Adjustable reference voltage sources offer the circuit designer great flexibility as this reference voltage is no longer limited to the manufacturer’s preset value. From the output to the feedback pin, the adjustable output is typically configured with a voltage divider, as shown in Figure 1. To regulate the output, the voltage at the feedback pin is compared to an internal reference voltage (shown in this post as VREF_INT), typically 1.2 V. The device will adjust the output voltage until VFB and VREF_INT match.
Some adjustable shunt references (such as the LM4041) make VFB pass through R1; others will make VFB pass through R2, such as the TLV431. while I am mainly looking at the LM4041, the concept applies to other adjustable shunt references as well, by converting R1 and R2 in the equation. On this blog post I will describe a way to change the resistive divider and the reference voltage with a digital signal.
Figure 1: A typical VREF feedback divider
A digital potentiometer is used in this method instead of two fixed resistors. This is conceptually illustrated in Figure 2, where the feedback pins are connected to the arc brushes of the potentiometer, with the high end connected to VREF and the low end connected to GND.
Figure 2: Potentiometer central tap (arc brush) connected to the feedback pins
Figure 3 shows the redrawn circuit, using the TPL0102 digital potentiometer as a voltage divider. A digital potentiometer can be fitted as a voltage divider by connecting the voltage across the high and low pins of the internal resistor, connecting the output to the arc brush pins. The position of the arc brush affects the resistance ratio between the arc brush and the high and low pins and in addition this position can be controlled digitally by sending a code to the device. the TPL0102 uses the I2C interface, the other potentiometers use the serial peripheral interface or the parallel interface.
Figure 3: VREF when the TPL0102 digital potentiometer is used as a feedback divider
Because the resistor ratio sets the output voltage, the absolute value of the crossover resistor is not critical. This makes it easy to replace the resistive crossover with a digital potentiometer. The relationship between the regulated output, VREF_OUT and the resistor ratio is shown in equation 1.
This is important in this application because the absolute resistance value of the digital potentiometer varies considerably, whereas the resistance ratio is very accurate. For example, to produce a reference voltage of 3.3 volts, a resistance ratio of 1.66 is required from R2 to R1.
Equations for calculating the output of a particular code voltage divider are provided in the potentiometer data sheet, as shown in equation 2 and equation 3. Where VHW represents the voltage from the high pin (H) to the arc brush and VWL represents the voltage from the arc brush (W) to the low pin (L).
I mentioned in the introduction that VFB passes through R1, so let’s go ahead and use equation 2 to calculate the voltage between the high pin and the arc brush pin. The arc brush is connected to the feedback pin of the device and VFB is forced to be VREF_INT. equation 4 shows the equation used to find the numeric code required for VREF_OUT.
Continuing with the example in equation 5, where NTAPS is 256, VREF_INT is 1.24 volts and VREF_OUT is 3.3 volts, you need to write the decimal code 160 to derive the values of the R1 and R2 resistors as 37.50 kW and 62.50 kW respectively. more importantly, the ratio calculated using equation 1 for these two resistors is also 1.66.
If you need to change the reference voltage, you simply write out the I2C transaction and move the arc brush position accordingly. The voltage at the feedback pin therefore changes and regulates VREF_OUT. You can also adjust the reference voltage digitally using a potentiometer. The more taps on the potentiometer, the higher the resolution of the resistivity and the higher the resolution of the output reference voltage.
A major drawback of using a digital potentiometer in this application is that the voltage limit of a digital potentiometer IC cannot normally exceed 5.5 volts. It is certain that resistivity will not occur with VREF_OUT greater than 5.5 volts. For a digital potentiometer with 256 taps and an internal reference voltage of 1.24 volts in parallel, the decimal code cannot exceed 200. Figure 4 shows the effect of the input code on the reference voltage of a potentiometer with 256 taps and a 1.24 volt reference device.
Figure 4: Reference voltage with digital potentiometer code
Installing and using a potentiometer prior to energising the parallel reference will ensure that the resistive divider is placed in the correct position. If this is not feasible, a large resistor can be added in parallel to the resistor that does not lower the VFB. This will result in 1MW from the feedback pin (via R2) to ground for the LM4041 or 1MW from the feedback pin (via R1) to the output port for the TLV431.
To avoid creating another parallel resistor, fixed resistors R1 and R2 can be fitted to the device and a digital potentiometer connected in series with one of the resistors. The potentiometer then needs to be installed as a varistor, as shown in Figure 5. This mounting method depends on the absolute resistance value of the digital potentiometer, which is not as accurate as when used as a ratiometric divider, and requires feedback to the microcontroller to make the final choice of digital code.
Figure 5: Fixed resistor with a digital potentiometer as a varistor
Now that you understand how to set the output voltage with an adjustable reference voltage, you know how to use it to change the output voltage at any time. If you are already using a microcontroller in your application, using a digital potentiometer in conjunction with it can add some functionality to a seemingly simple component.
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