DCDC converter and LDO for Embedded System - Page 1
Forum section: electronics / projects

author: maurosmartins
author_rank: Regular Contributor
timestamp: July 15, 2013, 10:03:42 am
content: Hello all,On a current project tha I'm working on, I need my board to be battery operated.The board consists of a zigbee module (with microcontroller) and a GSM modem.While the module has very low consumption that is not the case of the modem. I'm using Li-SOCl2, 3.6V lithium primary cells and I'm afraid that the voltage of the cell gets to low (for example due to ambient temperature) and won't be able to activate the modem (minimum 3.3V). Because of this I want to use a higher voltage (cells in series) and for that I need a Voltage regulator.I would like to use a DCDC converter and disable it while the modem is not needed and enable it only when I need to transmit, This way, I would reduce the No Load Current of the DCDC from about 7mA to 80uA.But.... How do I keep the microcontroller powered (in sleep mode)? I thought using a LDO just to power the microcontroller and having a pin toggled to activate the DCDC converter to enable it, and use Diodes or preferable Mosfets to "OR" both supplies.Would this be a good solution? could anyone provide other alternatives or tell me how this should be done?Best regards, Mauro.

author: ovnr
author_rank: Frequent Contributor
timestamp: July 15, 2013, 10:20:05 am
content: How much current does your GSM modem require (peak)? I've heard they tend to guzzle up a lot of current (couple amps) when transmitting.Running the microcontroller off of a LDO (from, say, a 2-cell pack) while the power-hungry peripherals run off a separate regulator is no trouble at all. You do need to take care to set all pins going into the powered-off region of your circuit to ground or tristate (input mode); if not, you'll be trying to power the GSM module with the microcontroller IO pins, which will probably cause all kinds of issues.I should note that you can get very low power DC/DC regulators too - theLT3995will keep its output in regulation while only consuming 3 microamps, and it can deliver up to 3 amps when running flat out. Compared to some older LDO designs, the DC/DC option can actually save power.Edit: I should also point out that if you really want to stick to a single cell, you can use a buck-boost converter to run either the whole mess (micro and GSM modem), or just the GSM Modem. It will however involve a higher quiescent current - on the order of 40-50 microamps. Getting high output currents is also trickier, but should be doable.How critical is battery life for your project?

author: amyk
author_rank: Super Contributor
timestamp: July 15, 2013, 10:42:06 am
content: Quote from: ovnr on July 15, 2013, 10:20:05 amDepends what power level is being used, which varies with distance from the associated tower, but 2-3A peaks in short bursts are common. Good decoupling is essential.

author: maurosmartins
author_rank: Regular Contributor
timestamp: July 15, 2013, 10:55:14 am
content: Hello ovnr,thank you very much for your reply.My modem consumes from 2 to 2.8 Amps but these are almost pulse currents, not constantly drawing such high current.I appreciated the info about the LT3995, the only DCDC converters I knew capable of delivering that current have High no Load currents, this can be constantly on!Best regards, Mauro.Edit: Yes amyk, just as you described, thank you.

author: itdontgo
author_rank: Regular Contributor
timestamp: July 15, 2013, 11:23:19 am
content: You definitely want to use that battery then? GSM modules are designed to run off Lithium batteries directly. They normally have battery measurement commands to do just the job and range, like you say from 3.3 to 4.5v supply.The problem you will run into using the DC-DC converter is really you're going to need a linear post regulator as the DC-DC reg won't be very suited to drive the module. You either have to have a forgiving module or a quiet high freq buck regulator.And like you say every 4.6ms in a call for 0.6ms the module will draw about 2A at 850MHz. You need to make sure your power supply doesn't overshoot.http://blog.adventcontrols.co.uk/2013/07/13/alpha-gsm-gate-opener-and-alarm-dialer-schematic/We used an AP5004 at 350kHz with a big capacitor and a fat trace right by the input. If I had the option of a battery though I'd get the battery to match.The other thing too is you want to make sure you shut the module down properly is you're going to power it off. Shut it down then cut the power.

author: maurosmartins
author_rank: Regular Contributor
timestamp: July 15, 2013, 11:43:47 am
content: Hello itdontgo,thank you for your reply, my project needs to be up for about three years, those batteries were the ones with better capacity (35Ah)/price I could find.Contacting the battery manufacturers they provided me with a project for a battery pack and it includes diodes in series with the cells when using several cells in parallel.Even being schotcky diodes, this reduces the voltage of the pack and I would no longer have a reliable voltage to drive the modem, that's why I want to use cells in series.Are there other possibilities? If you have a different strategy, please let me know.Best regards, Mauro.Best regards, Mauro.

author: itdontgo
author_rank: Regular Contributor
timestamp: July 15, 2013, 11:49:20 am
content: Whatever you do make sure you put a big meaty zener diode over the Vbatt and RF ground!

author: maurosmartins
author_rank: Regular Contributor
timestamp: July 15, 2013, 01:03:34 pm
content: Hello itdontgo,Quote from: itdontgo on July 15, 2013, 11:49:20 amCould you point me out the reason?Best regards, Mauro.

author: itdontgo
author_rank: Regular Contributor
timestamp: July 15, 2013, 05:53:54 pm
content: I don't mean over the batteries I mean over the +ve input to the module and the RF ground. These things can latch up really easily if you get an overshoot from your voltage regulator. It not easy to replace a module and expensive too so well worth the investment. Most modules will have a zener or MOV in them anyway but make sure you've got one on the outside.Look in this pic - on the left a zener, on the right a MOV