ZXLD1350_07 Datasheet, PDF (16/24 Pages) Zetex Semiconductors – 350mA LED driver with internal switch

ZXLD1350
Inductor selection
Recommended inductor values for the ZXLD1350 are in the range 47␮H to 220␮H.
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 ZXLD1350 are listed in the table below:
Part No.
DO1608C
MSS6132ML
CD104-MC
NP04SB470M
L
(␮H)
47
47
68
100
220
47
DCR
(⍀)
0.64
0.38
0.58
0.82
0.55
0.27
I_SAT Manufacturer
(A)
0.5
0.56
CoilCraft
0.47
0.39
0.53 Sumida
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_ON
=
----------------------------------------^L----^Δ----^I---------------------------------------
V_IN – V_LED – I_avg(R_S + rL + R_LX)
Note: T_ONmin>200ns
LX Switch 'Off' time
T_OFF
=
-------------------------------^L----^Δ----^I-------------------------------
V_LED + VD + I_avg(R_S + rL)
Note: T_OFFmin>200ns
Where:
L is the coil inductance (H)
rL is the coil resistance (⍀)
I_avg is the required LED current (A)
⌬I 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 (⍀)
VD is the diode forward voltage at the required load current (V)
Issue 6 - April 2007
16
© Zetex Semiconductors plc 2007
www.zetex.com