Table 48. Design Parameters

Table 49. Application Circuit Component List

8.2.2.1 Boost Converter Maximum Output Power (Boost)

The LM3533 maximum output power is governed by two factors: the peak current limit (I_CL = 880 mA minimum), 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 may not be met.

8.2.2.2 Peak Current Limited

In the case of a peak current limited situation, when the peak of the inductor current hits the LM3533 current limit, the NFET switch turns off for the remainder of the switching period. If this happens each switching cycle the LM3533 regulates the peak of the inductor current 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 + 0.4 V regulated headroom voltage), 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 16.

For DCM the peak inductor current is given by:

Equation 17.

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 is operating in CCM. If ΔI_L is greater than I_IN then the device is operating in DCM.

Equation 18.

Typically at currents high enough to reach the LM3533 device's peak current limit, the device is operating in CCM. When choosing the switching frequency and the inductor value, Equation 16 and Equation 17 must be used to ensure that I_PEAK stays below I_{CL_MIN} (see Electrical Characteristics).

8.2.2.3 Output Voltage Limited

In the case of a output voltage limited situation, when the boost output voltage hits the LM3533 OVP threshold, the NFET turns off and stays off until the output voltage falls below the hysteresis level (typically 1 V below the OVP threshold). This results in the boost converter regulating the output voltage to the programmed OVP threshold (16 V, 24 V, 32 V, or 40 V), causing the current sinks to go into dropout. The default OVP threshold is set at 16 V. For LED strings higher than typically 4 series LEDs, the OVP has to be programmed higher after power-up or after a HWEN reset.

8.2.2.4 Maximum Output Power (Charge Pump)

The maximum output power available from the LM3533 charge pump is determined by the maximum output voltage available from the charge pump. In 1× gain the charge pump operates in Pass Mode so that the voltage at CPOUT tracks V_IN (less the drop across the charge pumps pass switch). In this case the maximum output power is given as:

Equation 19.

where

• R_CP is the resistance from IN to CPOUT
• I_{LVLED_TOTAL} is the maximum programmed current in the LVLED strings.

In 2× 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 20.

Equation 19 and Equation 20 both 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 Characteristics.