LM3409 Datasheet, PDF (13/30 Pages) National Semiconductor (TI) – PFET Buck Controller for High Power LED Drives

Design Considerations
OPERATION NEAR DROPOUT
Because the power MosFET is a PFET, the LM3409/09HV
can be operated into dropout which occurs when the input
voltage is approximately equal to output voltage. Once the
input voltage drops below the nominal output voltage, the
switch remains constantly on (D=1) causing the output volt-
age to decrease with the input voltage. In normal operation,
the average LED current is regulated to the peak current
threshold minus half of the ripple. As the converter goes into
dropout, the LED current is exactly at the peak current thresh-
old because it is no longer switching. This causes the LED
current to increase by half of the set ripple current as it makes
the transition into dropout. Therefore, the inductor current rip-
ple should be kept as small as possible (while remaining
above the previously established minimum) and output ca-
pacitance should be added to help maintain good line regu-
lation when approaching dropout.
LED RIPPLE CURRENT
Selection of the ripple current through the LED array is anal-
ogous to the selection of output ripple voltage in a standard
voltage regulator. Where the output voltage ripple in a voltage
regulator is commonly ±1% to ±5% of the DC output voltage,
LED manufacturers generally recommend values for Δi_LED-
PP ^{ranging from ±5% to ±20% of I}LED^{. For a nominal system}
operating point, a larger Δi_LED-PP specification can reduce the
necessary inductor size and/or allow for smaller output ca-
pacitors (or no output capacitors at all) which helps to mini-
mize the total solution size and cost. On the other hand, a
smaller Δi_LED-PP specification would require more output in-
ductance, a higher switching frequency, or additional output
capacitance.
BUCK CONVERTERS W/O OUTPUT CAPACITORS
Because current is being regulated, not voltage, a buck cur-
rent 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 fre-
quency PWM dimming the LED load. When no output capac-
itor 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 mag-
netics 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 allow-
able 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 in-
ductance or V_IN that might otherwise push the maximum
Δi_LED-PP too high.
300856f2
FIGURE 9. 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 cal-
culated as the slope of the LED’s exponential DC character-
istic at the nominal operating point as shown in Figure 9.
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:
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 out-
put capacitors due to their high ripple current rating, low ESR,
low cost, and small size compared to other types. When se-
lecting a ceramic capacitor, special attention must be paid to
the operating conditions of the application. Ceramic capaci-
tors can lose one-half or more of their capacitance at their
rated DC voltage bias and also lose capacitance with ex-
tremes in temperature. Make sure to check any recommend-
ed deratings and also verify if there is any significant change
in capacitance at the operating voltage and temperature.
OUTPUT OVER-VOLTAGE PROTECTION
Because the LM3409/09HV controls a buck current regulator,
there is no inherent need to provide output over-voltage pro-
tection. If the LED load is opened, the output voltage will only
rise as high as the input voltage plus any ringing due to the
parasitic inductance and capacitance present at the output
node. If a ceramic output capacitor is used in the application,
it should have a minimum rating equal to the input voltage.
Ringing seen at the output node should not damage most ce-
ramic capacitors, due to their high ripple current rating.
13
www.national.com