LM3429 Datasheet, PDF (22/62 Pages) National Semiconductor (TI) – N-Channel Controller for Constant Current LED Drivers

LM3429, LM3429-Q1
SNVS616H – APRIL 2009 – REVISED JULY 2015
8 Application and Implementation
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NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
8.1.1 Inductor
The inductor (L1) is the main energy storage device in a switching regulator. Depending on the topology, energy
is stored in the inductor and transfered to the load in different ways (as an example, buck-boost operation is
detailed in the Current Regulators section). The size of the inductor, the voltage across it, and the length of the
switching subinterval (t_ON or t_OFF) determines the inductor current ripple (Δi_L-PP). In the design process, L1 is
chosen to provide a desired Δi_L-PP. For a buck regulator the inductor has a direct connection to the load, which is
good for a current regulator. This requires little to no output capacitance therefore Δi_L-PP is basically equal to the
LED ripple current Δi_LED-PP. However, for boost and buck-boost regulators, there is always an output capacitor
which reduces Δi_LED-PP, therefore the inductor ripple can be larger than in the buck regulator case where output
capacitance is minimal or completely absent.
In general, Δi_LED-PP is recommended by manufacturers to be less than 40% of the average LED current (I_LED).
Therefore, for the buck regulator with no output capacitance, Δi_L-PP should also be less than 40% of I_LED. For the
boost and buck-boost topologies, Δi_L-PP can be much higher depending on the output capacitance value.
However, Δi_L-PP is suggested to be less than 100% of the average inductor current (I_L) to limit the RMS inductor
current.
L1 is also suggested to have an RMS current rating at least 25% higher than the calculated minimum allowable
RMS inductor current (I_L-RMS).
8.1.2 LED Dynamic Resistance (r_D)
When the load is a string of LEDs, the output load resistance is the LED string dynamic resistance plus R_SNS.
LEDs are PN junction diodes, and their dynamic resistance shifts as their forward current changes. Dividing the
forward voltage of a single LED (V_LED) by the forward current (I_LED) leads to an incorrect calculation of the
dynamic resistance of a single LED (r_LED). The result can be 5 to 10 times higher than the true r_LED value.
Figure 25. Dynamic Resistance
Obtaining r_LED is accomplished by referring to the manufacturer's LED I-V characteristic. It can be calculated as
the slope at the nominal operating point as shown in Figure 25. For any application with more than 2 series
LEDs, R_SNS can be neglected allowing r_D to be approximated as the number of LEDs multiplied by r_LED.
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