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Author Topic: Choosing a 3.3V Voltage Regulator  (Read 4312 times)
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I'm currently working on a project in which the components must be powered by a 3.3v source. I want to use a 3.7V rechargeable lithium polymer battery and a voltage regulator to do this.

Looking at various sources for electrical components online, the number of different voltage regulators with a 3.3V output is massive. There are also many different attributes for each regulator such as allowable input voltage, temperature range, voltage dropout, etc. I was hoping someone could give me some guidance as to what I need to look for.

From what I understand, the voltage dropout should not exceed 0.4V since 3.7 - 3.3 = 0.4. However, I don't quite know what the value of the current output should be. One of the components in this particular project is a bluetooth module that has a varying range of current draw depending on whether it is dormant, connected, transmitting, etc. Many of the regulators have limits (minimum or maximum) on current output. If I were to use one with a 250mA minimum would that imply that the draw could not be less than 250mA at any given time? Will a regulator be effective in providing 3.3V when current draw varies?

Any help would be greatly appreciated.
« Last Edit: August 08, 2011, 10:49:46 am by isometrik » Logged

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You'll need a "low-dropout" regulator, because (as you mentioned) there's only 0.4V between your battery and your desired output voltage. But, what will the battery voltage be when it's nearly discharged? I think a LiPo cell can go below 3.7V, and maybe as low as 3V when fully discharged.  It'll be over 4V when on charge.

You will definitely need to know how much current your circuit will draw from the battery/regulator. Regulators are normally specified in terms of the maximum allowed current, but you will also need to consider the heat produced by that current flowing when, say, the battery is fully charged (Watts = Volts x Amps). Some regulators (e.g.LM317T) specify a minimum current, below which they cannot regulate and will produce too high a voltage. It's usually very small, and can be satisfied by choosing low resistors for the voltage-setting divider.
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If the voltage drops below the required voltage plus the rated voltage dropout, will it cause damage to the components or will they just cease to function?

If I know the maximum current draw of the circuit, is it sufficient to choose a voltage regulator with a maximum current output that exceeds it (as long as heat is not an issue)?
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If the voltage drops below the required voltage plus the rated voltage dropout, will it cause damage to the components or will they just cease to function?

Had to say for sure. The circuit may carry on working, because voltage ratings always have a certain amount of margin. But individuial parts vary, so you can't really rely on this. Probably won't damage anything permanently, but you may find that you need to switch off the circuit and switch it on again to get it to start working again.

If I know the maximum current draw of the circuit, is it sufficient to choose a voltage regulator with a maximum current output that exceeds it (as long as heat is not an issue)?

Yes. The regulator may have a built-in shutdown mode that will protect it if the current becomes too much. So, as long as it's rated for more current than you'll be drawing, you should be OK.
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Try taking a look at the regulators used on Arduino's with ATmega8MU2s (pretty much anything new after the Deumilanove).
They are low dropout types.
LP2985-33DBVR
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I'm currently working on a project in which the components must be powered by a 3.3v source. I want to use a 3.7V rechargeable lithium polymer battery and a voltage regulator to do this.


Any help would be greatly appreciated.

Strongly recommended IC's are

LP2950  whose dropout voltage is 380mv at 100ma.It is avaiable in a 3 pin to92 (transistor like) package which means its easily solderable.
 
LE33CZ - 0.2v drop out, max current 100ma, again avaiable in the easily solderable to92 package

LP2981 - This will drop only 0.2v at 100ma which is the maximum current in can supply.
This is a SOT-23 which means its a surface mount part and good soldering skills are needed.


The 3.3v with the lowest dropout which i  use is Richtek's RT9167
It can supply  300ma with a dropout of 0.35v at that current. at say 50ma current the drop is only 50mv!!
It's problem is its again a  SOT-23 package which means its a surface mount chip with small leads. It will require good soldering skills.


Please use one which suits you.
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I'm currently working on a project in which the components must be powered by a 3.3v source. I want to use a 3.7V rechargeable lithium polymer battery and a voltage regulator to do this.

