LM3407 Datasheet, PDF (13/22 Pages) National Semiconductor (TI) – 350 mA, Constant Current Output Floating Buck Switching Converter for High Power LEDs

LM3407
www.ti.com
SNVS553B – JANUARY 2008 – REVISED MAY 2013
APPLICATION INFORMATION
SWITCHING FREQUENCY SELECTION
The selection of switching frequency is based on the consideration of the conversion efficiency, size of the
passive components, and the total solution cost. In general, increasing the switching frequency will allow the use
of smaller external components but will decrease the conversion efficiency. Thus, the selection of switching
frequency is a compromise between the system requirements and may vary from design to design. The LM3407
switching frequency can be set in the range from 300 kHz to 1 MHz by adjusting the value of R_FS. The switching
frequency is inversely proportional to the value of R_FS. In order to ensure good operation stability, a resistor with
1% tolerance between 40 kΩ and 96 kΩ and with good thermal stability is suggested.
The switching frequency is estimated by the expression below:
40 Meg
f_SW =
+ 40 in kHz
R_FS
(2)
In the equation, f_SW is the oscillator frequency and R_FS is the frequency setting resistance. The above equation is
only valid for oscillator frequencies in the range of 300 kHz to 1 MHz, so the frequency setting resistance will be
in the range of about 40 kΩ to 150 kΩ.
LED CURRENT SETTING
The LED current setting is important to the lifetime, reliability, and color temperature of the LED string. The LED
current should be properly selected according to the characteristics of the LED used. Over-driving the LED array
can cause the color temperature to shift and will shorten the lifetime of the LEDs. The output current of the
LM3407 can be set by R_ISNS, which is calculated from the following equation:
0.198V
R_ISNS = _IOUT
(3)
To ensure the accuracy of the output current, a resistor with 1% tolerance should be used for R_ISNS. It is also
important for the designer to ensure that the rated power of the resistor is not exceeded with reasonable margin.
For example, when I_OUT is set to 350 mA, the total power dissipation on R_ISNS in steady state is (0.35A)² x
0.565Ω, which equals 69 mW, indicating a resistor of 1/8W power rating is appropriate.
INPUT AND OUTPUT CAPACITORS
The input capacitor supplies instantaneous current to the LM3407 converter when the internal power switch Q1
turns ON. The input capacitor filters the noise and transient voltage from the input power source. Using low ESR
capacitors such as ceramic and tantalum capacitors is recommended. Similar to the selection criteria for the
output capacitor, ceramic capacitors are the best choice for the input to the LM3407 due to their high ripple
current rating, low ESR, and relatively small size compared to other types. A 4.7 µF X7R ceramic capacitor for
the input capacitor is recommended
The output capacitor C_OUT is used to reduce LED current ripple, filter noise, and smooth output voltage. This
capacitor should have low ESR and adequate capacitance. Excessively large output capacitances create long
enable and disable times, which is particularly significant when a high dimming frequency is used. Since the
loading and input conditions differ from design to design, a 2.2 µF X7R ceramic capacitor is a good initial
selection. A DC voltage rating equal to or higher than twice the forward voltage of the LED string is
recommended.
C_OUT is optional and can be omitted for applications where small brightness variation is acceptable. Omitting
C_OUT also helps reduce the cost and board size of the converter. With the absence of C_OUT, the LED forward
current equals the inductor current. In order to ensure proper operation of the converter the peak inductor current
must not exceed the rated forward current of the LEDs. Otherwise the LEDs may be damaged.
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