# A voltage divider is used in the power supply design to achieve the desired output voltage

In the power supply design, the desired output voltage can be set manually. In most integrated power circuits and switching and linear regulator ICs, this can be accomplished with a voltage divider. To be able to set the desired output voltage, the ratio of the two resistors must be appropriate. Figure 1 shows a voltage divider.

In the power supply design, the desired output voltage can be set manually. In most integrated power circuits and switching and linear regulator ICs, this can be accomplished with a voltage divider. To be able to set the desired output voltage, the ratio of the two resistors must be appropriate. Figure 1 shows a voltage divider.

Figure 1. The voltage divider in a voltage regulator is used to regulate the output voltage.

The internal reference voltage (VREF) and the desired output voltage determine the resistance ratio of the resistors, see Equation 1:

The reference voltage VREF is defined by a switching regulator or linear regulator IC and is typically 1.2V, 0.8V or 0.6V. This voltage represents the lowest voltage value at which the output voltage (VOUT) can be set. With the reference and output voltages known, there are two more unknowns in the equation: R1 and R2. One of the two resistor values ​​can now be chosen relatively freely, typically less than 100kΩ.

If the resistor value is too small, the power dissipation due to the constant current VOUT/(R1+R2) flowing during operation is extremely high. If both R1 and R2 have a value of 1kΩ, the continuous leakage current flowing at an output voltage of 2.4V will be 1.2mA. This equates to a power dissipation of 2.88mW from the voltage divider alone.

Depending on how accurately the output voltage needs to be set and the amount of current in the power supply error amplifier at the FB pin, Equation 1 can be used more precisely by taking this current into account.

However, the resistor value should also not be too large. If the resistor values ​​are all 1MΩ, the power dissipation is only 2.88µW. A major disadvantage of setting the resistor value too high is that it results in a very high feedback node impedance. The current flowing into the feedback node can be very low, depending on the voltage regulator. Therefore, noise can couple into the feedback node and directly affect the control loop of the power supply. This stops regulation of the output voltage and causes instability in the control loop. Especially in switching regulators, this characteristic is critical, because the rapid switching of current can cause noise and couple to the feedback node.

The effective resistance value of R1+R2 is between 50kΩ and 500kΩ, depending on the expected noise of the other circuit segments, the output voltage value, and the need to reduce power dissipation.

Another important aspect is the placement of the voltage divider in the board layout. The feedback node should be designed to be as small as possible so that the noise coupling into this high impedance node is extremely low. Resistors R1 and R2 should also be placed very close to the feedback pins of the power IC. The connection between R1 and the load is usually not a high impedance node, so longer traces can be designed. Figure 2 shows an example of placing a resistor close to the feedback node.

Figure 2. An example of a properly configured voltage divider in a power supply.

To reduce the power consumption of the voltage divider, especially in ultra-low power applications such as energy harvesting, some ICs such as the ADP5301 buck regulator are equipped with an output voltage setting function that checks its Variable resistor value on the VID pin. This value is then saved for subsequent work without the continuous flow of current through the voltage divider. This is a very smart solution for efficient applications.

Figure 3. Regulating the output voltage without incurring continuous power dissipation in the voltage divider.

The ADP5301, a low-power buck regulator with industry-leading ultra-light-load power conversion efficiency, extends battery life in portable devices. The ADP5301 buck regulator, rated at 90% efficiency and only 180 nA quiescent current, delivers maximum power for longer than previous devices, making it ideal for Internet of Things (IoT) applications including wireless sensor networks and wearables , such as fitness bands and smartwatches.

Author: Yoyokuo