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LDS8621
parallel to provide maximum current through one LED
in flash mode, the lower average current value in
torch mode may be achieved using PWM. PWM
signal may be applied to one of the PWM pins, while
other channel is off, or to both PWM pins. The
average LED current is proportional PWM duty cycle
and number of channels that are in ON state. For
instance, if both channels are connected in parallel to
provide 196 mA through one LED in flash mode,
5. Input Voltage Under-Voltage Lockout
If V IN falls below 2.2 V (typical value), LDS8621
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 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
turning one channel of in torch mode redjuces current
disconnected, LDS8621
starts subsequently
to 96 mA, while applying PWM signal with 50% duty
cycle to this channel reduces average LED current to
48 mA. LDS8621 allows modulation frequiencies in
the range from 100 Hz to 100000 kHz with duty cycle
from 100% to 1%. Lower than 100 Hz modulation
frequiencies is not recommended especialy at short
duty cycles because LED flicker may be visible.
The LDS8621 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
(V OUT - 1.5V) range, LDS8621 recognizes this
condition as “LED Short” and disables this channel
with 750 μA control current. If LED pin voltage is less
than (Vout – 1.5V), LDS8621 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 LDS8621 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 LDS8621 requires restart
after die temperature falls below 130°C.
changing modes (2x – 1x – 1.33x -1.5x – 2x -…) in
an attempt to compensate insufficient voltage. As a
result, average current at all other channels that are
ON may be below regulated level.
LED Selection
LEDs with forward voltages (V F ) ranging from 1.6V to
4.5V may be used. Charge pumps operate in the
highest efficiency mode when V F voltage is close to
Vin voltage multiplied by switching mode, i.e. Vin x 1,
Vin x 1.33 and so on. If the voltage source is a Li-ion
battery, we recommend selecting LED with V F = 2.7 –
3.6 V to extend the battery life and achieve highest
efficiency.
External Components
The LDS8621 requires two external 1μF ceramic
capacitors (X5R or X7R type) for decoupling input,
output, and two 0.22 μF capacitors the same type for
the charge pump. In all charge pump modes, the
input current ripple is very low, and 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 four capacitors. A ground plane
should cover the area under the driver IC as well as
the bypass capacitors. Short connection to ground on
capacitors Cin and Cout can be implemented with the
use of multiple via. A copper area matching the
TQFN exposed pad (PAD) must be connected to the
ground plane underneath. The use of multiple via
improves the package heat dissipation.
? 2009 IXYS Corp.
Characteristics subject to change without notice
5
Doc. No. 8621DS, Rev. N1.0
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