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Step up or step down converter

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adnan_merter

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Hello I am designing a battery operated circuit and it consists of one pic 18f25k20 chip and an oled display. Here is my question,

The mcu is preferably works at 1.8v for power saving, and oled runs at 3.3v I made a quick search on the net, it seems not possible to make oled run under 3.3v because of internal boost pump so I have to use dc to dc converters.

One option is to use two AA batteries in parallel and use two step up converters

Second option is two AA batteries in series and use one step up and one step down converter

Which one is more convenient? Or any other sugeestions?
 

Hi,

what are the power requirements of both devices?
I assume there's no way around the step up for the oled, but for the microcontroller maybe a linear regulator is the better option.

Klaus
 

Power cosumption is not more than 10ma @3.3v, but the mcu datasheet says mcu consumes 100na @1.8v at sleep state when ext 32khz osc operated, there is much difference so I need to reduce the voltage.
 

Hi,

Oled with just 10mA... that's really low.

But an microcontroller in sleep mode will process nothing. You should rethink the real, averaged microcontroller current consumption.
For just 100nA a stepdown is not suitable.

Klaus
 

The microcontroller does nothing most of the time, it sleeps and it wakes up after 5 minutes read some values and sleeps again. The oled only works if user wants to see values. Most of the time oled is powerless at all
 

look under micrel.com and linear.com for low voltage input boost controllers which you can use in boost or sepic mode for your requirement.
Some can operate under two aa's in series as an input.
 

Hi,

What's the Oled standby current?
Maybe it makes sense to power off the OLED when not used. Switch OFF Oled power supply.

Check if a linear regulator really brings a benefit, instead of powering the Mcu directely from the battery.

Klaus
 

I completely agree that is why I connect oled vdd to one of cpu's io pin. When it it is necessary, mcu power it and init. My question is not about my components it is abot which converter comsumes less energy
 

Hi,

that is why I connect oled vdd to one of cpu's io pin.
How can this be? Because your cpu supply voltage is 1.8V, but your Oled needs 3.3V..
If you do so, then all the
Oled supply current needs to be supplied from the cpu supply.

My question is not about my components
For sure the power supply device selection depends on your devices. I can't see a way round, if you look for a suitable solution.

Otherwise you may find a good step down regulator that has 100 times the no load current of the used cpu. --> no good solution.
You additionally need to know the expectable peak supply current of your (sorry) devices to find a suitable regulator.

Klaus
 

Thanks for advice
My oled vdd is currently connected mcu pin and both operate 3.3v when I use different voltages, I have to switch on-off oled via transistor

Before using regulators I will try to adjust oled's internal boost pump output voltage by changing external boost pump capasitors. Maybe I can succeed to run oled with 1.8v because 1.8v enough for core. After my experiments I willhave write the results here.
 

pic18f25k20 should run from two AA cells 2.4 - 3.2V without a voltage regulator for lowest quiescent current. Boost converter can be enabled on request. Running the pic from single AA voltage through low power boost involves considerably higher idle current, bad idea.
 

pic18f25k20 should run from two AA cells 2.4 - 3.2V without a voltage regulator for lowest quiescent current. Boost converter can be enabled on request. Running the pic from single AA voltage through low power boost involves considerably higher idle current, bad idea.
Thanks for answer
Pic18f25k20 draws minimum current when operating 1.8v but how I can manage it to run lowest power state while applying more than 1.8v
 

I'm designing very similar system with similar problems. I decided to use 2 serial elements and 1 step up converter for led (not a display, just a laser module). Mcu uses direct power from batteries. FvM right, using this design is not a bad idea.
Efficiency of dc/dc varies with a load and may be very low for low current. And besides that, current for oled is much higher than for mcu. Even if you use display for several seconds it will consume much more energy than mcu for hours of idle state.
 

Hi,

What we try to explain:
In your posts you forget that the regulator itself needs power. And it seems that in your case the (idle) power consumption of the regulator(s) is more than the power consumption of your devices.

In my eyes you need to find the optimum power consumption of all things combined.
Therefore a microcontroller driven with 3V without regualtor may consume less overall power than a microcontroller running with a 1.8V regulator

Klaus
 

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