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Topic: 3.3V LDO Regulator to use with 3.7V Li-Po Battery (Read 11252 times) previous topic - next topic

riaanc

Hey Guys

I need to power some 3.3v logic from a 3.7v Li-Po battery. I used SparkFun's LiPower (https://www.sparkfun.com/products/10255) before, but it is limited to 200mA current delivery at 3.3V. I need about 300 - 350mA. I contacted the guy the who designed the LiPower and he said that an LDO will be sufficient for my needs. I have very limited knowledge of using regulators except for the 7805's etc.

Can someone please help me pick a regulator that:


  • Is easy enough to prototype on a breadboard with (this isn't THAT important, but it would help)

  • Has a very low drop voltage

  • Is capable of providing 3.3v about 300mA peak for about 30 seconds every few minutes, and idling at about  100mA the rest of the time without any significant thermal considerations



If you have the time, please help me understand:
With the 7805 regulators: They become quite hot, depending on the input voltage e.g. if the supply is 12V, and you draw 500mA, the regulator becomes extremely hot. I guess this means that the "excess" power is converted to heat?

How will a LDO regulator differ in this instance? As I am working from a battery, every bit of wasted current is important. Is there a more efficient way than using a LDO regulator to get 3.3V?
What does the LiPower unit do different?

dc42

I didn't find any suitable LDO regulators in non-SMD packages, but I found this one: http://uk.farnell.com/torex/xc6222d331mr-g/ic-ldo-700ma-3-3v-sot-25/dp/1830952. It has maximum dropout voltage of 120mV @ 300mA, and maximum output current of 700mA.

A switching regulator might be slightly more efficient than a LDO linear regulator, however as the voltage you need to drop is only 0.4V, the difference will be small. The LDO regulator will be about 89% efficient when delivering your 300-350mA, and it will be difficult to get more than a few % improvement with a switching regulator.

The power dissipation of a linear regulator is basically (output current) * (voltage drop in regulator). As the voltage drop in this case is only 0.4V, even at 350mA output the power dissipation will only be 140mW, so no worries about overheating. Whereas the 7805 in your example is dissipating 3.5W, so no wonder it needs a heatsink.
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jremington

An LDO regulator doesn't regulate if the voltage drops below the (output voltage + dropout voltage) which is guaranteed to happen with a LiPo battery. A better option is an up/down switching regulator, which will maintain a constant output voltage over a very wide range of input voltages above and below the output voltage. Pololu offers some very small and inexpensive modules that do just that.

See for example this 3.3V (also available in 5 V output) http://www.pololu.com/product/2122

dc42


An LDO regulator doesn't regulate if the voltage drops below the (output voltage + dropout voltage) which is guaranteed to happen with a LiPo battery.


Whether that is guaranteed to happen depends on what the current draw is, and how deeply you discharge it. For example, this discharge curve http://revolution-robotics.com/wordpress/wp-content/uploads/2013/03/lipo_discharge_curve_sm.png suggests that you can discharge by 80% of the full capacity at a discharge rate of 6C before the voltage drops below 3.5V.

However, I agree that the Pololu unit appears to be well-suited to this application.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

jremington

That is certainly true. I forgot to add that voltage monitoring is necessary, because if LiPo cells are discharged even once below about 2.5V, they usually become useless.

mnhpias

#5
Sep 30, 2015, 04:30 pm Last Edit: Sep 30, 2015, 04:31 pm by mnhpias
I found this LT1763 - 500mA, Low Noise, LDO Micropower Regulators . It has Wide Input Voltage Range: 1.8V to 20V . Do you recommend this for 3.7V Lipo battery ?

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