EYE ON NPI - TI BQ25798 I2C Controlled, 1 to 4-Cell, 5-A Buck-Boost Battery Charger for Solar Panels

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3d ago 11:37

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This week's EYE ON NPI is a follow up to one we did a few years ago on the similarly-named BQ25792 (https://blog.adafruit.com/2021/05/06/eye-on-npi-ti-bq25792-i2c-controlled-1-4-cell-5a-buck-boost-battery-charger-eyeonnpi-adafruit-digikey-adafruit-digikey-txinstruments/). The BQ25798 (https://www.digikey.com/short/vnr279pz) builds on the '92 by adding selectable dual inputs and true MPPT solar support. This chip is inexpensive, powerful and can handle almost any battery and power source matching you desire. Let’s look at some specifications: High power density, high integration buck-boost charger for 1-4 cell batteries supporting USB PD 3.0 profile – Integrates four switching MOSFETs, BATFET – Integrates input and charging current sensing Highly efficient – 750-kHz or 1.5-MHz switching frequencies – 5-A charging current with 10-mA resolution 96.5% efficient: 16-V battery at 3A from 20V Supports a wide range of input sources Autonomously sampled open circuit voltage (VOC) maximum power point tracking (MPPT) for charging from a photovoltaic panel – 3.6-V to 24-V wide input operating voltage range with 30-V absolute maximum rating – Detects USB BC1.2, HVDCP and non-standard adapters Dual-input power mux controller (optional) Narrow voltage DC (NVDC) power path Backup Mode with Ultra-fast switchover to adjustable voltage Powers USB port from battery (USB OTG) – 2.8-V to 22-V OTG output voltage with 10-mV resolution to support USB-PD PPS – OTG output current regulation up to 3.32 A with 40-mA resolution Flexible autonomous and I2C mode for optimal system performance Integrated 16-bit ADC for voltage, current, and temperature monitoring Like the '92, the BQ25798 (https://www.digikey.com/short/vnr279pz) supports any size battery. We have lots of battery packs in the Adafruit shop, and in particular we use 1S batteries – if there are more batteries, they are wired in series. But there’s lot of folks who are building robotics that require higher voltages, so they have 2S, 3S, or 4S batteries. This charger can handle any of ’em, and you can configure the battery pack size using a simple resistor on the PROG port. In this case it also allows the chip to run in 'standalone' mode without the use of I2C to configure. The biggest improvement you get with the BQ25798 (https://www.digikey.com/short/vnr279pz) is true solar MPPT support. The BQ25792 had VINDPM and IINDPM – the ability to track the input voltage to make sure it is not drooping from overdraw. While this lets you get pretty-close-to-MPPT it isn't true power-point-tracking which requires perturbation around the voltage to adjust as light and temperature affect the solar panel's efficiency. The '98 does this 'right' and even has a K Factor adjustment register - you can tweak this to get the best results based on different weather/temperature (https://www.ti.com/video/6287049638001)- or stick to the default value for good results. Another new feature is 'selectable dual-inputs' what this means if you can set up two power inputs - say DC plug and Solar - and then have the chip switch between them. This is particularly useful because you can't just use two OR'ing diodes to select the power source: the solar panel might have a higher initial open-voltage but can't supply as much current as a DC plug. I2C lets you select which one is priority! The BQ25798 (https://www.digikey.com/short/vnr279pz) also has many of the cool features we liked in the BQ25792: On-The-Go mode where you can turn the buck-boost around and have it generate a variable voltage output, say 5V for powering other USB devices. Another thing that works is powering over USB where you can have the BQ negotiate 'high voltage' support from USB 3 ports. Note that this isn't USB Type C power negotiation, for that you'll want to get a separate USB Type C PD negotiation chip like the TPS25750D (https://www.tij.co.jp/jp/lit/ml/slpp103/slpp103.pdf)...we're hoping there's a future version with PD built in! There's also a built in 16-bit ADC that you can use to monitor various voltages and current draw. While you can charge the battery in 'standalone' mode - you really do need I2C to get the best performance and capabilities. Thankfully there's not a huge number of registers, and SDA/SCL can be 3 or 5V logic signals so you should be able to get it working on anything from an ATmega328 to a Raspberry Pi. We like the high integration: you really only need a few passives and an inductor to get a fantastic all-in-one charger for any lithium ion battery pack. If you're intrigued and would like more information, you've come to the right place! DigiKey has the BQ25798 (https://www.digikey.com/short/vnr279pz) in stock right now for immediate shipment. Order today and you can start designing your solar-powered products of the future by tomorrow afternoon.

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