I'm trying to pick the best (cost-effective, small footprint, reliable) dc-dc converter, to convert 24V -> 5V for my device. In the future I'm planning to switch to 3.3V.
The device consists of microcontroller, some logic ICs, RS485 transceiver and a few leds.
I need at most 300mA at 5V.
So far I tested three setups:
LM2576, running at 57kHz. 680uH inductor, 330uF input capacitor, 1000uF output capacitor. However, this setup is pretty big. Big inductor, two very large capacitors, and D2PAK converter use too much space on the board. Original IC is pretty expensive and designed for much higher currents (3A) than my needs (300mA).
Mornsun B2405S-1W, integrated DC-DC, 100kHz, 220uF input and output capacitor. It's almost ideal, but the input voltage may not vary more than 10% from 24V. There is also no 3.3V version availible. It is isolated (what I don't need) and quite hard to buy. The datasheet recommends 22uF electrolytic capacitor at the output, which also consumes some space.
RT8259, this was my last try. It's designed to be quite cheap and efficient. It is elastic enough to provide 5V and 3.3V. The datasheet recommends 22uF output, 10uF input ceramic capacitors and 6.8uH inductor. I used 10uH inductor. No electrolytic capacitor was an advantage for me. It runs at 1.4MHz. I used small CD32 inductor and SS31 diode.
The problem is, that with RT8259, I'm getting really unstable output voltage:
To compare - from LM2576:
and from B2405S-1W:
What I tried to do:
I tried adding 100nF ceramic, 4.7uF tantalum caps at the output of RT8259, but there was NO change.
The layout is exactly as in the datasheet.
- How bad is such voltage for powering digital circuits, like my? How can I improve the output voltage?
- If I increase the inductance, should it help?
- What is going to happen when the inductance is to high / to low?