Mikroe Buck 5 Click with PSoC5 VDAC

Summary

This post examines alternate means for driving the MikroElektronica Buck 5 Click hardware. First, the onboard digital Potentiometer was rewired in a new configuration and second the Potentiometer was replaced with Cypress PSoC.

Buck 5 Click - Courtesy MikroElektronica

Buck 5 Click with Digital Potentiometer and PSoC5

MikroElektronica's Buck 5 Click (MIKROE-3100) hardware presented itself as a cost-effective and off the shelf unit for testing the MAX17506 DC-DC Converter. The Buck 5 Click was designed primarily for use with other MikroElektronica hardware. The input voltage range of the Maxim buck converter (4.5V to 60 V DC) is not fully utilised on the Buck 5 Click (5V to 30 V DC). In the same manner, the output voltage appears capped at 20 V DC maximum. Even with the voltage limits the output range of the hardware was able to be tweaked. 

Wiring a Cypress CY8CKIT-059 prototyping kit to drive the Buck 5 Click required three connections for the SPI and two for power. The Buck 5 Click schematics indicated the board logic operated with 3.3 V DC however, the board was powered from 5 V DC. The supply range of the MAX5401 digital Potentiometer (pot) was 2.7 V to 5.5 V DC.

To control the digital pot, a PSoC Creator application using an SPI Master component was created. 

SPIM_WriteTxData(Buck5_Dig_Pot_Val);
while (0u == (SPIM_ReadTxStatus() & SPIM_STS_SPI_DONE)) 
{ }

The subsequent step of increasing the output voltage range of the Buck 5 Click was accomplished although the configuration of feedback resistor network (R8) and digital pot (U2) for the DC-DC converter (MAX17506) needed to switch places. The digital pot operating voltage was the reason for the change. The pot wiper was connected directly into the feedback pin of the MAX17506. Below is a working board with modifications.

Buck 5 Click Modified with Increased Output Range

The output voltage range of the modified board was approximately 4V to 18 V DC. The digital pot, with 255 positions, provided output voltage steps of 55 mV.

When applying power to the digital pot, the default setting is the middle position which may not be ideal in all instances. To control the behaviour of the buck regulator on power-up, a connection from the PSoC to the Enable line of the Buck 5 Click was used. A pull-up resistor (R4) was removed from the board to prevent the Buck 5 Click from starting when power was applied

Buck 5 Click with Cypress PSOC5 VADC, No Digital Potentiometer

Other solutions for adjusting the output voltage of a DC-DC converter, through control of the feedback pin, use external PWM or DAC sources.

The Cypress PSoC provides control of DC-DC converters either through PWM (Trim and Margin component) or a DAC (Current or Voltage component). This post used the PSoC VDAC8 component which provided 255 steps. For additional resolution the dithered VDAC component with 4096 steps could be used.

For calculating the output voltage for VDAC solution, Maxim published a notable tutorial deriving the output equation shown below from first principles.

V_OUT = V_REF(1 + (R1/R2)) + (V_REF - V_DAC) (R1/R3)                        Eq (1)

For connection of the VDAC to the feedback loop of the DC-DC Converter, one additional resistor (R3) was required.

DC-DC Converter Output Voltage Control Using a DAC

A spreadsheet was used to implement Equation (1) which allowed the three unknown resistor values to be manually changed and the output voltage range of the DC-DC converter observed.

DC-DC Converter Vout Spreadsheet Calculations

Equation (2) was used to ensure that the maximum current seen by the PSoC (DAC) was less than 25 mA.

i₃ = (V_REF - V_DAC)/R3                       Eq. (2)

Quadrature Encoder

A quadrature encoder (Bourns PEC11R) was added to the design to allow for manual voltage adjustments. The QuadDec component was used to convert the encoder AB lines. The QuadDec returned value was limited to between 0 and 255. The reading from the QuadDec was then used to adjust the output of the VDAC component.

