LM3431_08 Datasheet, PDF (12/24 Pages) National Semiconductor (TI) – 3-Channel Constant Current LED Driver with Integrated Boost Controller

Where I_OUT is the sum of all LED string currents at 100% dim-
ming. I_L(AVE) will be highest at the minimum input voltage.
Then determine the minimum L based on Δi_L with the follow-
ing equation:
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FIGURE 3. Switching Frequency vs R_T
For a given application, the maximum switching frequency is
limited by the minimum on time. When the LM3431 reaches
its minimum on-time, pulse skipping will occur and output rip-
ple will increase. To avoid this, set the operating frequency
below the following maximum setting:
A good starting point is to set Δi_L to 150% of the minimum I_L
(AVE) ^{and calculate using that value. The maximum recom-}
mended Δi_L is 200% of I_L(AVE) to maintain continuous current
in normal operation. In general a smaller inductor (higher rip-
ple current) will give a better dimming response due to the
higher dI/dt. This is shown graphically below.
INDUCTOR SELECTION
Figure 4 shows how the inductor current, I_L, varies during a
switching cycle.
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FIGURE 5. Inductor Current During Dimming
The resulting peak to peak inductor current is:
And the resulting peak inductor current is:
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FIGURE 4. Inductor Current, SW Voltage, and V_OUT
The important quantities in determining a proper inductance
value are I_L(AVE) (the average inductor current) and Δi_L (the
peak to peak inductor current ripple). If Δi_L is larger than 2 x
I_L(AVE), the inductor current will drop to zero for a portion of the
cycle and the converter will operate in discontinuous conduc-
tion mode. If Δi_L is smaller than 2 x I_L, the inductor current will
stay above zero and the converter will operate in continuous
conduction mode.
To determine the minimum L, first calculate the I_L(AVE) at both
minimum and maximum input voltage:
Peak inductor current will occur at minimum V_IN.
The inductor must be rated to handle both the average current
and peak current, which is the same as the peak switch cur-
rent. As switching frequency increases, less inductance is
required. However, some minimum inductance value is re-
quired to ensure stability at duty cycles greater than 50%. The
minimum inductance required for stability can be calculated
as:
Where R3 is the sense resistor determined in the next section.
Although the inductor must be large enough to meet both the
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