9.2.1.1 R_FB1, R_FB2, and C_FB

The nominal voltage of the LED string with 10 LEDs is 30 V, and the minimum voltage of the IOUTn pin (n = 1, 2) is 1.25 V when I_LED is 250 mA. As a result, V_OUT(nom) is 31.25 V. Design V_OUT(max) to be 50 V. From Equation 5, V_FB(nom) is approximately 1.8 V, which falls in the recommended operation range from 1.05 V to 2 V. Also, design R_FB2 to be 16.2 kΩ. From Equation 3, R_FB1 is calculated to be 265.1 kΩ, and a standard resistor value of 261 kΩ is selected. C_FB is selected to be 10 pF as recommended.

9.2.1.2 L₁

The main parameter affected by the inductor is the peak to peak inductor current ripple (I_LR). To maintain a continuous conduction mode (CCM) operation, ensure that the average inductor current I_L1 is larger than half of I_LR. For a boost converter, I_L1 equals to the input current I_IN. Hence,

Also,

If V_IN is maximum, which is 24 V in this example, and only one LED string is turned on (because the two channels of the LM3492HC device are individually dimmable), I_IN is minimum. From Equation 6 to Equation 8, it can be calculated that I_IN(MIN), t_on, and L₁ are 0.326 A, 0.77 µs, and 28.5 µH. However,, from Equation 6, I_IN is maximum when V_IN is minimum, which is 10 V in this example, and the two LED strings are turned on together. Hence I_IN(max) is 1.56 A. Then, I_LR is

From Equation 7, t_on is 2.27 µs. From (9), I_LR is 0.80 A. The steady-state peak inductor current I_L1(PEAK) is

As a result, I_L1(PEAK) is 1.96 A. A standard value of 27 µH is selected for L₁, and its saturation current is larger than 1.96 A.

9.2.1.3 D₁

The selection of the boost diode D₁ depends on two factors. The first factor is the reverse voltage, which equals to V_OUT for a boost converter. The second factor is the peak diode current at the steady state, which equals to the peak inductor current as shown in Equation 10. In this example, a 100-V 3-A schottky diode is selected.

9.2.1.4 C_IN and C_OUT

The function of the input capacitor C_IN and the output capacitor C_OUT is to reduce the input and output voltage ripples. Experimentation is usually necessary to determine their value. The rated DC voltage of capacitors used should be higher than the maximum DC voltage applied. Owing to the concern of product lifetime, TI recommends ceramic capacitors. But ceramic capacitors with high rated DC voltage and high capacitance are rare in general. Multiple capacitors connecting in parallel can be used for C_IN and C_OUT. In this example, two 10-µF ceramic capacitor are used for C_IN, and two 2.2-µF ceramic capacitor are used for C_OUT.

9.2.1.5 C_VCC

The capacitor on the VCC pin provides noise filtering and stabilizes the LDO regulator. It also prevents false triggering of the VCC UVLO. C_VCC is recommended to be a 1-µF, good quality and low ESR ceramic capacitor.

9.2.1.6 C_CDHC

The capacitor at the CDHC pin not only affects the sensitivity of the DHC but also determines the soft-start time t_SS, the time for the output voltage to rise until power good. t_SS is determined from the following equation:

Equation 11.

In this example, C_CDHC is recommended to be a 0.47-µF good quality and low ESR ceramic capacitor.

9.2.1.7 R_RT and R_IREF

The resistors R_RT and R_IREF set the switching frequency f_SW of the boost converter and the LED current I_LED respectively. From Figure 19, if f_SW is 300 kHz, R_RT is selected to be 442 kΩ. From Figure 24, if I_LED is 250 mA, R_IREF is selected to be 4.99 kΩ.

9.2.1.8 R_COMM

Because the COMM pin is open drain, a resistor R_COMM of 52.3 kΩ is used to connect the VCC and COMM pins to act as a pullup function.