Table 37. Application Circuit Component List

8.2.2.1 Boost Converter Maximum Output Power (Boost)

Maximum output power of the LM3633 is governed by two factors: the peak current limit (I_CL = 880 mA min), and the maximum output voltage (V_OVP). When the application causes either of these limits to be reached it is possible that the proper current regulation and matching between LED current strings is not met.

8.2.2.1.1 Peak Current Limited

In the case of a peak current limited situation, the NFET switch turns off for the remainder of the switching period when the inductor current peak hits the LM3633 current limit. If this happens each switching cycle the LM3633 regulates the inductor current peak instead of the headroom across the current sinks. This can result in the dropout of the boost output connected current sinks, and the LED current dropping below its programmed level.

The peak current in a boost converter is dependent on the value of the inductor, total LED current in the boost (I_OUT), the boost output voltage (V_OUT) (which is the highest voltage LED string + V_HR ), the input voltage (V_IN), the switching frequency, and the efficiency (Output Power/Input Power). Additionally, the peak current is different depending on whether the inductor current is continuous during the entire switching period (CCM), or discontinuous (DCM) where it goes to 0 before the switching period ends. For Continuous Conduction Mode the peak inductor current is given by:

Equation 12.

For Discontinuous Conduction Mode the peak inductor current is given by:

Equation 13.

To determine which mode the circuit is operating in (CCM or DCM) it is necessary to perform a calculation to test whether the inductor current ripple is less than the anticipated input current (I_IN). If ΔI_L is less than I_IN then the device operates in CCM. If ΔI_L is greater than I_IN then the device is operating in DCM.

Equation 14.

Typically at currents high enough to reach the LM3633 peak current limit, the device operates in CCM.

The following figures show the output current and voltage derating for a 10-µH and a 22-µH inductor. These plots take equations (1) and (2) from above and plot V_OUT and I_OUT with varying V_IN, a constant peak current of 880 mA (I_{CL_MIN}), 500-kHz switching frequency, and a constant efficiency of 85%. Using these curves gives a good design guideline on selecting the correct inductor for a given output power requirement. A 10-µH inductor is typically a smaller device with lower on resistance, but the peak currents are higher. A 22-µH inductor provides for lower peak currents, but to match the DC resistance of a 10-µH inductor, a larger-sized device is required.

Figure 24. Maximum Output Power (22 µH)

Figure 25. Maximum Output Power (10 µH)

8.2.2.2 Boost Inductor Selection

The boost circuit operates using a 4.7-μH to 22-μH inductor. The inductor selected must have a saturation current greater than the peak operating current.

8.2.2.3 Output Capacitor Selection

The LM3633 inductive boost converter requires a 1.0-µF (X5R or X7R) ceramic capacitor to filter the output voltage. The voltage rating of the capacitor depends on the selected OVP setting. For the 16-V setting a 16-V capacitor must be used. For the 24-V setting a 25-V capacitor must be used. For the 32-V setting, a 35-V capacitor must be used. For the 40-V setting a 50-V capacitor must be used. 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 0.4 µF. This might require placing two devices in parallel in order to maintain the required output capacitance over the device operating range, and series LED configuration.

8.2.2.4 Schottky Diode Selection

The Schottky diode must have a reverse breakdown voltage greater than the LM3633 maximum output voltage (see Overvoltage Protection (Inductive Boost) section). Additionally, the diode must have an average current rating high enough to handle the LM3633 maximum output current, and at the same time the diode peak current rating must be high enough to handle the peak inductor current. Schottky diodes are required due to their lower forward voltage drop (0.3 V to 0.5 V) and their fast recovery time.

8.2.2.5 Input Capacitor Selection

The input capacitor on the LM3633 filters the voltage ripple due to the switching action of the inductive boost and the capacitive charge-pump doubler. A ceramic capacitor of at least 2.2-µF (X5R or X7R) must be used to filter the input voltage.

8.2.2.6 Maximum Output Power (Charge Pump)

The maximum output power available from the LM3633 charge pump is determined by the maximum output voltage available from the charge pump. In 1X gain the charge pump operates in Pass Mode so the voltage at CPOUT tracks V_IN (less the drop across the charge-pump pass switch). In this case the maximum output power is given as:

Equation 15.

where R_CP is the resistance from V_IN to CPOUT and I_{LVLED_TOTAL} is the maximum programmed current in the LVLED strings.

In 2X gain the voltage at CPOUT (V_{CPOUT_2X}) is regulated to typically 4.4 V. In this case the maximum output power is given by:

Equation 16.

Both equations assume there is sufficient headroom at the top side of the low-voltage current sinks to ensure the LED current remains in regulation (V_{HR_LV}) in the electrical table.

8.2.2.7 Charge Pump Flying Capacitor Selection

The charge pump flying capacitor must quickly charge up to the input voltage and then supply the current to the output every switching cycle (1 MHz). This fast switching action requires a 1.0-µF (X5R or X7R) ceramic capacitor connected to the C+ and C– pins with a low inductive connection.

8.2.2.8 Charge Pump Output Capacitor Selection

The charge pump output capacitor filters the switched charge from the flying capacitor every switching cycle (1 MHz). This fast switching action requires a 1.0-µF (X5R or X7R) ceramic capacitor connected to the CPOUT pin with a low-inductive connection.

8.2.2.9 Charge Pump Input Capacitor Selection

The input capacitor for the LM3633 charge pump is the same one used for the LM3633 inductive boost converter (see Input Capacitor Selection).