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LDS8720 Datasheet(PDF) 7 Page - IXYS Corporation

Part No. LDS8720
Description  184 WLED Matrix Driver with Boost Converter
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Manufacturer  IXYS [IXYS Corporation]
Direct Link  http://www.ixys.com
Logo IXYS - IXYS Corporation

LDS8720 Datasheet(HTML) 7 Page - IXYS Corporation

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LDS8720
© 2011 IXYS Corp.
7
Doc. No. 8720DS, Rev. N1.1
Characteristics subject to change without notice
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 VF ≤3.6 V in string if VIN
voltage is above 3.0 V and not more that 8 LEDs if
VIN may fall up to 2.7 V.
External Components Selection
The LDS8720 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 COUT =
1 µF.
The inductor should allow around 20% higher peak
current than LDS8720 Switch Current Limit ILIM (see
table Electrical Operating Characteristics on page 2).
However,
the
maximum
ripple
current
through
inductor IR should not exceed
IN
d
F
LED
LIM
R
V
V
NV
I
I
I
2
, where
VF - is a LED forward voltage, V
N – is number of LEDs per string
Vd - is a current regulator voltage drop = 0.2 V,
VIN - is an input voltage, V
We recommend continuous conduction mode for
inductor to achieve highest efficiency. That limits IR
value as
IN
d
F
LED
R
V
V
NV
I
I
2
Inductor value L is a function of switching frequency,
input and output voltage and is determined by
following equation:
IN
IN
PM
d
F
R
V
V
V
V
NV
f
I
L
1
1
1
, where
VPM - is a voltage drop across synchronous rectifier
(PMOSFET) = 1.5 ohms x ILED, (A),
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:
f
V
V
NV
I
V
V
NV
C
R
d
F
OUT
IN
d
F
OUT
)
(
)
(
, where
VR – is a ripple voltage at the output.
For example:
If VIN = 2.7 V, N = 9, VF = 3.3 V, Vd = 0.25 V, f = 0.7
MHz, IOUT = 90 mA, (3 strings in parallel x 30 mA)
and ripple voltage VR = 0.1 V, COUT = 0.83 µF so
1 µF is a good choice.
We recommend COUT = 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 CIN and COUT 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.
Figure 3. Recommended layout
.


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