LDS8865 IXYS Corporation, LDS8865 Datasheet - Page 8

• Manufacturer Part Number: LDS8865
• Description: Lds8865 6-channel Ultra Low Dropout Led Driver
• Manufacturer: IXYS Corporation

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
x 2/0.9), assuming that charge pump’s efficiency
alone
preset current is 30 mA per LED.
When PWM LED current control is first enabled (at
cold start, for example), the LDS8865’s V
V
cycle through all operating modes (if necessary) so
that V
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
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
V
the voltage drop generated by charge pump’s R
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
channel is switched off and a 250μ A test current is
placed in the channel to sense when the channel
moves below V
Protection Mode
The LDS8865has follow protection modes:
© 2009 IXYS Corp.
Characteristics subject to change without notice
PCR
OUT
IN
monitors cause the LDS8865’s charge pump to
increases slightly by an amount proportional to
OUT
would then be 400 mA (= 30 mA/LED x 6LEDs
at 2x mode is 90% and maximum factory
is high enough to maintain regulated LED
OUT
IN
and the V
– 1.5 V.
OFF
F
of the LEDs chosen, the
). As shown in Figure 3,
OUT
IN
, V
, then the
OUT
, and
OUT
8
1. LED short to V
If LED pin is shorted to V
becomes as short circuit, or LED pin voltage is within
from V
recognizes this condition as “LED Short” and disables
this channel. If LED pin voltage is less than (Vout –
1.5V),
particular channel to programmed value.
2. V
The charge pump’ output voltage V
limits at about 6.2 V maximum. This is to prevent the
output pin from exceeding its absolute maximum
rating.
3. V
If V
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
enters shutdown mode Device restarts when input
voltage rises above 2.3 V and PWM signal is applied.
6. Low V
If, in 2x mode, V
LED current for given LED V
open circuit, or if any LED at active channels is
disconnected,
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.
OUT
OUT
OUT
IN
Figure 4. Application circuit with 5 LEDs
falls below 2.2 V (typical value), LDS8865
Over-Voltage Protection
Short Circuit Protection
OUT
LDS8865 restores LED current at this
is shorted to ground before LDS8865 is
IN
or High LED V
to (V
LDS8865
IN
OUT
OUT
is too low to maintain regulated
protection
-
F
Voltage Detection
1.5V) range, LDS8865
Doc. No. 8865DS, Rev. N2.1
F
OUT
starts
, or LED becomes an
, LED burned out
OUT
automatically
subsequently