Here is the third of three short tech nuggets, by Rich Nowakowski from TI, before our “Ask the Experts” session on Wednesday, July 23, at 1:00 p.m. ET (10:00 a.m. PT). Hopefully these brief blogs will help stir your curiosity and questions, which you can post on our chat site here either before or during the session.
Deep sub-micron process technology is impacting the power management solution of performance DSPs, FPGAs, and ASICs. Advanced processes require tighter core voltage regulation accuracy, even during load transients. Faster switching frequencies and newer control modes have been shown to help reduce voltage over- and under-shoots and during load transient events. Here is an article that discusses improving DC output voltage accuracy with proper feedback resistor selection, but perhaps at the expense of system efficiency.
Using large feedback resistances to increase efficiency is common. However, choosing resistances that are too large affects the converter’s output-voltage accuracy because of leakage current going into the converter’s feedback pin. Figure 1 shows the current paths at the resistive feedback divider (R1 and R2). For a fixed feedback leakage current (IFB ), current through R1 (IR1) decreases as the values of R1 and R2 increase. Therefore, an increase in divider resistance means that a larger percentage of IR1 leaks into the feedback pin, and the current through R2 (IR2) decreases, causing a lower feedback-pin voltage (VFB ) than expected. Since VFB is compared to an internal reference voltage to set the output voltage, any inaccuracies in the feedback voltage create inaccuracies in the output voltage.
Equation 1 can be derived from Kirchhoff’s Current Law, showing VFB as a function of R1 and R2. Note that IFB is not fixed in a real system and can vary from device to device and over the operating conditions. To generate a worst-case estimate of the output-voltage change that is due to the leakage current, the specified maximum value of IFB is used in the calculations.
How important is output voltage accuracy in your application, and what other techniques or features can help improve accuracy?