8.1.1 Setting the Switching Frequency

Both the LM3414 and LM3414HV are PWM LED drivers that contain a clock generator to generate constant switching frequency for the device. The switching frequency is determined by the resistance of an external resistor R_FS in the range of 250 kHz to 1 MHz. Lower resistance of R_FS results in higher switching frequency. The switching frequency of the LM3414/HV is governed using Equation 5.

Equation 5.

Figure 20. Switching Frequency vs R_FS

To ensure accurate current regulation, the LM3414/HV should be operated in continuous conduction mode (CCM) and the ON time should not be shorter than 400 ns under all operation condition.

8.1.2 Setting LED Current

The LM3414/HV requires no external current sensing resistor for LED current regulation. The average output current of the LM3414/HV is adjustable by varying the resistance of the resistor, R_IADJ that connects across the IADJ and GND pins. The IADJ pin is internally biased to 1.255 V. The LED current is then governed by Equation 6.

Equation 6.

where

• 350 mA < I_LED < 1A

Figure 21. LED Current vs R_IADJ

The LED current can be set to any level in the range from 350 mA to 1A. To provide accurate LED current, R_IADJ should be a resistor with no more than 0.5% tolerance. If the IADJ pin is accidentally shorted to GND (R_IADJ = 0), the output current is limited to avoid damaging the circuit. When the overcurrent protection is activated, current regulation cannot be maintained until the overcurrent condition is cleared.

8.1.3 Inductor Selection

To ensure proper output current regulation, the LM3414/HV must operate in Continuous Conduction Mode (CCM). With the incorporation of PLM, the peak-to-peak inductor current ripple can be set as high as ±60% of the defined average output current. The minimum inductance of the inductor is decided by the defined average LED current and allowable inductor current ripple. The minimum inductance can be found by the equations shown in Equation 7 through Equation 8.

Because:

Equation 7.

Thus:

Equation 8.

The LM3414/HV can maintain LED current regulation without output filter capacitor. This is because the inductor of the floating buck structure provides continuous current to the LED throughout the entire switching cycle. When LEDs are driven without filter capacitor, the LED peak current must not set exceeding the rated current of the LED. The peak LED current is governed by Equation 9.

Equation 9.

8.2.1 LM3414/HV Design Example

Figure 22. LM3414/HV Design Example Schematic

8.2.1.2 Detailed Design Procedure

8.2.1.2.1 Calculate Operating Parameters

To calculate component values the operating duty cycle (D) must be calculated using Equation 10.

Equation 10.

8.2.1.2.2 Calculate R_IADJ

To get the desired LED current calculate the value for R_IADJ using Equation 11.

Equation 11.

8.2.1.2.3 Calculate R_FS

Calculate the value of R_FS for the desired switching frequency using Equation 12.

Equation 12.

8.2.1.2.4 Calculate L_MIN

Calculate the minimum inductor value required for the desired LED current ripple using Equation 13.

Equation 13.

8.2.1.2.5 Calculate C_IN-MIN

Calculate the minimum input capacitor value for the desired input voltage ripple using Equation 14.

Equation 14.

8.2.2 LM3414/HV Design Example (I_OUT = 1 A)

Figure 23. LM3414/HV Design Example (I_OUT = 1 A) Schematic

8.2.2.2 Detailed Design Procedure

8.2.2.2.1 Calculate Operating Parameters

To calculate component values the operating duty cycle (D) for this application can be calculated be calculated using Equation 15.

Equation 15.

8.2.2.2.2 Calculate R_IADJ

For 1A LED current calculate the value for R_IADJ using Equation 16.

Equation 16.

Choose a standard value of R_IADJ = 3.24kΩ.

8.2.2.2.3 Calculate R_FS

Calculate the value of R_FS for 500-kHz switching frequency using Equation 17.

Equation 17.

Choose a standard value of R_FS = 40.2kΩ.

8.2.2.2.4 Calculate L_MIN

Calculate the minimum inductor value required for 500 mA or less peak-to-peak LED current ripple using Equation 18.

Equation 18.

Choose a higher standard value of L = 47µH.

8.2.2.2.5 Calculate C_IN-MIN

Calculate the minimum input capacitor value for 200 mV or less input voltage ripple using Equation 19.

Equation 19.

Choose a higher standard value of C_IN = 2.2µF.

8.2.2.3 Application Curve

Figure 24. PWM Dimming Top = DIM. Bottom = LED Current.