This week’s EYE ON NPI (video) is going to give you life, we’re looking at Nexperia’s Battery Life Booster ICs, an interesting solution for dealing with common coin cell batteries as an inexpensive power supply for your small electronic products.
The marketing copy for this chip series is pretty eye-catching, with copy that they are “designed to extend the life of a typical lithium coin cell battery by up to an order of magnitude (10x) while also increasing its peak output current capability by up to 25x”. Quite a claim! So let’s take a look at what this chip can really do, and why we need it to help with our coin battery-powered designs.
Primary Lithium LiMnO2 coin batteries are hard to beat when you want ultra-dense, low-cost power that is very small. They are the smallest form factor of user-serviceable battery. And the power density of coins – plus their flat shape – makes them great for wearable devices, medical sensors or hearing aids, wireless nodes, anything that is smaller than a finger.
With a nominal voltage of 3VDC, and 280 Wh/kg density, Lithium LiMnO2 are more dense than a AA alkaline. LiSOCl2 batteries are even more dense, with 3.6V nominal and 500 Wh/kg – twice the density of LiMnO2 or Alkaline. Both are non-rechargeable batteries, and while fairly inexpensive when bought in bulk, they can add up in cost when buying from a grocery store, so making sure that they last a long time is essential for a good customer experience.
The Energizer Lithium Coin Handbook and Application Manual is chock full of good advice for folks who want to use a CR2032 or similar coin battery to power their products. It’s written with engineers in mind, so a lot of the document covers how the nominal voltage and current, 3V and 240mAh, are deeply affected by shelf life, internal resistance, temperature, and pulse effects. The biggest downside you will see on coin batteries is their relatively high internal resistance – IR. Whereas alkaline and LiPoly batteries have internal resistance well below 1 ohm, the IR of a CR2032 is 9 ohms.
So say you want to draw 100mA from a CR2032, even if just for a few millisecs – that 0.1 A turns into a .9V drop across the IR which means what you thought was a 3V power supply is now 2V – too low for your microcontroller or radio to run. Since the power lost across the IR is 2*I*R and R is fixed, it’s doubly important that you keep that I low, so that you can minimize IR loss and use as much of that 700mWh stored energy. In our 100mA example, 30% of the battery energy is lost to resistor heat dissipation!
That’s where Nexperia’s Battery Life Booster ICs come in. They are designed to be used with products that don’t need continuous battery drain, that is they have a very low duty cycle where the user presses a button a few times a day or maybe the sensor node transmits only once a minute. During that short transmit or usage cycle, a radio or LCD display may need up to 200mA but its for a very small amount of time and the voltage needs to be stable. The NBM family of chips do this by slowly sipping a small amount of current from the battery, keeping that I low so that the 2*I*R power loss is minimized, then use that constant current to boost a voltage onto a large storage capacitor. The capacitor is chosen to have high capacitance and high voltage for a large total Joule storage capacity.
For example: say we need 100mA from 3V for 10ms. That’s 100mA * 10ms * 3V = 3000uJ. Add 20% safety margin, for 3000 * 1.2 = 4320uJ. The max voltage is 11V, and the minimum is 3.5V so that we have a little dropout to 3V – our capacitance is 2 * 4320uJ / (11V^2 – 3.5V^2) = 79.4uF. Add another 20% margin since capacitance can vary, for 100uF and 16V to handle the max voltage. See the datasheet section 11.7 for this math in more detail. Once the capacitor is charged up to the high voltage, it can then be discharged by a second buck converter inside the NBM, which brings the 3.5 to 11Vcap down to 3V while supplying that 100mA desired. As long as the time for charging the capacitor is long enough to keep that drainage current under a few milliamps, we get the power density upside without the internal resistance downside.
To squeeze the most power out, it’s important to let the NBM chip know how fast you want to let it charge the storage cap, the maximum voltage the cap can handle, the regulated buck output as well as the expected duty cycle and output needs.
That’s all done via I2C or SPI, so there are two variants to choose from, A and B. The other two variations are for whether the capacitor booster can go up to 5.5V for the 5 series or 11V max for the 7 series. If you don’t want to have to program in any registers at all, the I2C version can also act autonomously with setting resistors.
All four variants of the Nexperia NBM family, NBM7100A, NBM5100A, NBM7100B and NBM5100B are in stock right now at DigiKey for immediate shipment! Order today to pick up these chips that will pay for themselves in one battery life-cycle by adding a PMIC to your coin-battery powered products, and DigiKey will ship your chips immediately so you can start integrating them by tomorrow afternoon.