1
\$\begingroup\$

I have a LCD driver board and it uses LM2576 for voltage regulation/step-down. As being cheap ebay/aliexpress purchase the instructions state 12V DC input without specifying range. Datasheets I found for LM2576 state unregulated input voltage can be up to 40V.

So the question is whether the LCD driver would work in car environment where 12V is really something like noisy 12-15V. Any recommendations for extra components like capacitor before the board?

Edit: Managed to find more specifications of the board (PCB800099 was the keyword) and there 5-24V is mentioned.

\$\endgroup\$
  • 1
    \$\begingroup\$ The power environment depends on whether you are on the switched (not too bad) or unswitched (horrendous) power. \$\endgroup\$ – Peter Smith Jan 29 '18 at 9:50
3
\$\begingroup\$

Unless you have the schematic of the board, or make a bit of reverse-engineering, you can't assume there isn't something else than just the LM2576 connected to the input supply. There are at least the input capacitor (which has a rating most likely well under the 40V), and there may be some additional monitoring circuitry, or protection circuitry on the board too.

So the only thing you can trust here (assuming you can trust something from a cheap ebay/aliexpress product) is what it tells you in the description: 12V.

Moreover, the battery voltage in a car is much more unreliable than what you seem to think. It is not guaranteed to be within 12-15V. It can actually go up to 100V peak during load dumps. So I would choose components that can take at least 20-30V for normal operation, coupled with some beefy protection that will be able to clamp or disconnect any higher input voltage, up to 100V. This could eventually be implemented with an intermediate circuit between the battery and the LCD board you selected.

There are a lot of documentation material available about automotive load-dump protections. Here are a few application notes, for example:

\$\endgroup\$
  • \$\begingroup\$ So I think RBO40-40T or similar would be a good start? \$\endgroup\$ – Petteri Hietavirta Jan 29 '18 at 10:27
  • 1
    \$\begingroup\$ Something like that, yes. But this one clamps at ~35-40V so make sure you choose a board that can really stand 40V. If, as the description says, your board only withstands 12V, it will fry. So, either choose another board, or design a 12V regulator that will be able to stand 40V (at the current you need) and add it between the power source and your board, or design some circuit that will disconnect the power source when the voltage is greater than 12V+tolerance (if you go this way, you need the monitoring to be very accurate - a zener clamp won't do). \$\endgroup\$ – dim Jan 29 '18 at 14:57
  • \$\begingroup\$ I managed to find more info of the board and 5-24V is the input voltage. So a fuse and RBO40-40T with additional clamping should be fine. \$\endgroup\$ – Petteri Hietavirta Jan 29 '18 at 20:55
1
\$\begingroup\$

As you mentioned LM2576 is really for ranged input voltage. Be careful as there are different possible configurations. If you look on the oficial datasheet: Datasheet You can see that according to the voltage you want you have different configuration. In you case you need a stable 12V as you mentioned.

As for Cin / Cout you are right, these capacitors are important. They do the EMI function. Explenation. Another rason to use them is in order to prevent auto-oscilation(Not sure if this is the way you call it in English). The values should be high.

In your case TI already gives an example so you can choose the ones they give, however try to buy the ones which can work in wider V range, for example till 150V.

Hope this is useful.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.