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LDS8726
C OUT ?
( NV F ? V d ) V R f
? I NV F ? d ?
V IN ?
If the input supply voltage falls below the under-
voltage threshold, switch is disabled, and the device
enters shutdown mode.
LED Selection
The number of the LEDs in string is limited by
maximum output voltage that cannot exceed over-
voltage protection level. We recommend using not
more than 10 LEDs with V F ≤ 3.6 V in string if V IN
voltage is above 3.0 V and not more that 8 LEDs if
V IN may fall up to 2.7 V.
External Components Selection
The LDS8726 requires four external components
only. The recommended input capacitor value is
between 1.0 and 10uF, while the output capacitor
selection is function of desired output ripple, loop
stability, and inrush current. We recommend C OUT =
1 μF.
The inductor should allow around 20% higher peak
current than LDS8726 Switch Current Limit I LIM (see
table Electrical Operating Characteristics on page 2).
However, the maximum ripple current through
inductor I R should not exceed
I ,
I R ? 2 ? LIM ? LED ? where
? ?
V F - is a LED forward voltage, V
N – is number of LEDs per string
Vd - is a current regulator voltage drop = 0.2 V,
V IN - is an input voltage, V
We recommend continuous conduction mode for
inductor to achieve highest efficiency. That limits I R
value as
L - is an inductance, H, and
f - is a switching frequency, 700 kHz.
Inductor should have minimum DC resistance to
avoid driver’s efficiency degradation.
The equation for the output capacitor selection is:
( NV F ? V d ? V IN ) I OUT
, where
V R – is a ripple voltage at the output.
For example:
If V IN = 2.7 V, N = 7, V F = 3.3 V, Vd = 0.25 V, f =
1 MHz, I OUT = 90 mA, (3 strings in parallel x 30 mA)
and ripple voltage V R = 0.1 V, C OUT = 0.63 μF so
1 μF is a good choice.
We recommend C OUT = 1 μF to achieve better
efficiency and driver’s stability.
Recommended Layout
In active mode, the driver switches internally at a high
frequency. We recommend minimize trace length to
all external capacitors and inductor. The input and
output ceramic capacitors (X5R or X7R type) should
located as close to the device’ pins as possible to
prevent from EMI distribution
A ground plane should cover the area under the
driver IC as well as the bypass capacitors. Short
connection to ground on capacitors C IN and C OUT can
be implemented with the use of multiple via. A copper
area matching the TDFN exposed pad (PAD) must
be connected to the ground plane underneath. The
use of multiple via improves the package heat
dissipation.
I R ? LED
2 I
? NV F ? V d ?
? V IN
Inductor value L is a function of switching frequency,
input and output voltage and is determined by
following equation:
L ?
I R f ?
?
?
?
1
? 1
V V ? V
? NV F ? d ? PM ? IN
1
V IN
, where
?
?
Figure 3. Recommended layout
V PM - is a voltage drop across synchronous rectifier
(PMOSFET) = 1.5 ohms x I LED , (A),
.
? 2010 IXYS Corp.
Characteristics subject to change without notice
7
Doc. No. 8726DS, Rev. N1.0
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