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LDS8681
Unused LED Channels
For applications with only one LED, unused channel
can be connected to V OUT or both channels may be
teed together to provide sum of the currents set at
each channel.
Protection Modes
The LDS8681 has follow protection modes:
1. LED short to V OUT protection
If LED pin is shorted to V OUT , LED burned out
becomes as short circuit, or LED pin voltage is within
from V OUT to (V OUT - 1.5V) range, LDS8681
recognizes this condition as “LED Short” and disables
this channel. If LED pin voltage is less than (Vout –
1.5V), LDS8681 restores LED current at this
particular channel to programmed value.
2. V OUT Over-Voltage Protection
The charge pump’ output voltage V OUT automatically
limits at about 5.9 V maximum. This is to prevent the
output pin from exceeding its absolute maximum
rating.
3. V OUT Short Circuit Protection
If V OUT is shorted to ground before LDS8681 is
enabled, input current may increase up to 1 A within
20 μs after enable and is limited to ~ 90 mA after
that.
4. Over-Temperature Protection
If the die temperature exceeds +160°C, the driver will
enter shutdown mode. Device restores normal
operation after die temperature falls below 135°C.
5. Input Voltage Under-Voltage Lockout
If V IN falls below 2.3 V but above 1.7 V (typical value),
LDS8681 will restart when input voltage rises above
2.3 V called lockout mode. If V IN continues fall below
1.7 V, LDS8681 device will enter shutdown mode..
To restart device, set EN pin logic low for more than
50 ms and logic high after that..
6. Open LED Definition
When LED becomes an open circuit, device will enter
into 2-x mode unless this unused LED channel is
connected to V OUT directly.
LED Selection
If the power source is a Li-ion battery, LEDs with
lowest forward voltages are recommended to achieve
highest efficiency and extended operation on a single
battery charge.
LEDs with forward voltages (V F ) ranging from 1.6 V to
4 V may be used. Charge pump operates in highest
efficiency when V F voltage is close to V IN voltage
multiplied by switching mode, i.e. V IN x 1 or V IN x 2
External Components
The driver requires three external ceramic capacitors
(C1, C IN , and C OUT ) X5R or X7R type. In 2-x charge
pump mode, 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 capacitors. 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 (see figure
below).
The ground plane on the top layer is connected to the
ground plane on the bottom layer through two vias to
imporove power dissipation.
? 2009 IXYS Corp.
Characteristics subject to change without notice
6
Doc. No. 8681_DS, Rev. N1.0
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