Table 2. Setting Boost Switching and PWM Dimming Frequencies With an External Resistor

Table 3. Recommended Boost Compensation Options Based
on the Boost Switching Frequency and Inductance

Table 4. Setting Full-Scale LED Current and Boost Compensation Using an External Resistor

Table 5. Available Boost Switching Frequencies

Table 6. Recommended Boost Compensation Options Based
on the Boost Switching Frequency and Inductance

Table 7. Available Boost Compensation Options

Table 8. Available PWM Dimming Frequencies

Table 9. Available Full-Scale LED Currents

Table 10. Recommended Inductance

Table 11. Recommended Output Capacitance⁽¹⁾

9.2.1.2.1 Boost Output Capacitor Selection

The LP8557 inductive boost converter typically requires two 4.7-µF output capacitors. The voltage rating of the capacitor must be 35 V or higher as the OVP threshold is at 29.6 V (typ). Pay careful attention to the capacitor tolerance and DC bias response. For proper operation the degradation in capacitance due to tolerance, DC bias, and temperature should stay above 2 µF. This might require placing more than two devices in parallel in order to maintain the required output capacitance over the device operating temperature and output voltage range.

9.2.1.2.2 Schottky Diode Selection

The Schottky diode must have a reverse breakdown voltage greater than the LP8557’s maximum output voltage. Additionally, the diode must have an average current rating high enough to handle the LP8557 maximum output current; at the same time the diode's peak current rating must be high enough to handle the peak inductor current. Schottky diodes are required due to their lower forward voltage drop (0.3V to 0.5V) and their fast recovery time.

9.2.1.2.3 Inductor Selection

The chosen inductor must be from 10 to 22 µH (for 500-kHz operation) or 4.7 to 10 µH (for 1-MHz operation) and must have a saturation rating equal to, or greater than, the circuit's peak operating current. I_PEAK can be found by the following calculation:

Equation 1.

This assumes the device is operating in continuous conduction mode (CCM) which is typically the case when operating near the peak current. For smaller rated inductors, and when the device is operating in discontinuous conduction mode (DCM), the peak current can be found from:

Equation 2.

The device operates in CCM when the following is true:

Equation 3.

9.2.1.2.4 Boost Input andVDD Capacitor Selection

The LP8557 VDD pin is typically tied to the same supply as the input of the boost power stage (V_IN node). A 10-µF input capacitor is recommended on that node. The voltage rating of the capacitor must be at least 10 V. If a supply powering the VDD pin is different from a supply powering the boost power stage, then 10-µF input capacitors are required on both VDD and VIN nodes.