【Shi Shuo Design】Detailed discussion of voltage dividers in power supplies

[Figure 1 shows a voltage divider]

Figure 1. A voltage divider in a voltage regulator is used to adjust the output voltage.

The internal reference voltage (VREF) and the desired output voltage determine the resistor value ratio, see Equation 1:

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

_{If the resistor values ​​are too small, the power dissipation during operation due to the constant current V OUT} /(R1+R2) flowing will be very high. If the values ​​of R1 and R2 are both 1kΩ, the continuous leakage current flowing at an output voltage of 2.4V will be 1.2mA. This is equivalent to a power dissipation of 2.88mW in the voltage divider alone.

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

However, the resistor values ​​should 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 values ​​too large is that it results in a very high feedback node impedance. Depending on the voltage regulator, the current flowing into the feedback node can be very low. Therefore, noise can couple into the feedback node and directly affect the control loop of the power supply. This can suspend the regulation of the output voltage and cause instability in the control loop. Especially in switching regulators, this characteristic is critical because the fast switching of the current induces noise that couples into the feedback node.

Effective resistance values ​​for R1+R2 are between 50kΩ and 500kΩ, depending on the expected noise from other circuit sections, the output voltage value, and the need to minimize 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 coupled into this high impedance node is very low. Resistors R1 and R2 should also be very close to the feedback pin of the power IC. The connection between R1 and the load is usually not a high impedance node, so a long trace can be designed. Figure 2 shows an example of placing the resistors close to the feedback node.

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

To reduce the power dissipation in 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 the variable resistor value on its VID pin only once during startup. This value is then stored for subsequent operation without current continuously flowing through the voltage divider. This is a very smart solution for high-efficiency applications.

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

The ADP5301 low-power buck regulator has industry-leading ultra-light load power conversion efficiency, which can extend the battery life of portable devices. With a rated efficiency of 90% and a quiescent current of only 180 nA, the ADP5301 buck regulator can provide maximum power for a longer time than previous devices, making it ideal for Internet of Things (IoT) applications, including wireless sensor networks and wearable devices such as fitness bands and smart watches.