Abstract: This circuit uses pulse-frequency-modulation (PFM) to generate +3.3V from a -5V power supply without need for a transformer. It is ideal for applications that have a stable -5V power supply and do not require isolation. The circuit of Figure 1 uses pulse-frequency-modulation (PFM) to generate +3.3V from a -5V power supply without need for a transformer. It is ideal for applications that have a stable -5V power supply and do not require isolation. Figure 1. The connections shown allow this dc-dc boost converter to derive +3.3V from a -5V supply. The switch-mode regulator (IC1) operates in the classic boost configuration, in non-bootstrapped mode. Conversion with respect to the IC is +5V to +8.3V, but the connections shown (GND and AGND to -5V and V+ to system ground) enable the circuit to produce an output of 3.3V relative to the system ground. Its conversion efficiency is over 90% for 1A loads and about 84% for light loads of ~10mA (Figure 2). Figure 2. Efficiency vs. load current for the Figure 1 circuit. Figure 3 shows the output voltage (set by external resistors) vs. load current. For a 3.3V output, load regulation is within 3mV for load currents up to 500mA, and within 3% of 3.3V for load currents up to 2A. You adjust the circuit output by selecting the values of R1 and R2: where the feedback resistor (R1) is between 10k and 500k, and VREF = 1.5V. Figure 3. Output voltage vs. load current for the Figure 1 circut. The input and output capacitors (C1 and C4) should have very low effective series resistance (ESR) to ensure high efficiency and minimum output-voltage ripple. As shown, C1 is a 100µF OS-CON capacitor with 25m ESR, and C4-C5 are 150µF OS-CON capacitors with 25m ESR. For light loads or in applications that can tolerate higher output ripple, capacitors with smaller values or higher ESR (or both) are acceptable. Because the -5V supply voltage serves as a ground reference for IC1, that voltage should be very stable. Any voltage change in the -5V rail appears with unchanged magnitude at the +3.3V output. This design idea appeared in the May 24, 2004 issue of ED magazine. 我们期待您的反馈！ 喜欢？不喜欢？有待改善？或为我们提供建议？请与我们联系 — 我们将根据您的意见或建议改善我们的工作。 网页评价或提供建议 自动更新 需要自动接收最新发布的应用笔记吗？请订阅EE-Mail™ (English only)。   下载，PDF格式 (37kB)  AN3325, AN 3325, APP3325, Appnote3325, Appnote 3325