench for it in LTspice. \$\endgroup\$
    – winny
    Commented Jul 3 at 14:07
  • \$\begingroup\$ If you use the LM60440, see section "9.2.2.5 Output Capacitor Selection". It has the equations and works through an example of sizing the capacitors for a known load. \$\endgroup\$
    – user1850479
    Commented Jul 3 at 14:51
  • \$\begingroup\$ @winny Yes, i am designing it with Webench currently. It wont let me change the Load to my specifications though. I have tried to edit the Load Transient Simulation but it wont let me add the output bulk capacitors (3x470uF) and the Load doesnt go as low as 0A. \$\endgroup\$
    – xdZapo
    Commented Jul 3 at 16:47
  • \$\begingroup\$ @user1850479 Yes, but the known load is constant. I want to specifically know the max current which gets drawn from the converter. The Bulk capacitors should logically provide a lot of current to the leds. I was hoping that i am able to design the converter with a smaller Max Current than what the LEDS actually need (4.2A Amplitude of Sq.Wave) \$\endgroup\$
    – xdZapo
    Commented Jul 3 at 16:50
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    \$\begingroup\$ Since the PWM on intervals will randomly line up from time to time, you probably want to size the converter to handle the actual max current he load can draw. Otherwise you may randomly glitch when the voltage drops too low. \$\endgroup\$
    – user1850479
    Commented Jul 3 at 21:48

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