Powering Atmega, DAC and Bluetooth module in car

[sidlauskas]

Hi,

I'm looking for a proper power supply schematic which would fit my needs and would have all required protections for use in car. I want to power Atmega 328p (3.3v@8MHz), PCM5102 and CSR64215 bluetooth module with it from the car's headunit (cd changer output). They all should be powered from 3.3V. I guess it would be great to isolate DAC from the rest as well to achieve as low noise at the ouput as possible. Any suggestions?

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[grifftech]

this is low noise, but not very efficient (<30%): http://www.ti.com/lit/ds/symlink/lm1117.pdf

[sidlauskas]

this is low noise, but not very efficient (<30%): http://www.ti.com/lit/ds/symlink/lm1117.pdf

As far as I know in cars LDO's should be able to suffer up to 60V peaks as various strange stuff could happen in the car, so I guess this one is not a good choice for my application or at least I have to implement a proper front-end stage to protect it from potential harms.

Ok, let's say the LDO part is clear enough for me. What is not clear for me is the front-end part of the circuit where the LDO and the rest of circuit including Atmega, Bluetooth an the DAC should be somehow protected from the harms in the car (overcurrent, overvoltage, reverse voltage, etc..). I already managed to burn Atmega and Bluetooth module while powering them from LDO with the few input/output caps and without any protection stage. Any thoughts?

[Lt_Flash]

Just use Zener diodes for overvoltage protection, P-MOSFET on input for reverse voltage protection and some PTCs or even fast fuses on input as the cheapest solution for overcurrent. Also I'd recommend to use switching power supply as LDO converting from 12V to 3.3V would have to dissipate A LOT of heat, especially under load. If you don't need more than ~1A of current - use TPS6300X or TPS6120X (I prefer fixed voltage ones - less components), if you need a lot of current - use LM3152MH with appropriate MOSFETs - you can basically choose any current you want. Also use a normal automotive fuse before your board - just in case.

[Benta]

Just use Zener diodes for overvoltage protection

A load dump would fry those in milliseconds. Use an automotive specified voltage regulator, there are plenty around.

[Lt_Flash]

Just use Zener diodes for overvoltage protection

A load dump would fry those in milliseconds. Use an automotive specified voltage regulator, there are plenty around.

LM315X is more than enough, it can handle up to 42V. I meant - Zeners on data pins, if required, didn't think to put them on power rails of course.

[sidlauskas]

So I've decided to use DC-DC switching regulator for the first stage and multiple LDO's at the end which will bring the voltage down to 3.3V and power rest of the circuit. For the first stage I chose BD9778F switching regulator with built-in power MOSFET which features wide input voltage range (7V-35V). For the end part I'll probably use AMS1117 LDO's. I already draw the schematic for the first power stage and would like to get some feedback from you if possible. Thank you!