This week’s EYE ON NPI is back from hiatus – Adafruit is completing its move from Manhattan to Brooklyn – and in honor of Brooklyn’s Biggie Smalls we’re showing off a Boostie-Buck with Big current output, and Small BOM size…it’s Analog Devices’ MAX77859 Buck-Boost Converter with 2.5~22V input and 3~20V output with an internal H-bridge that can handle 7.8A switch current.
We laid eyes on this part while looking for something to pair with USB Type C power delivery chips: you can get a maximum of 3A out of USB C PD supplies, but that current limit is the same whether you’re drawing 5V or 12V or even 20V. That means if you have a PD supply that can give you higher voltages, a buck converter will let you get high currents at lower voltages. That is, draw 12V 3A and convert that down to 5V 6A for powering huge quantities of LEDs. While it’s common to find buck converters for high currents and high voltages, we wanted something that could also generate 12V from 5V so that LED strips with 3-diode-series subsections would also work. And of course sometimes it’s nice to be able to tweak the voltage up to 6V to account for the voltage drops on long strip runs.
For simple ‘fixed voltage output’ modes, you can use the MAX77859 in standalone mode. This mode simply requires a few resistors to set the voltage output and current-limit (if you desire one).
A Power Good output lets you know that the supply is able to run, and the Enable input disconnects the output. In this mode, the SCL/SDA pins are tied high to prevent noise from accidentally triggering the I2C receiver.
The MAX77859 can also work the other way around: providing a variable voltage output based on a USB PD downstream device’s requests. For supply usage, you can use the chip in I2C control mode. Use the same resistors from the standalone configuration to set up 5V 1A output. Then, connect to the I2C peripheral from any controller to set the registers for voltage and current-limit settings. You can also set things like the PWM frequency, slew rate, and internal feedback compensation resistor. Status bits will tell you whether the chip has detected a short, or is in thermal limit mode. With 20mV-step-size output voltage, and 50mA-step-size output current limit control, you could easily use this board to create a dynamic programmable power supply!
The datasheet mentions having four variants available: WLP/BGA and 4×4 QFN. The two A variants have current limiting capability. The WLP is definitely smaller but you will need plugged vias since its 0.4mm pitch.
Right now only the WLP is stocked, so if you want the QFN, check with DigiKey’s sales rep to request notification when ADI releases that version. If you want to get started immediately, there’s an eval board MAX77859WEVKIT available that exposes all of the control lines.
If you need a powerful buck-boost with plenty of juice for your next power supply design, the Analog Devices MAX77859 Buck-Boost Converter will do an excellent job at an excellent price: $1.25 in quantity. It’s also in stock right now at DigiKey for instant shipping, so you can book your MAX77859 samples or an eval board today and have it in your hands by tomorrow morning.
