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LDS8866
6LEDs x 2/0.9), assuming that charge pump’s
efficiency alone at 2-x mode is 90% and maximum
factory preset current is 30 mA per LED.
When PWM LED current control is first enabled (at
cold start, for example), the LDS8866’s V IN , V OUT , and
V PCR monitors cause the LDS8866’s charge pump to
cycle through all four operating modes (if necessary)
Protection Modes
The LDS8866 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
so that V OUT is high enough to maintain regulated
from V OUT to (V OUT -
1.5V) range, LDS8866
LED current. To prevent nuisance switching of the
charge pump during this initial start-up sequence, a
recognizes this condition as “LED Short” and disables
this channel. If LED pin voltage is less than (Vout –
0.8ms transition filter is applied at each charge-pump
1.5V),
LDS8866 restores LED current at this
mode. Depending on V IN and the V F of the LEDs
chosen, the maximum cold-start delay to regulated
LED current operation can be up to 2.4ms. (See
Timing Diagram Figure 3)
Once the LDS8866 reaches steady-state operation,
its charge pump remains in operation even when the
LED current is turned off (t OFF ). As shown in Figure 3,
V OUT increases slightly by an amount proportional to
the voltage drop generated by charge pump’s R OUT
and the total LED current load. The LDS8866’s
efficiency and LED current regulation are not affected
because the LEDs are off during this time.
If the PWM1/PWM2 pins are held high or low longer
than 30ms (Time to Shutdown), the LDS8866 turns
LEDs off. If PWM1/PWM2 pins are low, shutdown
mode is enabled and the supply current drops to 1 μA
or less. If PWM1/PWM2 pins are logic high, the
LDS8866 charge pump remains active with an overall
quiescent current ~ 1 mA.
Unused LED Channels
For applications with only two or four LEDs, unused
LED banks can be disabled via the appropriate PWM
pin connected to the ground.
For applications requiring 1, 3, or 5 channels, the
unused LED pins should be tied to V OUT (see Figure
4). If LED pin voltage is within 1 V of V OUT , then the
channel is switched off and a 250 μ A test current is
placed in the channel to sense when the channel
moves below V OUT – 1.5 V.
Figure 4. Application circuit with 5 LEDs
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 LDS8866 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 LDS8866 requires restart
after die temperature falls below 130°C.
5. Input Voltage Under-Voltage Lockout
If V IN falls below 2.2 V (typical value), LDS8866
enters shutdown mode. Device requires restart when
input voltage rises above 2.3 V.
6. Low V IN or High LED V F Voltage Detection
If, in 2-x mode, V IN is too low to maintain regulated
LED current for given LED V F , or LED becomes an
open circuit, or if any LED at active channels is
disconnected, LDS8866 starts subsequently
changing modes (2-x – 1-x – 1.33-x -1.5-x – 2-x - …)
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 (V F ) ranging from 1.6 V to
3.6 V may be used. Charge pumps operate in highest
efficiency when V F voltage is close to V IN voltage
multiplied by switching mode, i.e. V IN x 1, V IN x 1.33,
and so on. If the power source is a Li-ion battery,
LEDs with VF = 2.7 V - 3.3 V are recommended to
achieve highest efficiency performance and extended
operation on a single battery charge.
? 2009 IXYS Corp.
Characteristics subject to change without notice
8
Doc. No. 8866DS, Rev. N2.0
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