Detailed Design Procedure for LM51571

Custom Design With WEBENCH® Tools for LM51571

Click here to create a custom design using the device with the WEBENCH® Power Designer.

• Start by entering the input voltage (V_IN), output voltage (V_OUT), and output current (IOUT) requirements into WEBENCH® . For this design V_IN = 12 V (typical), V_OUT = 24 V, I_OUT = 500 mA
• Optimize the design for key parameters such as efficiency, footprint, and cost using the optimizer dial in WEBENCH Power Designer.
• Run electrical simulations to see important waveforms and circuit performance (If supported by the device). Otherwise download the spice model from website and simulate the circuit using PSpice® for TI

Design Notes for LM51571

• When selecting the inductor (L1), consider three key parameters: inductor current ripple ratio (RR), falling slope of the inductor current, and Right Half Plane (RHP) zero frequency (f_RHP). RR is selected to have a balance between core loss and copper loss. The falling slope of the inductor current must be low enough to prevent subharmonic oscillation at high duty cycle (additional RSL resistor is required, if not). Higher f_RHP (lower inductance) allows a higher crossover frequency and is always preferred when using a small value output capacitor. The inductance value can be selected to set the inductor current ripple between 30% and 70% of the average inductor current to achieve a good compromise between RR, f_RHP, and inductor falling slope
• To set the output regulation target, select the feedback resistor values as shown in the following equation:
  Equation 1. $V_{OUT} = V_{REF }\times \frac{R_{FB1}}{R_{FB2}}$

  where:

  • V_REF = 1 V (typical)
• There are a few ways to select the proper value of the output capacitor (C_OUT). Select the output capacitor value based on output voltage ripple, output overshoot, or undershoot due to load transient. The ripple current rating of the output capacitors must be enough to handle the output ripple current. By using multiple output capacitors, the ripple current can be split allowing for a capacitor with a lower ripple current to be chosen.
• A Schottky diode is the preferred type for a D1 diode due to its low forward voltage drop and small reverse recovery charge. Low reverse-leakage current is an important parameter when selecting the Schottky diode. The diode must be rated to handle the maximum output voltage plus any switching node ringing. Also, it must be able to handle the average output current.
• The switching frequency of the device can be set by a single resistor connected between RT and the GND pins using the following equation:
  Equation 2. $R_T = \frac{2.21 \times { 10}^{10}}{f_{RT\left(TYPICAL\right)}} - 955$

See the How to Design a Boost Converter Using LM5155 application report for calculating compensation components or using WEBENCH Power Designer.