Generating a Higher Output Voltage than 1.525V Using the MAX77812

Introduction

The MAX77812 supports programmable output voltage from 0.25V to 1.525V in 5mV steps through an I²C interface. For some applications, an output voltage higher than 1.525V is required. The MAX77812 supports the higher output voltage with the addition of an external voltage-divider network.

External Voltage-Divider Network

A buck converter regulates the output voltage to the target value by comparing the sensed output voltage (V_SNSxP) to the internal reference. If V_SNSxP is lower than the actual output voltage (V_OUTx), V_OUTx will be higher than the nominal output voltage set through the I²C interface.

As shown in Figure 1, the external voltage-divider network consists of feedback resistors (R_FB1 and R_FB2) and a feed-forward capacitor (C_FF). The resistors divide V_OUTx to the lower value V_SNSxP at the remote sense input (SNSxP):

The internal sensing resistor at SNSxP is R_SNS. Voltage V_SNSxP is then compared to the internal reference set by the output voltage setting register (Mx_VOUT[7:0]). Therefore, the relation between the actual and the nominal output voltages is:

An output voltage higher than 1.525V can be achieved by adjusting Mx_VOUT[7:0], and the voltage-dividing ratio is:

Value Selection

The selection of R_FB1 and R_FB2 must guarantee the accuracy of output regulation and minimize the power loss on these resistors. The resistance of R_FB2 should be significantly smaller than R_SNS to be dominant. Because the resistance of R_SNS is approximately 350kO, the recommended value for R_FB2 is around 51.1kO. To minimize the difference between the actual and the nominal output voltages, Mx_VOUT[7:0] is selected as close to 1.525V as possible. Once R_FB2 and Mx_VOUT[7:0] are fixed, R_FB1 can be selected based on equation (2). The accuracy of the output voltage highly relies on the accuracy of the voltage-dividing ratio, thus ±1% or better resistors are recommended for R_FB1 and R_FB2.

The external voltage-divider network creates an additional pole and zero at (R_FB2 ||R_SNS˜R_FB2) for simplified calculation:

To maintain the loop stability, the recommended value for C_FF is around tens of picofarads and is determined by the values of R_FB1 and R_FB2.

Table 1 shows a few examples of the value selection recommendation for common output voltages.

Considerations

Although the output voltage can be higher than 1.525V, it is still limited by the input voltage and load current. Theoretically speaking, the constant on-time is proportional to the ratio of the actual output voltage (V_OUTx) to the input voltage (V_IN). The MAX77812 calculates the on-time by sensing V_SNSxP and V_IN. Because V_SNSxP is lower than V_OUTx, the on-time is insufficient. As a result, more switching cycles are needed and thus the switching frequency (f_SW) increases. The constant on-time control architecture also exhibits higher f_SW when the load current increases. Therefore, supporting a high V_OUTx under the heavy load condition leads to a substantial increase of f_SW, which may be ultimately limited by the control architecture. For the same V_IN, the higher the actual output voltage is, the lower the maximum load current that can be supported. Table 1 provides examples of the maximum load current measured on the bench. To mitigate the switching frequency increase, Mx_VOUT[7:0] needs to be selected as close to 1.525V as possible for a longer on-time.