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LDS8680
drop Vd across PCR at every channel in ON state. If
this voltage falls below 150 mV (typical) at any one
channel, (channel with LED with highest forward
voltage), the Mode Control Block changes charge
pump mode to the next multiplication ratio.
Vd (LEDA/B) = V IN x M – V F – Rcp x I OUT , where Rcp
is a Charge Pump Output Resistance at given mode,
I OUT is sum of all LED currents, and M is a charge
pump’ multiplication ratio.
If the input voltage is insufficient or falls to a level
where Vd ≤ 150 mV, and the regulated currents
cannot be maintained, the low dropout PowerLite ?
Current Regulator switches the charge pump into 2-x
mode (after a fixed delay time of about 800 μ s). In 2-x
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),
LDS8680 will restart when input voltage rises above
2.3 V called lockout mode. If V IN continues fall below
1.7 V, LDS8680 device will enter shutdown mode and
reset all data register. To restart device, set EN/SET
pin logic low for more than 50 ms and program new
mode, the
charge pump’
output voltage is
I LED current value using Smart OneWire ? interface.
approximately equal to 2 times the input supply
voltage (less any internal voltage losses).
LED Current Setting
The current in each of the two LED channels is
programmable through the Smart OneWire ?
EN/SET digital control input. By pulsing this signal
according to a shown above protocol, a set of internal
registers can be addressed and written into allowing
to configure each of LEDs with the desired current.
See recommended commands at the Table
“CURRENT SETTING COMMANDS”
Unused LED Channels
For applications with only one LED, unused LED can
be disabled via the register internally and connect to
V OUT .
Protection Modes
The LDS8680 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, LDS8680
recognizes this condition as “LED Short” and disables
this channel. If LED pin voltage is less than (Vout –
1.5V), LDS8680 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 6.2 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 LDS8680 is
enabled, input current may increase up to 1 A within
6. Open LED Definition
When LED becomes an open circuit, device will enter
into 2-x mode. Unless this unused LED channel was
disabled via Smart OneWire ? commands.
7. LED protection
LDS8680 uses timeout ~250 ms to protect LEDs from
thermal damage at currents above 200 mA. This
function may be disabled in command mode.
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, the input current ripple is very low, and
an input bypass capacitor of 1μF is sufficient.
In 1-x 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
? 2009 IXYS Corp.
Characteristics subject to change without notice
8
Doc. No. 8680_DS, Rev. N1.0
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