Certain of the new, high-power microprocessors for portable applications (Strong ARM and others) require dual power-supply voltages for operation. As an example, these voltages can be 3.3V for the I/O circuitry and 1.5V for the processor core. A battery of two AA cells, rechargeable or primary (available at most corner shops), is popular for these applications because it offers very high power density. A compact circuit for this application (Figure 1) is based on a dual DC-DC boost controller (IC1). Output 1, configured as in a standard application, provides the 3.3V rail and delivers output currents up to 1A. If necessary, the output voltage can be adjusted by adding external feedback to the FB1 terminal. Figure 1. Powered by two AA cells, this circuit generates dual supply rails for a portable µP system. IC1 is powered by a charge-pump doubler (IC2) that generates 6.6V from the 3.3V rail. (You can omit this device if your application includes a suitable bias voltage.) Connecting IC2's FC pin to VCC sets the chip to run at its lowest operating frequency (6kHz), for which the quiescent current is minimum (~70µA at 3V supply). Locate the C6 bypass capacitor very close to IC1's VDD pin. Output 2 is configured as a step-down switcher instead of as a boost, which is the standard topology for both Output 1 and Output 2. The IC's internal MOSFET driver turns on the external, high-side MOSFET (Q1B) by pulling its gate to 6.6V. With its source connected to the battery, Q1B's VGS ranges from 3.6V (battery fresh) to 4.6V (battery end-of-life). Connecting CS2 to ground disables current sensing for controller 2, and the inductor's value (47µH) ensures discontinuous conduction without saturation. Feedback to the FB2 terminal via R5 and R6 sets the output-voltage level: VOUT = VREFR6/(R5+R6), where VREF = 1.25V. Quiescent current for the circuit is 465µA with a 2V battery and 280µA with a 3V battery. Efficiency can exceed 80% for the step-up output (Figure 2), and 90% for the step-down output (Figure 3). (For each graph, the other output is unloaded.) active-low SHDN1 and active-low SHDN2 provide independent shutdown control for the two outputs, but output 2 (1.5V) will be absent unless output 1 (3.3V) is present. R1 and R2 set the threshold (VTH) for low-battery detection: VTH = VREFR2/(R1+R2), where VREF = 1.25V. Figure 2. Efficiency for Figure 1's step-up output (3.3V) varies with input voltage and output current. Figure 3. Efficiency for Figure 1's step-down output (1.5V) also varies with input voltage and output current. 相关型号   APP 1783: Nov 11, 2002 MAX860 50mA、频率可选的开关电容电压变换器 完整的数据资料 (PDF, 496kB) 免费样品 MAX863 双路、高效率、PFM、升压型DC-DC控制器 完整的数据资料 (PDF, 312kB) 免费样品 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)。 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   下载，PDF格式 (51kB)  AN1783, AN 1783, APP1783, Appnote1783, Appnote 1783