Zxld1360, Application information – Diodes ZXLD1360 User Manual
Page 17
ZXLD1360
ZXLD1360
Document number: DS33471 Rev. 4 - 2
17 of 25
March 2011
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Application Information
(cont.)
Inductor Selection
Recommended inductor values for the ZXLD1360 are in the range 33µH to 100µH.
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 VIN 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 ZXLD1360 are listed in the table below:
Part No.
L
(µH)
DCR
(V)
ISAT
(A)
Manufacturer
MSS1038-333 33
0.093
2.3
MSS1038-683 68
0.213
1.5
NPIS64D330MTRF 33
0.124 1.1
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
(
)
LX
L
S
avg
LED
IN
ON
R
r
R
I
V
V
I
L
t
+
+
×
−
−
Δ
=
Note:
t
ONmin
> 240ns
(
)
L
S
avg
D
LED
OFF
r
R
I
V
V
I
L
t
+
×
+
+
Δ
=
Note:
t
OFFmin
> 200ns
Where:
L is the coil inductance (H)
r
L
is the coil resistance (
Ω)
R
S
is the current sense resistance (
Ω)
I
avg
is the required LED current (A)
ΔI is the coil peak-peak ripple current (A) {Internally set to 0.3 x Iavg}
V
IN
is the supply voltage (V)
V
LED
is the total LED forward voltage (V)
R
LX
is the switch resistance (
Ω) {=0.5Ω nominal}
V
D
is the diode forward voltage at the required load current (V)