Power Supply Design with LiPo batteries

With the growing popularity of portable and wearable electronics, need for efficient power supply more necessary than ever before. For handheld devices, such as phones or PDAs, smart watches or any other smart gadgets… power supply starts to become a major design challenge mainly because of the fact that these devices use Lithium power sources. The characteristics of a Lithium battery make it near impossible to effectively use popular linear regulators such as 7805. Why?

Why linear regulators will not work

Linear regulators can be considered to be of 2 types – a regular linear regulator (e.g. 7805) and LDO (Low drop-out) linear regulator (such as the AMS-1117). The regular linear regulators have a high voltage dropout, i.e. they need input voltage much higher than the output voltage to regulate the output properly. This is why you may not be able to get 3.3V output from 5V using a regular linear regulator.

The solution? Use LDO regulator! These devices use a MOS pass transistor instead of a BJT and thus works better with a lower dropout voltage, usually about 1 volt. Now it is possible to obtain 3.3V from a source of just 5V.

But what if your LiPo (Lithium Polymer) battery has a voltage of 4.2V and near-discharged state voltage of 3.6V? If you need 3.3V, your LDO will need a minimum battery voltage of about 4.5V to operate!

A crude solution

You can put 2 LiPo batteries in series and now your battery voltage will vary from, say 8.4V to 7.2V… now you can use a linear regulator to get 3.3V – simple!
Not really. Why? Because if you are drawing, for example, 500mA at 3.3V, the power dissipated across the linear regulator will be
(8V-3.3V)(0.5A) = ~2.3W !!
Now consider your actual usable power which is 3.3V*0.5A = ~1.6W !
Conclusion:

  • power regulation efficiency is not even 50%!
  • You are using 2 LiPo cells even though you could do with 1 – double cost!
  • You will need a big heat sink to dissipate the 2.3 watts
  • Heat sink increases system cost and size

Solution: Use a DC-DC converter!

DC-DC converters can be of 3 types:

  • Buck converter (step-down, voltage output is lower than input)
  • Boost converter (step-up, voltage output is higher than input)
  • Inverting converter (inverts the voltage, i.e. negative output for positive input)

A DC-DC converter uses inductance as a means of storing energy and is extremely versatile in terms of output voltage and input voltage. One of the most inexpensive DC-DC converter chips is the MC34063 which costs a couple of cents only (INR 10 in India!). What other characteristics would you associate with DC-DC converters? Here are the pros and cons:

Pros:

  • High efficiency, almost always over 90%!
  • Versatile voltage output specifications
  • No heating issues when designed correctly
  • High current capabilities in small packages

Cons:

  • Ripple in the output, may worsen with load current!
  • May interfere with nearby circuitry (EM interference)
  • High frequency converters are expensive
  • Takes some design effort to get things working well

A simple DC-DC converter example

A DC-DC converter may be a complex device. But with the availability of thousands of high power density converter ICs, it has become easy to simply select parts and put together a converter with very good specifications. For example, a simple converter circuit may look like this:

Adjustable voltage output DC-DC converter based on LM3242
Adjustable voltage output DC-DC converter based on LM3242

 

As for the part size, it could be down to a couple of millimeters. However, the inductor typically takes up more space that the switching regulator chip itself. For example, here, the inductor L1 is bulkier than the chip below it! The solution to this problem is the use of higher switching frequency – but that increases the system cost. Therefore, this is a troublesome trade-off in portable electronics design.

 

STMicroelectronics STBB2 switching-regulator evaluation board.
STMicroelectronics STBB2 switching-regulator evaluation board

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