A small and simple circuit (Figure 1) derives +5V from the -48V rail widely used in telecom applications. Useful for gate bias and other purposes, the 5V supply delivers up to 5mA. A shunt reference (U1) defines -5V as ground reference for a charge pump (U2), and the charge pump doubles this 5V difference (between system ground and charge pump ground) to produce +5V with respect to the system ground. Figure 1. This small circuit (six components) produces 5V at 5mA from a -48V input. The shunt reference maintains 5V across its terminals by regulating its own current (IS), which in turn is determined by the value of R. Current through R (IR) is fairly constant, and varies only with the input voltage. IR, the sum of the charge-pump and shunt-reference currents (IR = ICP + IS), has maximum and minimum values set by the shunt reference. The shunt reference sinks up to 15mA, and requires 60µA minimum to maintain regulation. Maximum IR is determined by the maximum input voltage. To prevent excessive current in the shunt reference with no load on the charge-pump output, use the maximum input voltage (-48V -10% = -52.8V) when calculating the minimum value of R. The maximum reference sink current (15mA) plus the charge pump's no-load operating current (230µA) equals the maximum IR value (15.23mA). Thus, RMIN = (VIN(MAX) - VREF) / IR(MAX) = (52.8V - 5V) / 0.01523A = 3.14kΩ. Choose the next-highest standard 1% value, which is 3.16kΩ. Guaranteed output current for the charge pump is calculated at the minimum line voltage: -48V + 10% = -43.2V. The charge pump's maximum input current is: ICP = (VIN(MIN) - VREF) / R - ISH(MIN) = (43.2V - 5V) / 3.16kΩ - 90µA = 12mA, where 90µA is the minimum recommended operating current for the shunt reference. Assuming 90% efficiency in the charge pump, the output current is IOUT = (ICP / 2) × 0.9 = (12mA / 2) × 0.9 = 5.4mA. Charge-pump input current is halved, because output voltage is twice the input voltage. Power is dissipated via the shunt reference under no-load conditions, so be sure that R can handle the resulting wattage. A 1W resistor suffices in this case. A similar version of this article appeared in the September 19, 2002 issue of EDN magazine. 関連製品   APP 1803: Dec 05, 2002 MAX1683 スイッチトキャパシタ電圧ダブラ フルデータシート (PDF, 112kB) 無料 サンプル MAX6138 複数の逆方向降伏電圧付き、0.1%、25ppm、SC70シャント電圧リファレンス フルデータシート (PDF, 336kB) 無料 サンプル 自動アップデート お客様が関心のある分野でアプリケーションノートが新規に掲載された際に自動通知Eメールの受信を希望する場合は、EE-Mail™にご登録ください。 We Want Your Feedback! フィードバックをお寄せください。 内容に満足されましたか、あるいは満足されていませんか？もっと良いページにできると思いますか？あるいは、単なるコメントでも結構です。フィードバックをお待ちしています。—マキシムはお客様からいただく訂正、提案を元に改善していきます。 このページを評価し、フィードバックを送信する。   ダウンロード、PDFフォーマット(31kB)  AN1803, AN 1803, APP1803, Appnote1803, Appnote 1803