LM3431A National Semiconductor, LM3431A Datasheet - Page 12

• Manufacturer Part Number: LM3431A
• Description: 3-Channel Constant Current LED Driver with Integrated Boost Controller
• Manufacturer: National Semiconductor

www.national.com
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:
INDUCTOR SELECTION
Figure 4 shows how the inductor current, I
switching cycle.
The important quantities in determining a proper inductance
value are I
peak to peak inductor current ripple). If Δi
I
cycle and the converter will operate in discontinuous conduc-
tion mode. If Δi
stay above zero and the converter will operate in continuous
conduction mode.
To determine the minimum L, first calculate the I
minimum and maximum input voltage:
L(AVE)
FIGURE 4. Inductor Current, SW Voltage, and V
, the inductor current will drop to zero for a portion of the
FIGURE 3. Switching Frequency vs R
L(AVE)
L
is smaller than 2 x I
(the average inductor current) and Δi
L
, the inductor current will
L
L
is larger than 2 x
30041126
, varies during a
L(AVE)
30041124
T
at both
OUT
L
(the
12
Where I
ming. I
Then determine the minimum L based on Δi
ing equation:
A good starting point is to set Δi
(AVE)
mended Δi
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.
The resulting peak to peak inductor current is:
And the resulting peak inductor current is:
Peak inductor current will occur at minimum V
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
and calculate using that value. The maximum recom-
FIGURE 5. Inductor Current During Dimming
L(AVE)
OUT
L
is the sum of all LED string currents at 100% dim-
is 200% of I
will be highest at the minimum input voltage.
L(AVE)
to maintain continuous current
L
to 150% of the minimum I
30041129
L
with the follow-
IN
.
L