Remember that 3.7v is the nominal voltage, when li-ion batteries are fully charged there voltage is 4.2v, they reach 3.7 when they are roughly 70% discharged.  so you have plenty of room to operate. When they reach 3.4v its time to switch the circuit off, although the lowest recommended voltage is 3.0v beyond which most li-ion batteries safety circuits will cut the power off.
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What components are you using that can only take 3.3v? Are there no alternatives? There are many devices (including the AVR microcontrollers) that can take anything from 3v or less to 5.5v, allowing you to run off a Li-Ion battery without regulation.
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What components are you using that can only take 3.3v? Are there no alternatives? There are many devices (including the AVR microcontrollers) that can take anything from 3v or less to 5.5v, allowing you to run off a Li-Ion battery without regulation.

Most 3.3v CMOS logic designed new devices (micro sd cards etc, VS1053 etc) can only take a maximum voltage of + 0.3v ie say 3.6v maximum! If you give higher voltages the chip can get fried, or it may not work, ie may latchup, or it's lifetime will be drastically reduced. So a 3.3v voltage regulator is essential, and one would select a LDO which will only drop say 0.1v, so that you can use it till the battery voltage is 3.4V, at that voltage the li-ion is about 90% discharged so no point in depleting it further, you can just auto switch off.
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Remember that 3.7v is the nominal voltage, when li-ion batteries are fully charged there voltage is 4.2v, they reach 3.7 when they are roughly 70% discharged.  so you have plenty of room to operate. When they reach 3.4v its time to switch the circuit off, although the lowest recommended voltage is 3.0v beyond which most li-ion batteries safety circuits will cut the power off.

3.7V is 10% charged, and 3.8V is the lowest it should be discharged for good health.
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Remember that 3.7v is the nominal voltage, when li-ion batteries are fully charged there voltage is 4.2v, they reach 3.7 when they are roughly 70% discharged.  so you have plenty of room to operate. When they reach 3.4v its time to switch the circuit off, although the lowest recommended voltage is 3.0v beyond which most li-ion batteries safety circuits will cut the power off.

3.7V is 10% charged, and 3.8V is the lowest it should be discharged for good health.

This is wrong info.  Li-ions can be safely used to 3.0v, beyond which the cutout circuit in the battery will prevent further discharge. I quote "If the voltage drops below 2.50 V per cell, the battery protection circuit may also render it unchargeable with regular charging equipment. Most battery protection circuits stop at 2.7–3.0 V per cell." http://en.wikipedia.org/wiki/Li_ion_battery
For example The lithium ion battery protection IC UCC3952-1 datasheet says that it cuts off at 2.65 volts.

See also http://focus.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slua015&fileType=pdf
This contains the discharge profile of a li-ion and you can see that the lowest voltage is 2.7, so 3.7 is NOT 10% capacity as you mentioned.  You can safely use Li-ions till the voltage is as low as 3v


Infact there are buck boost ic's which will give 3.3 v even when the voltage of the lithium in is less than 3.3v, for example the wonderful sc632a ic can give 3.3v even at a battery voltage of 2.95 volts.

So to summarise the safe voltage for li-ion or li-poly batteries are 3.0 to 4.2 volts
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Just to add a new Idea: What about a SEPIC converter with a DC/DC converter like LT1615.
The datasheet of the LT1615 contains such a circuit which generates 3.3 Volt from any value between 2.5 to 4.2 Volt.

Oliver
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Just to add a new Idea: What about a SEPIC converter with a DC/DC converter like LT1615.
The datasheet of the LT1615 contains such a circuit which generates 3.3 Volt from any value between 2.5 to 4.2 Volt.

Oliver

Yes, you can use such a citcuit, there are also buck and boost regulators like the sc632a, they act like a linear regulator till the batteris votage is near 3.3v, then when the batteries voltage goes lower, they use a capacitor or inductor to boost the voltage to 3.3v. They can give 3.3v output for 300ma max from a input of 2.95 to 5v.  LInear technologies has plenty of such buck boost ic's too....
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Searching Farnell's catalog found only _one_ TO92 regulator with >=300mA current and <=400mV drop out!  The L4931CZ33-AP - there are lots of surface mount parts, the SOT223 ones aren't too hard to solder and they have much lower dropout voltages too.
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