NCP5007 ,Compact White LED DriverAPPLICATION NOTEINTRODUCTION Depending upon the type of flash involved, the amount ofenergy stored ..
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NJM311 , PRECISION VOLTAGE COMPARATOR
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NCP5007-NCP5007SNT1G
Compact White LED Driver
AND8135/D
Efficient High Power
Flash Light
Prepared by: Michael BairanzadeON Semiconductor
INTRODUCTIONAlthough the xenon lamps dominate the standard camera
market, they are not applicable in the portable phone. As a
matter of fact, the large reservoir capacitor and high voltage
associated with the xenon flash make such a concept not
suitable for the cellular phone equipments. The
semiconductor based White LED devices provide the right
choice when limited flash light becomes necessary to
illuminate a photographic scene. This paper depicts the
basics of the xenon concept and details a typical White LED
flash application.
XENON LAMP CONCEPTA low pressure of a rare gas mixture fills a glass envelope
with two ends electrodes on both sides. In steady state, the
voltage across the electrodes is set to a value well below the
trigger voltage as depicted Figure 1. At this point, no current
flows and the system is stable until a trigger voltage is
applied to the third electrode. This high voltage pulse, in the
1 kV range, comes from a transformer built with a small
magnetic tore triggered by an abrupt discharge of the
capacitor C2 (see Figure 2).
Figure 1. Xenon Flash Breakdown VoltageThe gas is ignited and the plasma generates a bright flash,
the typical duration being 2 ms for consumer applications.
Depending upon the type of flash involved, the amount of
energy stored into capacitor C1 can be a low 10 Joule (small
camera) to thousand of Joule for professional applications.
+260 V
The net advantage of such a concept is the very short
pulse, making easy snap shot photos to capture moving
stuffs. The drawbacks are the large physical size of the
reservoir capacitor and the recycle time needed to recharge
the capacitor between two shots (in the 5 sec range for
consumer applications). Clearly, these drawbacks make the
xenon based flash not suitable for hand held cellular phone,
with limited size and energy supply.
To overcome the physical size limitation, the flash
concept is to make profit of the high efficiency, in term of
light, coming from the modern white LED.
With a 4 V forward drop voltage, such diodes do not need
extra high voltage trigger pulse, they are extremely fast to
turn ON/OFF and all the associated electronic circuit can be
housed inside a standard portable phone. Since the white
LED have electrical characteristics similar to the standard
LED (see Figure 3), one must provide a constant forward
current to control the device.
