Detailed Design Procedure for LM25576

Custom Design With WEBENCH® Tools for LM25576

• RT sets the oscillator switching frequency. Generally, higher-frequency applications are smaller but have higher losses. Operation at 500 kHz was selected for this example as a reasonable compromise for both small size and high efficiency. RT is calculated using the following equation:
  Equation 3. $R_{T }= \frac{\left(\frac{1}{f_S}\right) - 580\times {10}^{-9}}{135 \times {10}^{-2}}$
• The inductor value is determined based on the operating frequency, load current, ripple current, and the minimum and maximum input voltage (V_IN(min), V_IN(max)). To keep the circuit in continuous conduction mode (CCM), the maximum ripple current (I_RIPPLE) should be less than twice the minimum load current, or 0.1 A_p-p. Using this value of ripple current, the value of inductor (L1) is calculated using the following equation:
  Equation 4. $L1 = \frac{V_{OUT} \times \left(V_{IN\left(MAX\right)} - V_{OUT}\right)}{I_{RIPPLE} \times f_S \times V_{IN\left(MAX\right)}}$
• With the inductor value selected, the value of C_RAMP necessary for the emulation ramp circuit is calculated using the following equation:
  Equation 5. $C_{RAMP }= L1 \times {10}^{-5}$
• A Schottky type re-circulating diode is required for all LM25576 applications. Ultra-fast diodes are not recommended and may result in damage to the device due to reverse recovery current transients. The near-ideal reverse recovery characteristics and low forward voltage drop are particularly important diode characteristics for high input voltage and low output voltage applications common to the LM25576. The reverse recovery characteristic determines how long the current surge lasts each cycle when the buck switch is turned on. The reverse recovery characteristics of Schottky diodes minimize the peak instantaneous power in the buck switch occurring during turn-on each cycle. The resulting switching losses of the buck switch are significantly reduced when using a Schottky diode. The reverse breakdown rating should be selected for the maximum V_IN, plus some safety margin.
• The regulator supply voltage has a large source impedance at the switching frequency. Low ESR ceramic input capacitors are necessary to limit the ripple voltage at the V_IN pin while supplying most of the switch current during the on-time. When the buck switch turns on, the current into the V_IN pin steps to the lower peak of the inductor current waveform, ramps up to the peak value, then drops to zero at turn-off.

For more information, see the Working with Inverting Buck-Boost Converters application report.