This is a quick build around an AL8843 step-down led driver from Diodes inc

It uses the reverse buck topology, where the switching mosfet is referenced to ground, and the output is referenced to Vcc. This works well for loads that don’t need a ground connection – LED’s for example.

It also simplifies the chip internals because there’s no need for a floating gate driver and a charge pump to power it.

The chip uses hysteretic mode control, where Ton and Toff are determined by the input voltage, inductor, load and current sense resistor.

The chip tries to keep the voltage across Rset to 0.1V with 13% hysteresis. The switching frequency can go up to 1MHz

The CTRL pin allows for analog or pwm dimming – analog when it’s voltage is between 0.4 and 2.5V, pwm when it’s switched between 0.3 (OFF) and 2.6 (ON) volts.

 

My board uses a 33uH inductor from the SRR1260 series – the same footprint can be used to update the board for different led configurations.

The board uses 2mm pitch JST-PH connector for the power and CTRL signals. There are also large vias for directly soldering wires.

The PCB is a simple 2 layer one, where the bottom layer is mostly GND – if the JST headers are not populated, it can be used as a heat spreader.

I’ve got some scope snapshots of the board powering a set of 21 LM301B leds in a 3S7P configuration.

Rset is 0.1Ω – the leds are driven with 1A, for 9.3W of output power.

The oscilloscope is connected to the SW node, and the input voltage is stepped from 10 to 18V

The internal mosfet in conducting when the signal is close to ground.

 

At 10V the Ton time dominates, and the frequency is just under 100KHz:

 

At 12V the inductor starts storing more energy and Ton slowly decreases.

Fsw reaches 240KHz:

 

The same thing happens when the input voltage is further increased:

 

At higher voltage Ton and Toff are equal, and the frequency increases:

 

As usual, the design docs are published on github: https://github.com/ealex/AL8843