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Topic: Choosing a 3.3V Voltage Regulator (Read 6 times) previous topic - next topic

kalum



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

olikraus

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

kalum


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....

MarkT

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.
[ I won't respond to messages, use the forum please ]

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