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circuit design for automotive application

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david90

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I'm designing a circuit that uses a 555 timer that has a vddmax of 18v. Is it ok to input 12V from a car to run this circuit or should I get a voltage regulator?
 

From the datasheet it says supply voltage from 4.5V. to 16V.
So you can supply with 12V. But in the car, the dynamo will charge the battery up to 15V. I suggest that you have to check the operation is within the requirement along the supply voltage range (12V. to 15V.)
 

say that it is within limit, what can I do about the voltage spike? Will a capacitor in parallel with supply be good enough?
 

Just capacitor could be not enough. Look there how old transistor ripple filter is done :
**broken link removed**
(r1 r2 c1 Q1)

Never worked with car supply bus , but suspect that spectrum of ripple and spike is too broad so C1 must be shunted with some capacitor enough to damp high frequency parts of noise spectrum.
For 100 protection put tvs to Q1 emitter to ground with voltage more than at filter output (defined by R1 and R2 ratio) and low ohmic resistor between this scheme and filter input - to prevent over current through Q1 .
I dont take into account current your device will sink.

Yet you can use coil of course instead of electronic inductor above.
 

Is it better to use linear over switching regulator for automotive apps?
 

david90 said:
Is it better to use linear over switching regulator for automotive apps?
Click to expand...
Use what depending your specific applications and experience. For instant, if your application needs only a few or even up to hundreds miliamperes, so linear is a good choice as it is simple, and easy to build.

For supplying from car power, you should consider the spike as you said, even without spike, car voltage of 14.5...15V is dangerous enough to your chip with maximum voltage of 16V.

Try to use the components with the supply voltage up to maximum 80% of the maximum allowed value. For your case, you can use the 555 at 12V on car power supply, but you need the stepdown regulator, and pay attention to capacitors and/or MOVs/zener to immune the spike.

nguyennam
 

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