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LDS8865
When LED current control is enabled at the
PWM1/PWM2 inputs, the LDS8865’s maximum input
current is determined by the factory preset maximum
LED current multiplied by number of LED used, the
charge pump operating mode (1x, 1.5x, or 2x), and
divided by charge-pump driver’s efficiency. For
example, if six LEDs are used and the charge pump
is operating in 2x mode, the maximum pulse current
at V IN would then be 400 mA (= 30 mA/LED x 6LEDs
x 2/0.9), assuming that charge pump’s efficiency
alone at 2x 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 LDS8865 ’s V IN , V OUT , and
V PCR monitors cause the LDS8865’s charge pump to
cycle through all operating modes (if necessary) so
that V OUT is high enough to maintain regulated LED
current. To prevent nuisance switching of the charge
pump during this initial start-up sequence, a 0.8ms
transition filter is applied at each charge-pump 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 1.6 ms. (See Timing Diagram
Figure 3)
Once the LDS8865 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 LDS8865 ’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 LDS8865 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
LDS8865 charge pump remains active with an overall
quiescent current ~ 1 mA.
Unused LED Channels
For applications with only four or two 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 Vout (see Figure
4). If LED pin voltage is within 1V 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.
Protection Mode
The LDS8865has follow protection modes:
Figure 4. Application circuit with 5 LEDs
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, LDS8865
recognizes this condition as “LED Short” and disables
this channel. If LED pin voltage is less than (Vout –
1.5V), LDS8865 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 LDS8865 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 LDS8865 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), LDS8865
enters shutdown mode Device restarts when input
voltage rises above 2.3 V and PWM signal is applied.
6. Low V IN or High LED V F Voltage Detection
If, in 2x 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, LDS8865 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.
? 2009 IXYS Corp.
Characteristics subject to change without notice
8
Doc. No. 8865DS, Rev. N2.1
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