LM3409MY-NOPB Datasheet, PDF (16/43 Pages) Texas Instruments – PFET Buck Controller for High Power LED Drivers

LM3409, LM3409HV, LM3409-Q1
SNVS602J – MARCH 2009 – REVISED MAY 2013
www.ti.com
BUCK CONVERTERS W/O OUTPUT CAPACITORS
Because current is being regulated, not voltage, a buck current regulator is free of load current transients,
therefore output capacitance is not needed to supply the load and maintain output voltage. This is very helpful
when high frequency PWM dimming the LED load. When no output capacitor is used, the same design equations
that govern Δi_L-PP also apply to Δi_LED-PP.
BUCK CONVERTERS WITH OUTPUT CAPACITORS
A capacitor placed in parallel with the LED load can be used to reduce Δi_LED-PP while keeping the same average
current through both the inductor and the LED array. With an output capacitor, the inductance can be lowered,
making the magnetics smaller and less expensive. Alternatively, the circuit can be run at lower frequency with the
same inductor value, improving the efficiency and increasing the maximum allowable average output voltage. A
parallel output capacitor is also useful in applications where the inductor or input voltage tolerance is poor.
Adding a capacitor that reduces Δi_LED-PP to well below the target provides headroom for changes in inductance or
V_IN that might otherwise push the maximum Δi_LED-PP too high.
Figure 29. Calculating Dynamic Resistance r_D
Output capacitance (C_O) is determined knowing the desired Δi_LED-PP and the LED dynamic resistance (r_D). r_D can
be calculated as the slope of the LED’s exponential DC characteristic at the nominal operating point as shown in
Figure 29. Simply dividing the forward voltage by the forward current at the nominal operating point will give an
incorrect value that is 5x to 10x too high. Total dynamic resistance for a string of n LEDs connected in series can
be calculated as the r_D of one device multiplied by n. The following equations can then be used to estimate Δi_LED-
PP ^{when using a parallel capacitor:}
'i_{LED-PP =}
'i_{L - PP}
1+
r_D
Z_C
(18)
Z_C
=
1
2 x S x f_SW
x C_O
(19)
In general, Z_C should be at least half of r_D to effectively reduce the ripple. Ceramic capacitors are the best choice
for the output capacitors due to their high ripple current rating, low ESR, low cost, and small size compared to
other types. When selecting a ceramic capacitor, special attention must be paid to the operating conditions of the
application. Ceramic capacitors can lose one-half or more of their capacitance at their rated DC voltage bias and
also lose capacitance with extremes in temperature. Make sure to check any recommended deratings and also
verify if there is any significant change in capacitance at the operating voltage and temperature.
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