PSoC Creator Top Design

The project required the QuadDec, DAC and OpAmp components as shown below.

PSoC Creator Project Top Design

The calculations in Excel used a voltage range from 0.1 V to 4.08 V which was the option selected in the VDAC component.

VDAC Component Settings

While not essential, an OpAmp follower was added to the design.

OpAmp Component Settings

To maintain 255 positions the QuadDec component was changed from the default 8 bit to 16 bits.

QuadDec Component Settings

The remaining signals that are shown on the PSoC Creator project page (Top Design) were related to the DC-DC converter (PSU_Enable) and the on-board switch (Switch1). Voltage ramps were generated in code on start-up then Switch1 was pressed.

To facilitate testing of the encoder, a UART component was added to the project.

PSoC Creator Pin Mapping

Pin mapping for the design is shown below.

PSoC Project Pin Mapping

PSoC Creator Code

Listed below is an extract of some quick and grubby code used for testing the Mikroe with the PSoC Creator prototyping board.

/**

* @brief Main init and update VDAC based on quadrature encoder value

*/

int main()

{

   uint16_6 Encoder_Count, Encoder_Count_last = 0;

   CyGlobalIntEnable();

   QuadDec_Start();

   UART_Start();

   Opamp_Start();

   VDAC_Start();

   CyDelay(2000);     /* Time to stabalise */

   PSU_Enable_Write(true);    /* Switch DC DC On */

   pulse_on_switch1();

   for(;;)

   {

      Encoder_Count = QuadDec_GetCounter();

      if (Encoder_Count != Encoder_Count_Last)

      {

         if ((Encoder_Count <= MAX_ENC) && (Encoder_Count >= MIN_ENC))

         {

            write_to_uart(Encoder_Count);

            VDAC_SetValue(Encoder_Count);

         }

         else if (Encoder_Count > MAX_ENC)

         {

            Encoder_Count = MAX_ENC;

            QuadDec_SetCounter(MAX_ENC);

          }

         else if (Encoder_Count < MIN_ENC)

         {

            Encoder_Count = MIN_ENC;

             QuadDec_SetCounter(MIN_ENC);

         }

         Encoder_Count_Last = Encoder_Count;

      }

   }

}

The functionality of the test code was to read the Quadrature Decoder, ensures the value read was within the range, then write the value to the VDAC.

Hardware Setup

Shown in the image below is the hardware setup which consisted of the Buck 5 Click, PSoC board and the rotary encoder.

Buck 5 Click with Increased Output and PSoC VDAC

Output Voltage Range and Regulation

The calculated output voltage range was for 1.62 V to 20.33 V DC and the measured range was 2.39 V to 20.42 V DC.

Rudimentary load tests were conducted with a resistive load driven by the Buck 5 Click. Tests indicated a 0.25 % voltage regulation with low loads (1W) and better than 0.1 % with higher loads (65 W). The DC-DC converter used in the Buck 5 Click (MAX17506) has a capability of 5 A. The full load current was not load tested.

Output Voltage Adjustment using PSoC

To change the output voltage of the DC-DC converter without the encoder, the value written to the VDAC was controlled in software. When the switch located on the Cypress CY8CKIT-059 prototyping kit was pressed on power-up, the code generated several pulse types.

Buck 5 Click Output Voltage Waveform driven from PSoC VDAC

The capture above shows the Buck 5 Click output voltage ramp up and down with a 220 R resistive load.

Final Thoughts

The MikroElektronica Buck 5 Click was a reliable hardware module which allowed a Cypress PSoC to be interfaced for testing. Using a DAC to adjust the output voltage provided an alternative method to the on-board Potentiometer. The option to use PWM control for voltage adjustment was not reviewed in this post although this method should be considered as another solution.

Downloads

PSoC Creator 4.3 Power Supply Project (PSU) Project.

PSoC Creator 4.3 PSU Project