Here is proposed circuit for 3rd channel that can be also used as a USB charger. A SMPS controller
LT3741 (IC35) is used that offer current limitation and is synchronous buck therefore overall thermal dissipation should be smaller. Selected mosfet (Q6, Q31) comes in interesting LFPAK package that is, pretty small, simple for soldering and have good thermal resistance.
Projected output voltage when calibrated should be 6 V and current up to 5 A on the CH3 binding posts or 2.6 A on the USB A socket while used as a USB charger. Max. power will be limited by software to 25 W.
For voltage and current control I first planned to use Arduino DUE DAC outputs but it's too limited (cannot without extra circuit set voltage down to 0 V) and I ended up with SPI 12-bit dual DAC
MCP4822 that has internal voltage reference (2.048 V). It's possible to use the 10-bit version that is pin compatible (MCP4812).
Voltage programming is accomplished by injecting DAC output 2.048 to 0 V for 0 to 6.3 V via R185. For current programming another DAC output is used. Its 0 to 1.5 V limits output current in range from 0 to 5 A. Max. output current is defined with voltage drop on the Rsense that is 50 mV. Therefore R33 of R010 is selected.
For voltage monitoring MCU ADC input is used (AUX_U_MON). Voltage divider R187, R188 is set to provide almost full scale for max output voltage. For current monitoring (AUX_I_MON)
INA282 (gain 50V/V) is used that gives 2.5 V for max output current (50 mV x 50). INA282 has zero-drift but only 10 kHz bandwidth that is not critical in our case since its used for passive monitoring and not as a part of the current control loop.
Possibly I will add dual RRIO op-amp (e.g. OPA2344 or similar) into the picture one for buffering voltage monitoring divider output (R187, R188) and another one to extend current shunt monitor output from 2.5 V to MCU's ADC full scale (3.3 V).
Switching frequency is defined by MCU and it will we about 640 kHz or two times higher then what is used for CH1 and CH2 power pre-regulators. Therefore a simple divider by 2 that also offer 180 deg phase shifting will be used:
For USB charging a separate USB A-type socket will be used. Only one output can be selected at the time: binding posts (J7, J8) or X22 connector (vertical, PCB mounted). K_USB relay MCU controlled is used for that purpose. Maybe it can be replaced with two power mosfets.
TPS2511 is used to detect connected USB device and provide up to 2.6 A of output current. Finally as an option is added IC38 for protection.
Adding CH3 require bigger Arduino shield and that is mainly dictated with dimensions and spacing of binding posts and USB-A socket. Additional 50 mm of horizontal space will be enough for that. Because of new AUX PS module and Arduino Shield positions of 4 PCBs on the PCB panel have to be rearranged for better usage:
If someone wants to play with LT3741 here is the LTspice model (see also attached file):
Your comments and ideas are welcome.