# How should I power these components at different voltages?

I'll be using a 12V power source, as this is an automotive application.

I have the following components:

• ATmega2560, 1.8V-5.5V, 200mA (actually an Arduino Mega 2560, as I couldn't find a through-hole AVR with four UARTs)
• GPS module, 3.3V-5V with 5V-safe inputs, 20mA
• GSM module, 3.4V-4.5V (4.0V is recommended), 3.3V logic, 2A peak
• MCP2515 CAN controller, 2.7V-5.5V, 5mA
• MCP2551 CAN tranceiver, 4.5V-5.5V, 75mA

Everything would be a nice, easy 5V if it weren't for the GSM module, which wants around 4V. So should I use two regulators (a 5V one and an adjustable one set to 4V)? Or if I already have a 5V regulator, is it possible to just use a voltage divider after that for the 4V?

I plan to use a 4050 for level conversion from the MCU to the GSM module (the MCU at 5V reads 3.3V as high).

Would I also require a heatsink? The Arduino's regulator doesn't have one, but it gets hot given a 12V supply.

• It looks like all the parts you list are in spec with 4.5 V supply. If this is for a hobby project, why not just use a 4.5 V (say, +/- 1%) supply for everything? – The Photon Jun 22 '13 at 17:59
• It's not a hobby project, it's a prototype for a production project. Is it okay to constantly drive the GSM module at the top of its supply range? And how would I derive the 3.3V needed for the 4050? – Isvara Jun 22 '13 at 19:40
• For a production project, I think I would use separate supplies --- because no supply will be exactly 4.5 V to satisfy the specs of all your subsystems. Or search for an alternate for either the GSM module or CAN transceiver so that everything works together. – The Photon Jun 22 '13 at 19:50

I have similar project.

I did it with two voltages. First I put the switching regulator to drop to 3.9V for GSM. Then LDO to drop from 3.9V to 3.3V for MCU, GPS, CAN.

I see the GPS module you pointed out possibly has it's own LDO on board and that's why its voltage ranges from 3.3V. Take a look on those modules alone http://www.gtop-tech.com/en/category/GPS-Antenna-Module/A01_MT3339.html they all go from 3.0V so both voltages could be used (3.9V or 3.3V).

For CAN tranceiver I used SN65HVD230D from Texas Instruments, which is a little more expensive but works with 3.3V (that eliminated necessity of third 5V supply).

Don't use voltage dividers for supplying power. The load connected to this divider acts like another resistor in parallel with one from divider. If the load you connect is mcu or some module, the impedance is unknown and variable in time - you wont be able to achieve desired voltage.

Voltage dividers are good if you connect some high impedance line to it. Then almost no current flow through that line and the voltage at the divider stays at the level it was set. This should be fine in case you need to lower some logic levels (you mentioned 4050 which is a good solution).

You might also need some protection circuit at the power input, before the voltage regulator. Automotive power can have load dumps (100V on power line for about 400ms), positive and negative voltage transients. Cold crank condition can lower the voltage to 3V for 15ms. Jump start can raise it to 24V.

I would use a 5V switching DC/DC converter with a TVS diode and a capacitor in front of it to guard against load dumps on the 12V rail. You then have the option of an adjustable LDO (with > 2A capacity!) from the 5V rail (which would put more load on the 5V DC/DC converter, too) or a separate 4V switching DC/DC converter with similar TVS and capacitor protection.

All the 5V things can easily be driven by a single Murata OKI-78SR, which is a TO-220 form factor switching DC DC converter that generates 5V/1.5A out, and costs < $5 in quantity 1. (Sweet!) The 4V is harder. 2A is a fair bit of current. You can burn it all off from 12V with a LM350 adjustable regulator (3A rated) with a heat sink. Or you can design a switching regulator on your own that targets 4V out. Or, if this is a large-quantity design, talk to suppliers like GE or Murata or Recom to see if they can make a custom part for you, so you don't have to. Another option is to get a 3A+ converter to get 5V out (they range about$40 up from DigiKey in quantity 1) and linear regulator down to 4V. Check out the CUI VYB10W-Q24-S5-T. Note that you'd need a LDO regulator with drop-out at 1.2V or lower to get to 3.8V. Finding one of those that can do 2A is a little harder. Try something like a MIC29302AWU perhaps, realizing that it only goes up to 16V for input, which a car 12V rail can temporarily go way above (see above about load dumps.)

All the ASIC's whether it be Atmega, GSM Module, GPS Module, CAN Transceiver and Controller will come with the ratings of MIN, MAX and TYPICAL supply. But, for the proper and best functioning of any of the modules or ASIC always feed the TYPICAL supply input mentioned in the datasheet.

If its some industrial design, don't use dividers to feed power supply to ASIC's or modules, as it may not deliver the expected power.

Go with the power consumption analysis of the system and design your power supply using LDO or DC-DC Converters.