USB controlled, precision power supply (or voltage reference)

[bloguetronica]

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

[bloguetronica]

Hi,

I've assembled the unit and then did some measurements. This power supply revealed to be very accurate, way more than the previous one. The use of the OPA703 in conjunction with a more accurate version of the LTC2640 DAC, plus external reference, made all the difference.

I've attached the photos of the measurements with the Aneng AN8008. The supply was set to 20, 50, 100, 200 and 500mV, and then to 1V, 2V and 4V.

Kind regards, Samuel Lourenço

[german77]

Nice, I will built it or buy it. But I'm just too lazy to find and order every component. On the other side whats the propose of the voltage sense. If only two terminals are shown on the case.  Sure it's very accurate if you measure directly on the terminals. But as soon as you put a load, your voltage reference is no longer that accurate .

The solution it's simple. The sense leads should be available on the outside of the case and they must be connected with another pair of leads.

[Mr. Scram]

Did you test the voltages under load too?

[bloguetronica]

Nice, I will built it or buy it. But I'm just too lazy to find and order every component. On the other side whats the propose of the voltage sense. If only two terminals are shown on the case.  Sure it's very accurate if you measure directly on the terminals. But as soon as you put a load, your voltage reference is no longer that accurate .

The solution it's simple. The sense leads should be available on the outside of the case and they must be connected with another pair of leads.

I will design a 1A power supply with that option. But anyway, if you use thick wire you can minimize these losses. This supply is limited to 200mA due to USB constraints, so the losses won't be very big. The built in sense circuitry only compensates for the internal wiring losses, but you can always change this.

Did you test the voltages under load too?

Did that, only to calculate the load regulation. Unfortunately, I had to use Analog Discovery for that, so the results weren't very accurate. Thus, I opted not to publish them. Anyway, the load regulation figure is pretty close and gives you an idea. IMHO, it should be lower.

Kind regards, Samuel Lourenço

[bloguetronica]

Hi,

I've decided to do more tests under load. Initially, I ran into some strange phenomenon with my setup, and though that the power supply under test was broken. The voltage was dropping very much under load. Such misbehavior was due to the setup not being properly grounded, while being supplied using SMPS adapters. Well, it so happens these adapters output a nasty residual current that is able to light up a small neon bulb. That current was probably messing up the error amplifier inside the power supply, despite all the protections.

Corrected the oversight, the circuit was grounded and the results were pretty much consistent. The supply was tested without load and with a 200mA constant current load, white set to 500mV, 1V, 2V and 4V. The pictures are attached.

Kind regards, Samuel Lourenço