Diodes ZXLD1352 - Manual (Page 15)

ZXLD1352
ZXLD1352
Document number: DS33469 Rev. 3 - 2
15 of 21
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December 2010
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Application Information (Continued)
Inductor Selection
Recommended inductor values for the ZXLD1352 are in the range 47µH to 220µH.
Higher values of inductance are recommended at higher supply voltages in order to minimize errors due to switching delays,
which result in increased ripple and lower efficiency. Higher values of inductance also result in a smaller change in output current
over the supply voltage range. (See graphs). The inductor should be mounted as close to the device as possible with low
resistance connections to the LX and V
IN
pins.
The chosen coil should have a saturation current higher than the peak output current and a continuous current rating above the
required mean output current.
Suitable coils for use with the ZXLD1352 are listed in the table below:
Part No.
L
( μH)
DCR
(
Ω)
I
SAT
(A) Manufacturer
DO1608C 47 0.64 0.5
CoilCraft
MSS6132ML
47 0.38 0.56
68 0.58 0.47
100 0.82 0.39
CD104-MC 220 0.55 0.53 Sumida
NP04SB470M 47 0.27 0.38 Taiyo Yuden
The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times within the specified limits over
the supply voltage and load current range.
The following equations can be used as a guide, with reference to Figure 1 - Operating waveforms.
LX Switch 'On' time
T
O N
L
ΔI
V
IN
V
LED
– I
a v g
R
S
r L R
LX
+ + ( )
–
---------------------------------------------------------------------------------------- =
Note: T
ONmin
>200ns
LX Switch 'Off' time
T
O FF
L
ΔI
V
L ED
VD I
av g
R
S
r L +
( )
+ +
----------------------------------------------------------------------- =
Note: T
OFFmin
>200ns
Where:
L is the coil inductance (H)
rL is the coil resistance (
Ω)
I
avg
is the required LED current (A)
DI is the coil peak-peak ripple current (A) {Internally set to 0.3 x I
avg
}
V
IN
is the supply voltage (V)
V
LED
is the total LED forward voltage (V)
R
LX
is the switch resistance (
Ω)
VD is the diode forward voltage at the required load current (V)