Table 24. EEPROM Setting Example

9.2.1.2.1 Inductor Selection

There are two main considerations when choosing an inductor; the inductor must not saturate, and the inductor current ripple must be small enough to achieve the desired output voltage ripple. Different saturation current rating specifications are followed by different manufacturers so attention must be given to details. Saturation current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of application should be requested from the manufacturer. Shielded inductors radiate less noise and are preferable. The saturation current must be greater than the sum of the maximum load current and the worst case average-to-peak inductor current. The equation below shows the worst case conditions.

Equation 9.

where

• I_RIPPLE: peak inductor current
• I_OUTMAX: maximum load current
• V_IN: minimum input voltage in application
• L: min inductor value including worst case tolerances
• f: minimum switching frequency
• V_OUT: output voltage
• D: Duty Cycle for CCM Operation
• V_OUT: Output Voltage

As a result the inductor must be selected according to the I_SAT. A more conservative and recommended approach is to choose an inductor that has a saturation current rating greater than the maximum switch current limit defined by <BOOST_IMAX_SEL[2:1]> EEPROM bits. A 22-µH to 33-µH inductor with a saturation current rating of at least 9 A is recommended for most applications. The inductor resistance must be less than 300 mΩ for good efficiency. See detailed information in Texas Instruments Application Note Understanding Boost Power Stages in Switch Mode Power Supplies (SLVA061). “Power Stage Designer™ Tools” can be used for the boost calculation: http://www.ti.com/tool/powerstage-designer.

9.2.1.2.2 Output Capacitor Selection

A ceramic capacitor with a 100-V voltage rating is recommended for the output capacitor. The DC-bias effect can reduce the effective capacitance by up to 80%, a consideration for capacitance value selection. Effectively the capacitance must be 33 µF for 600-mA loads. A different option is to use an aluminum electrolytic capacitor with low ESR and ceramic capacitor in parallel. Typically a 33-µF (ESR < 500 mΩ) with 10-µF (effective) ceramic capacitor in parallel is sufficient. If ESR is lower, capacitance for ceramic capacitor can be decreased.

For higher switching frequency (2.2 MHz) and boost output current below 400 mA, two 10-µF ceramic capacitors in parallel are sufficient.

9.2.1.2.3 Input Capacitor Selection

A ceramic capacitor with 50-V voltage rating is recommended for the input capacitor. The DC-bias effect can reduce the effective capacitance by up to 80%, a consideration for capacitance value selection. Effectively the capacitance must be 33 µF for 600-mA loads. A different option is to use an aluminum electrolytic capacitor with low ESR and ceramic capacitor in parallel. Typically a 33-µF (ESR < 500 mΩ) with 10-µF (effective) ceramic capacitor in parallel is sufficient. If ESR is lower, capacitance for ceramic capacitor can be decreased.

For higher switching frequency (2.2 MHz) and boost output current below 400 mA two 10-µF ceramic capacitors in parallel are sufficient.

9.2.1.2.4 Charge Pump Output Capacitor

A ceramic capacitor with at least 16-V voltage rating is recommended for the output capacitor of the charge pump. The DC-bias effect can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Typically a 10-µF capacitor is sufficient.

9.2.1.2.5 Charge Pump Flying Capacitor

A ceramic capacitor with at least 10-V voltage rating is recommended for the flying capacitor of the charge pump. Typically 1-µF capacitor is sufficient.

9.2.1.2.6 Diode

A Schottky diode must be used for the boost output diode. Peak repetitive current must be greater than inductor peak current (up to 9 A) to ensure reliable operation. Average current rating must be greater than the maximum output current. Schottky diodes with a low forward drop and fast switching speeds are ideal for increasing efficiency. Choose a reverse breakdown voltage of the Schottky diode significantly larger than the output voltage. Do not use ordinary rectifier diodes because slow switching speeds and long recovery times cause the efficiency and the load regulation to suffer.

9.2.1.2.7 Boost Converter Transistor

An nFET transistor with high enough voltage rating (V_DS at least 5 V higher than maximum output voltage) must be used. Current rating for the FET must be the same as the inductor peak current. Gate-drive voltage for the FET is VDD or about 2 x VDD, if the charge pump is enabled (EEPROM selection).

9.2.1.2.8 Boost Sense Resistor

A high-power 25-mΩ resistor must be used for sensing the boost SW current. Power rating can be calculated from the inductor current and sense resistor resistance value.

9.2.1.2.9 Power Line Transistor

A pFET transistor with necessary voltage rating (V_DS at least 5 V higher than max input voltage) must be used. Current rating for the FET must be the same as input peak current or greater. Transfer characteristic is very important for pFET. V_GS for open transistor must be less then V_IN. A 20-kΩ resistor between the pFET gate and source is sufficient.

If a pFET with high enough V_DS and low V_GS is not available, it is possible to use an nFET with extra external components with the EEPROM bit NMOS_PLFET_EN set high. See Charge Pump section (Figure 25) for using the nFET as a power-line FET.

9.2.1.2.10 Input Current Sense Resistor

A high-power 20-mΩ resistor must be used for sensing the boost input current. Power rating can be calculated from the input current and sense resistor resistance value.

9.2.1.2.11 Filter Component Values

Table 25 shows recommended filter component values for the VSYNC PLL filter (phase margin 60°). An external filter must be used only when external VSYNC is used; otherwise, the LP8860-Q1 uses internal compensation.

Figure 54. Filter Components

9.2.1.2.11.1 Critical Components for Design

Schematic on Figure 55 shows the critical part of circuitry: boost components, the LP8860-Q1 internal charge pump for gate driver powering and powering/grounding of LP8860-Q1 boost components. Layout example for this is shown in Figure 67.

Figure 55. Critical Components for Design

Table 26. Bill of Materials for Design Example

REFERENCE DESIGNATOR	DESCRIPTION	NOTE
R1	20 mΩ 3 W	Input current sensing resistor
R2	20 kΩ 0.1 W	Power-line FET gate pullup resistor
R3	10 Ω 0.1 W	Gate resistor for boost FET
R4	10 Ω 0.1 W	Current sensing filter resistor
R5	25 mΩ 3 W	Boost current sensing resistor
C1	1 μF 10 V ceramic capacitor	VDD bypass capacitor
C2	10 μF 16 V ceramic capacitor	Charge pump output capacitor
C3	33 μF 50 V electrolytic capacitor	Boost input capacitor
C4	10 μF 50 V ceramic capacitor	Boost input capacitor
C5	1 μF 10 V ceramic capacitor	Flying capacitor
C6	1000 pF 10 V ceramic capacitor	Current sensing filter capacitor
C7	39 pF 50 V ceramic capacitor	High frequency bypass capacitor
C8	33 μF 50 V electrolytic capacitor	Boost output capacitor
C9	10 μF 100 V ceramic capacitor	Boost output capacitor
L1	22 μH saturation current 9 A	Boost inductor
D1	60 V 15 A Schottky diode	Boost Schottky diode
Q1	60 V 10 A pMOSFET	Power-line FET
Q2	60 V 15 A nMOSFET	Boost nMOSFET

Table 27. EEPROM Setting Example

Table 28. EEPROM Setting Example

Table 29. EEPROM Setting Example

Table 30. EEPROM Setting Example

Table 31. EEPROM Setting Example