Adjusting the output voltages

Figure 3B shows how to use two external resistors to increase the REG3 and/or REG4. Follow the steps in the tab titled "REG3-4 Output Voltage Inc." in the spreadsheet to find the resistor values and the output voltage accuracy of this solution.

• The circuit of Figure 3B uses two external resistors (R1 and R2) to increase the standard output voltage. As shown in the spreadsheet, R_1 and R_2 increase the output voltage over the entire I2C-adjustable output-voltage range. If the converter's switching waveforms exhibit excessive jitter or pulse grouping compared to the standard configuration, add a feed-forward capacitor in the range of 10pF to 100pF. To find the best capacitor for your application, start with a 33pF and make adjustments from there.

Figure 3. Increasing the output voltage of REG3 and 4.

Decrease the V3 and V4 output voltage with external resistors

Figure 4B shows how to decrease the REG3 and/or REG4 output voltage (same technique as in Figure 2). Follow the steps in the tab titled "REG3-4 Output Voltage Inc." in the spreadsheet to find the resistor values and output voltage accuracy of the two solutions:

• The circuit of Figure 4B uses two external resistors (RFBI and RFBV) to decrease the standard output voltage. To decrease the output voltage, an "always-on" reference is needed. In this case we use V8. If accuracy is critical, the V8 reference can be substituted with a voltage reference like the MAX6029. Lastly, this configuration requires a minimum load on the regulator (RLMIN) to ensure that the output does not drift up to the reference.
• To eliminate the load on the reference voltage that occurs in Figure 4B when the regulator is shutdown, use the disconnect circuit scheme shown in Figure 2C.

Figure 4. Decreasing the output voltage of REG3 and 4.

Please contact the Maxim factory with output voltage questions that are beyond the scope of this application note. We appreciate your interest in the MAX8660/MAX8661, and have several other options for adjusting output voltages.