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Micrel, Inc.
L P is solved by re-arranging the equation for f 1 :
The diode dissipation is given by:
MAQ3203
L P =
( 2 π )
2
1
× C P × ( f 1 )
2
P D = I LED ×
(V IN ? V LED )
V IN
× V f
Step 3: Calculate the damping resistor. Critical damping
occurs at Q = 1:
V f is the forward voltage of the diode at I LED . A Schottky
diode forward voltage is typically 0.6V at its full rated current.
It is normal design practice to use a diode rated at 1.5 to 2
times output current to maintain efficiency. This derating
Q =
1
R S
L P
C S + C P
= 1
allows V f to drop to approximately 0.5V. When calculating
the “worst case” power dissipation, use the maximum input
voltage and the actual diode forward voltage drop at the
Solving for R S :
maximum operating temperature; otherwise the calculated
power dissipation will be artificially high. The forward voltage
drop of a diode decrease as ambient temperature is
R S =
L P
C S + C P
increased, at a rate of ? 1.0mV/ ° C.
Input Capacitor
The ceramic input capacitor is selected by voltage rating and
The snubber capacitor, C S , is charged and discharged
each switching cycle. The energy stored in C S is
dissipated by the snubber resistor, R S , two times per
switching period. This power is calculated in the
equation below:
P SNUBBER = f S × C S × V IN 2
ripple current rating. To determine the input current ripple
rating, the RMS value of the input capacitor can be found by:
I CIN(RMS) = I LED × D × ( 1 ? D )
The power loss in the input capacitor is:
P LOSS(CIN) = I
× C IN
where:
2
CIN(RMS)
ESR
f S is the switching frequency for each phase. V IN is the
DC input voltage.
An alternate method to reduce the switch node ringing is
to place a 2.2 ? resistor in series with the n-channel
MOSFETs gate pin. This will slow down both the rising
and falling edge of the switch node waveform.
Freewheeling Diode
The diode provides a conduction path for the inductor
current during the switch off time. The reverse voltage
rating of the diode should be at least 1.2 times the
maximum input voltage. A Schottky diode is recommend
for highest efficiency.
The Schottky diode can be the major source of power
loss, especially at the maximum input voltage. The
current through the diode is equal to the LED current
with a duty cycle of (V IN – V LED )/V IN .
The input capacitor current rating can be considered as
I LED /2 under the worst condition D = 50%.
LED Ripple Current
The LED current is the same as inductor current. If LED ripple
current needs to be reduced then place a 4.7μF/50V ceramic
capacitor across LED.
March 2011
14
M9999-032411-A
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