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LDS8868 Datasheet(PDF) 11 Page - IXYS Corporation

Part No. LDS8868
Description  6-Channel Ultra Low Dropout LED Driver
Download  14 Pages
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Manufacturer  IXYS [IXYS Corporation]
Direct Link  http://www.ixys.com
Logo IXYS - IXYS Corporation

LDS8868 Datasheet(HTML) 11 Page - IXYS Corporation

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LDS8868
© 2009 IXYS Corp.
11
Doc. No. 8868_DS, Rev. N2.1
Characteristics subject to change without notice
1.5V),
LDS8868
restores
LED
current
at
this
particular channel to programmed value.
2. VOUT Over-Voltage Protection
The charge pump’ output voltage VOUT automatically
limits at about 6.2 V maximum. This is to prevent the
output pin from exceeding its absolute maximum
rating.
3. VOUT Short Circuit Protection
If VOUT is shorted to ground before LDS8868 is
enabled, input current may increase up to 200 – 300
mA within 20 µs after enable and is limited to 35 – 40
mA after that.
4. Over-Temperature Protection
If the die temperature exceeds +150°C, the driver will
enter shutdown mode. The LDS8868 requires restart
after die temperature falls below 130°C.
5. Input Voltage Under-Voltage Lockout
If VIN falls below 2.2 V (typical value), LDS8868
enters shutdown mode and all registers data are
cleared. Device requires restart when input voltage
rises above 2.3 V. To restart device, set EN/SET pin
logic low, turn VIN off/on, set EN/SET pin logic high,
and program ILED using 1-wire interface.
6. Low VIN or High LED VF Voltage Detection
If, in 2x mode, VIN is too low to maintain regulated
LED current for given LED VF, or LED becomes an
open circuit, or if any LED at active channels is
disconnected,
LDS8868
starts
subsequently
changing modes (2x – 1x –1.5x – 2x -…) in an
attempt to compensate insufficient voltage. As a
result, average current at all other channels that are
ON may fall below regulated level.
LED Selection
LEDs with forward voltages (VF) ranging from 1.6 V to
3.6 V may be used. Charge pumps operate in highest
efficiency when VF voltage is close to VIN voltage
multiplied by switching mode, i.e. VIN x 1, VIN x 1.5,
and so on. If the power source is a Li-ion battery,
LEDs with VF = 2.7V - 3.3V are recommended to
achieve highest efficiency performance and extended
operation on a single battery charge.
External Components
The driver requires two external 1 µF ceramic capa-
citors (CIN and COUT) and two 0.22 µF ceramic capa-
citors (C1 and C2) X5R or X7R type. Capacitors C1
and C2 may be increased up to 1 µF to improve
charge pump efficiency by 3%. In all charge pump
modes, the input current ripple is very low, and an
input bypass capacitor of 1µF is sufficient.
In 1x mode, the device operates in linear mode and
does not introduce switching noise back onto the
supply.
Recommended Layout
In charge pump mode, the driver switches internally
at a high frequency. It is recommended to minimize
trace length to all four capacitors. 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
TQFN exposed pad (TAB) must be connected to the
ground plane underneath. The use of multiple via
improves the package heat dissipation.
Figure 4. Recommended layout


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