Hi,
This is an evolution of the project presented here:
https://www.eevblog.com/forum/projects/usb-controlled-mini-power-supply/Essentially, the project is very similar. however, for added precision, this supply uses an OPA703 instead of the cheaper OPA705. This new opamp boasts a much lower input offset voltage, providing a more precise tracking of the voltage produced by the DAC. The DAC itself is also more accurate (lower INL), and this time it is referenced to a more precise external voltage reference, the REF3440.
The major breakthrough is the output lead impedance compensation built in. This compensation is made via the voltage dividers constituted by R9/R10 (+sense) and R7/R8 (-sense). This circuit requires a single opamp for tracking and compensation. Due to the lead impedance compensation, the load regulation is minimized (although, I confess I was expecting a substantially lower value).
Some specs:
– Maximum output voltage (nominal): 4.09V (minimum is very close to zero)
– Voltage increment/step: 1mV
– Noise (V
out = 2V, I
L = 0mA): 1.37mV
rms– Noise (V
out = 2V, I
L = 200mA): 1.43mV
rms– Accuracy: ±(0.154% + 1.75mV)
– Load regulation: 1.72mV/A (measured with the intended internal wiring)
– Short-circuit current (I
SC): 257mA
Components:
C1/2/4-6/16/17/20-23 – 100nF 10V (0805) MLCC;
C3/13/18 – 1µF 10V (0805) MLCC;
C7 – NOJA475M006 (NOJA475M006RWJ ou equiv.) niobium electrolytic capacitor;
C8/9 – NOJA475M010 (NOJA475M010RWJ ou equiv.) niobium electrolytic capacitor;
C10 – 470pF 1KV (0805) MLCC;
C11/12/15 – 2,2µF 10V (0805) MLCC;
C14 – 10nF 10V (0805) MLCC;
C19 – 470nF 10V (0805) MLCC;
C24 – NOJC476M010 (NOJC476M010RWJ ou equiv.) niobium electrolytic capacitor;
D1 – WP1503CB/ID LED;
D2 – WP1503CB/YD LED;
D3 – WP1503CB/GD LED;
D4 – S1A rectifier diode;
D5 – MMSD4148 fast switching diode;
IC1 – ZEN056V075A48LS PolyZen ;
IC2/3 – SP4021-01FTG TVS diode;
IC4 – CP2130 (CP2130-F01-GM) USB-SPI bridge;
IC5 – RI3-0509S isolated DC-DC converter;
IC6 – LP2985-50 (LP2985-50DBV) voltage regulator;
IC7 – ADuM1310 (ADuM1310ARWZ) digital isolator;
IC8 – REF3440 (REF3440IDBV) voltage reference;
IC9 – LTC2640ACTS8-HZ12 DAC;
IC10 – OPA703 (OPA703NA) op-amp;
J1 – Molex 67068-9001 USB connector;
J2 – 2-pin header;
J3 – 2 hole terminal block;
L1 – XFL3012-103ME (XFL3012-103MEB or XFL3012-103MEC) power inductor;
L2 – XFL3012-223ME (XFL3012-223MEB or XFL3012-223MEC) power inductor;
Q1 – FDN327N power MOSFET;
Q2 – MJD31C power BJT;
Q3 – MMBT3904 signal BJT;
R1 – 1MΩ±5% 1/8W (0805) thick film resistor;
R2/3 – 4,7KΩ±5% 1/8W (0805) thick film resistor;
R4/5 – 82Ω±5% 1/8W (0805) thick film resistor;
R6 – 180Ω±5% 1/8W (0805) thick film resistor;
R7-10 – 1KΩ±0.05% 1/8W (0805) thin film resistor;
R11 – 470Ω±5% 1/8W (0805) thick film resistor;
R12 – 2,7Ω±5% 1/2W (1210) thick film resistor.
I'm pretty satisfied so far. The schematic and layout are pretty much set in stone. So, the purpose of this post is only to share this project.
Kind regards, Samuel Lourenço