LM3407 Datasheet, PDF (14/22 Pages) National Semiconductor (TI) – 350 mA, Constant Current Output Floating Buck Switching Converter for High Power LEDs

LM3407
SNVS553B – JANUARY 2008 – REVISED MAY 2013
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SELECTION OF INDUCTOR
In order to achieve accurate constant current output, the LM3407 is required to operate in Continuous
Conduction Mode (CCM) under all operating conditions. In general, the magnitude of the inductor ripple current
should be kept as small as possible. If the PCB size is not limited, higher inductance values result in better
accuracy of the output current. However, in order to minimize the physical size of the circuit, an inductor with
minimum physical outline should be selected such that the converter always operates in CCM and the peak
inductor current does not exceed the saturation current limit of the inductor. The ripple and peak current of the
inductor can be calculated as follows:
Inductor Peak to Peak Ripple Current:
1
I_L(ripple) =
V_IN - (n x V_F) - 0.198
1+
R_ISNS
x (n x V_F)
L x V_IN x f_SW
(4)
Peak Inductor Current:
0.198 ^IL(ripple)
I_L(peak) = _RISNS +
2
where
• n is the number of LEDs in a string
• V_F is the forward voltage of one LED.
(5)
The minimum inductance required for the specific application can be calculated by:
1
L_min =
V_IN - (n x V_F) - 0.198 x
1+
R_ISNS
x (R_ISNS x n x V_F)
0.197 x V_IN x f_SW
(6)
For applications with no output capacitor in place, the magnitude of the inductor ripple current should not be
more than 20% of the average inductor current, which is equivalent to the output current, I_OUT. However, in some
situations the physical size of the required inductor may be too large and thus not allowed. The output capacitor
can help absorb this current ripple to significantly reduce the ripple component along the LED string. With an
output capacitor C_OUT in place, the magnitude of the inductor ripple current can be relaxed to 80% of the output
current. Figure 24 illustrates the relationship between I_OUT, I_L(peak), and I_L(ripple).
I_L1
I_{L (peak)}
I_OUT
I_{L (ripple)}
t_ON
t_OFF
T
time
Figure 24. Relationship between I_OUT, I_L(peak) and I_L(ripple)
Table 1 provides the suggested inductance of the inductor for 500 kHz and 1 MHz switching frequency operation
with C_OUT = 4.7µF and I_L(ripple) = 0.8 x I_OUT
14
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