Table 12. Design Parameters

Table 13. Application Circuit Component List

9.2.2.1 LED Current Setting/Maximum LED Current

The maximum LED current is restricted by the following factors: the maximum duty cycle that the boost converter can achieve, the peak current limitations, and the maximum output voltage.

9.2.2.2 Maximum Duty Cycle

The LM3530 can achieve up to typically 94% maximum duty cycle. Two factors can cause the duty cycle to increase: an increase in the difference between V_OUT and V_IN and a decrease in efficiency. This is shown by Equation 12:

Equation 12.

For a 9-LED configuration V_OUT = (3.6 V x 9LED + VHR) = 33 V operating with η = 70% from a 3-V battery, the duty cycle requirement would be around 93.6%. Lower efficiency or larger V_OUT to V_IN differentials can push the duty cycle requirement beyond 94%.

9.2.2.3 Peak Current Limit

The LM3530 boost converter has a peak current limit for the internal power switch of 839 mA typical (739 mA minimum). When the peak switch current reaches the current limit, the duty cycle is terminated resulting in a limit on the maximum output current and thus the maximum output power the LM3530 can deliver. Calculate the maximum LED current as a function of V_IN, V_OUT, L, efficiency (η) and I_PEAK as:

Equation 13.

where

• ƒ_SW = 500 kHz
• η and I_PEAK can be found in the Efficiency and I_PEAK curves in the Specifications and Application Curves.

9.2.2.4 Output Voltage Limitations

The LM3530 has a maximum output voltage of 41 V typical (40 V minimum) for the LM3530-40 version and 24 V typical (23.6 V minimum) for the LM3530-25 version. When the output voltage rises above this threshold (V_OVP) the overvoltage protection feature is activated and the duty cycle is terminated. Switching will cease until V_OUT drops below the hysteresis level (typically 1 V below V_OVP). For larger numbers of series connected LEDs the output voltage can reach the OVP threshold at larger LED currents and colder ambient temperatures. Typically white LEDs have a –3mV/°C temperature coefficient.

9.2.2.5 Output Capacitor Selection

The LM3530’s output capacitor has two functions: filtering of the boost converters switching ripple, and to ensure feedback loop stability. As a filter, the output capacitor supplies the LED current during the boost converters on time and absorbs inductor energy during the switch off time. This causes a sag in the output voltage during the on time and a rise in the output voltage during the off time. Because of this, the output capacitor must be sized large enough to filter the inductor current ripple that could cause the output voltage ripple to become excessive. As a feedback loop component, the output capacitor must be at least 1 µF and have low ESR otherwise the LM3530 boost converter can become unstable. This requires the use of ceramic output capacitors. Table 14 lists part numbers and voltage ratings for different output capacitors that can be used with the LM3530.

9.2.2.6 Inductor Selection

The LM3530 is designed to work with a 10-µH to 22-µH inductor. When selecting the inductor, ensure that the saturation rating for the inductor is high enough to accommodate the peak inductor current. Equation 14 and Equation 15 calculate the peak inductor current based upon LED current, V_IN, V_OUT, and efficiency.

Equation 14.

where:

Equation 15.

When choosing L, the inductance value must also be large enough so that the peak inductor current is kept below the LM3530 switch current limit. This forces a lower limit on L given by Equation 16.

Equation 16.

I_{SW_MAX} is given in , efficiency (η) is shown in the Application Curves, and ƒ_SW is typically 500 kHz.

9.2.2.7 Diode Selection

The diode connected between SW and OUT must be a Schottky diode and have a reverse breakdown voltage high enough to handle the maximum output voltage in the application. Table 16 lists various diodes that can be used with the LM3530. For 25-V OVP devices a 30-V Schottky is adequate. For 40-V OVP devices, a 40-V Schottky diode should be used.