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摘要:本应用笔记解释了如何通过选择合适的外部元件来实现为其他模块供电的降压电压,从而构建集成降压转换器功率级。 介绍当系统具有高功率要求时,开关稳压器始终是首选。它们提供高效率和紧凑的 PCB 解决方案尺寸,以实现令人满意的电源。本应用笔记提供的方程式有助于在实现定制电压时选择降压型开关稳压器所需的外部元件。 降压转换器配置作为图1所示的基本降压开关稳压器配置,当高边开关在时钟开关周期开始时导通时,低边开关同时关断。V 同时,在高边开关导通时间间隔期间,电感电流I 图 1.降压转换器配置 图 2.降压转换器波形 选择电感器在连续导通模式下,电源开关在开关周期内的每个状态下消耗功率损耗。增加的电感电流仍然等于降低的电感电流。占空比按以下公式排列: 等式 (1) 其中参数为:V 等式 (2) 电感值越高,产生的电流纹波越小,ΔI 开关稳压器IC数据手册提供了电感选择范围,通常为固定的标称值\pm容差%,并且器件内部斜率补偿针对在可接受范围内选择的外部电感值进行了优化。一旦选择了电感值,峰值电流就导出为直流输出负载加一半的电感纹波电流。 电感纹波电流和峰值电流表示为: 等式 (3) Equation (4) where the parameters are:V Equation 5 where the parameters are:I Select inductors having a higher root mean square (RMS) current rating to avoid overheating impacting the efficiency and performance. The saturation current of the inductor stated in the data sheet should be larger than the maximum value of I Selecting the Input CapacitorWhen the high-side switch is in the ON state, the switching current flowing on the IC input path carries switching noise, and the input capacitor filters out most noise effectively and reduces input voltage ripple. The larger the load current drawn from the output, the larger the voltage ripple that is seen on the input. Place the input ceramic capacitor as close to the switching regulator as possible. Calculate the input capacitance for a specified input voltage ripple as: Equation (6) where the parameters are:I Equation (7) where the parameters are:I Equation (8) where the parameters are:V The I The actual ceramic capacitance should be larger than the value calculated above to give some margin on the capacitance loss by DC bias voltage derating. Follow the data sheet component selection guide for minimum ceramic capacitance needed to stabilize the input voltage. One 0.1µF ceramic capacitor should be preferably placed in parallel, decoupling high frequency noise on the input. Add aluminum bulk capacitance to provide large current during load transient and maintain an allowable input voltage deviation. Selecting the Output CapacitorThe output voltage ripple is contributed by the capacitor elements capacitance and ESR in steady state. Use low ESR, X7R ceramic capacitors to minimize the output voltage ripple. The output capacitance with a specific output voltage ripple requirement is expressed as: Equation (9) where the parameters are:ΔI Equation (10) where the parameters are:ΔI The capacitance is usually determined by the fast load transient response once the output voltage ripple is no longer a problem. Capacitance satisfies the voltage overshoot during the transient response while also meeting the undershoot requirement, because the inductor discharging slew rate is slower when releasing load relative to the faster inductor charging slew rate when applying load step on output. The output capacitance to maintain the overshoot voltage within a specified voltage change is expressed as: Equation (11) where the parameters are:ISTEP: Load step on outputL: Inductor valueΔV Select the recommended output capacitance as in the data sheet per its inductor value to get an optimal phase margin. Adjusting the Output VoltageOutput voltage is set either by the fixed internal resistor divider, or adjusted by the external resistor network, if applicable, with feedback to the switching regulator IC. The current I Follow the data sheet to choose the recommended bottom resistor R Equation (12) where the parameters are:R 图 3.可调电阻分压器网络配置 功耗在特定工况下,输出功率P 开关稳压器IC上的功耗表示为: 等式 (13) 其中参数为:V 器件内部的功耗转化为热量,增加结温。芯片 T 的结温 要估计给定环境 TA 中的结温,请执行以下操作: 等式 (14) 其中参数为:T 一些稳压器具有嵌入式芯片温度监控功能,无论稳压器在何种环境温度下工作,都可以直接在单个TEMP引脚上测量电压的方式指示结温。 当应用急需高功率电源时,电源开关稳压器解决方案的热管理具有挑战性。快速开关频率允许使用更紧凑的元件,以节省空间,以集成更多的功能解决方案,并在彼此之间引入更多的热传递。优化的 PCB 布局和热效率高的 IC 封装可冷却结温,使其更适合更高的环境工作条件。选择将功率平均分配到多个并联动力系上的多相配置可以缓解热压,而不是成为PCB上的热点,烧毁相邻元件。 |
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