Datasheet

DocID023432 Rev 4 25/40
LED2000 Application information
40
where  is the expected system efficiency, D is the duty cycle and I
O
is the output DC
current. Considering  = 1 this function reaches its maximum value at D = 0.5 and the
equivalent RMS current is equal to I
O
divided by 2. The maximum and minimum duty cycles
are:
Equation 30
and
Equation 31
where V
F
is the free-wheeling diode forward voltage and V
SW
the voltage drop across the
internal PDMOS. Considering the range D
MIN
to D
MAX
, it is possible to determine the max.
I
RMS
going through the input capacitor. Capacitors that can be considered are:
Electrolytic capacitors:
These are widely used due to their low price and their availability in a wide range of
RMS current ratings.
The only drawback is that, considering ripple current rating requirements, they are
physically larger than other capacitors.
Ceramic capacitors:
If available for the required value and voltage rating, these capacitors usually have
a higher RMS current rating for a given physical dimension (due to very low ESR).
The drawback is the considerably high cost.
Tantalum capacitors:
Small tantalum capacitors with very low ESR are becoming more widely available.
However, they can occasionally burn if subjected to very high current during charge.
Therefore, it is suggested to avoid this type of capacitor for the input filter of the device
as they may be stressed by a high surge current when connected to the power supply.
If the selected capacitor is ceramic (so neglecting the ESR contribution), the input voltage
ripple can be calculated as:
Equation 32
Table 7. List of ceramic capacitors for the LED2000
Manufacturer Series Capacitor value (µC) Rated voltage (V)
TAIYO YUDEN UMK325BJ106MM-T 10 50
MURATA GRM42-2 X7R 475K 50 4.7 50
D
MAX
V
OUT
V
F
+
V
INMIN
V
SW
–
------------------------------------ -=
D
MIN
V
OUT
V
F
+
V
INMAX
V
SW
–
--------------------------------------=
V
IN PP
I
O
C
IN
f
SW

----------------------- 1
D

----–


D
D

---- 1D–+=