Supercapacitor LED flash/torch controller upgrades conventional camera module architecture to deliver floodlight-strength flash power
An innovative IC from STMicroelectronics (NYSE: STM), a global semiconductor leader serving customers across the spectrum of electronics applications, will enable compact digital cameras and cameraphones to satisfy demands for increased power from the built-in flash unit while supporting more sophisticated user controls.
ST's new chip, the STCF04,
is an integrated camera flash and torch controller that
raises the maximum power of an LED flash module from a few
Watts, typical of today's designs, to over 40 Watts,
producing the brightness equivalent to an outdoor security
floodlight. The new device also provides extra flash and
torch brightness controls, with eight user-selectable
levels for the flash and 12 for the torch, as well as a
light-sensor input. Flash and torch safety timers,
temperature sensing and short-circuit protection features
are also built in.
The STCF04 is used in a unique configuration combining a
supercapacitor, a discrete high-current MOSFET switch and
high-power white LEDs, and is ideal for use in emergency
high-intensity flashing lights as well as camera and
cameraphone applications.
The discrete MOSFET is the key to the STCF04's power
advantage over other controllers that usually integrate a
MOSFET of lower rating on the chip. Its extra power enables
designers to provide the higher quantities of light energy
required by today's high-Mpixel camera sensors by
specifying an LED flash unit rather than a conventional
high-power xenon flash, thereby gaining the advantage of a
compact and simplified design.
Since ST began sampling the STCF04, two major partners -
Murata which is leading producer of supercapacitors
(MURATA) and OSRAM which is a top-tier supplier of flash
modules for leading smartphone platforms have begun using
the device to showcase the best capabilities of their
latest products.
Major features of STCF04:
• Maximum flash power >40W
• 1 x 320mA torch current
• 100mA privacy indicator red LED output; 12 adjustment
steps
• I2C bus with selectable address
• Temperature sensing, short-circuit, over-voltage and
flash/torch timer safety features
Available in the 25-bump 3 x 3mm TFBGA package, the STCF04
is currently sampling. Full production is planned at the
end of Q1 2012 priced at $2.00 for orders of 1000 pieces.
Alternative pricing options may be available for higher
quantities.
Click here for the high-resolution photo
About STMicroelectronics
STMicroelectronics is a global leader serving customers
across the spectrum of electronics applications with
innovative semiconductor solutions. ST aims to be the
undisputed leader in multimedia convergence and power
applications leveraging its vast array of technologies,
design expertise and combination of intellectual property
portfolio, strategic partnerships and manufacturing
strength. In 2011, the Company's net revenues were $9.73
billion. Further information on ST can be found at www.st.com.
Background: why today's digital cameras need a more
powerful flash
The image sensors of digital cameras and multi-function
gadgets such as cameraphones typically feature more
Megapixels with each successive product generation. Compact
DSCs have increased from 2-3Megapixels in early models to
8-10Megapixels today, and typical cell/smartphone cameras
have increased from 1-2Megapixels to around 8Megapixels, as
the overall sensor size has remained the same.
Increasing the pixel density of the image sensor enables
sharper, better quality images, particularly when
transferred to a large screen such as a plasma or LCD TV.
On the other hand, each individual pixel in the sensor is
smaller and so absorbs less light energy in a given
exposure time.
Although not usually a problem in normal daylight, since
the sun easily provides enough energy for each pixel,
smaller pixel sizes challenge photography in low-light
conditions and demand a high-power flash to ensure adequate
exposure. While it is true that typical pixel sensitivity
is increasing with each generation of sensors, the
improvements do not compensate fully for the reduction in
surface area.
Miniaturized xenon flash units, which use the same
technology as in a traditional camera flash, produce a
short burst of high-power illumination which is more than
adequate for most digital photography. However, the xenon
flash can be inconvenient in a small form-factor product
such as a phone or compact DSC. High-voltage circuitry such
as a charging supply of over 300V and a trigger operating
at several kilovolts are needed, and must be separated from
other components for safety. Also, the high-voltage
capacitors used in these circuits tend to be large and
bulky.
Flash modules featuring high-power white LEDs have emerged
as an alternative. These offer the advantage of much
simpler control circuitry, as well as lower operating
voltages that are intrinsically safer and call for less
bulky components. Although less powerful than a xenon flash
in peak intensity, an LED flash operates over a longer
duration so that the light energy emitted throughout the
exposure time is adequate for each pixel to detect the
image.
However, since today's high-Megapixel camera sensors demand
even more light to energize the pixels, small form-factor
cameras need higher power LED flash modules. Today's most
powerful units feature a supercapacitor to provide the fast
supply of energy needed to light the flash, which a small
battery such as a cameraphone battery is unable to provide.
A supercapacitor has high energy-storage capability in a
small volume, and has very low Equivalent Series Resistance
(ESR); a parasitic effect that limits the discharge rate of
a conventional capacitor. The battery charges the
supercapacitor over a relatively long period of about a
second. When fully charged the supercapacitor discharges
its energy rapidly into the LED circuit, which produces a
powerful pulse of light whose duration can be controlled
using external circuitry. Then the supercapacitor can be
recharged from the battery, to be ready for the next
flash.
A controller IC coordinates charging the supercapacitor and
triggering the discharge into the LED circuit. This IC
usually also controls functions such as adjustment of flash
brightness or duration, and the use of the LED in torch
mode. The torch, or flashlight, function is often
implemented as a value-added feature of the end product.
This combination of a supercapacitor, one or more white
power LEDs, and the controller IC is a more than
satisfactory alternative to the xenon flash in compact DSCs
and built-in cameras that eliminates the safety issues of
the xenon flash.
As camera Megapixel count continues to increase, and as
consumers expect better and better performance from
built-in cameras, producers of flash modules need enhanced
driver ICs capable of delivering ever greater quantities of
light energy. Product designers also appreciate extra
features such as more sophisticated torch-mode controls in
order to offer improved user experiences with each
successive product generation. A limitation of most
controller ICs, however, is that the power MOSFET needed to
control the transfer of energy into and out of the
supercapacitor is usually integrated in the device and has
limited current-handling capability. This allows only a few
Watts to be driven into the flash.
ST's STCF04 enables much higher power - in fact over 40W,
as described earlier - to be supplied to the LEDs and also
supports extra controls and user features, thereby
overcoming the two critical demands placed on designers of
flash modules and cameras to provide extra power and
functionality. ST has achieved this by designing the device
for use with an external MOSFET of much higher power rating
than can be achieved using an integrated device.
Moving the power switch off-chip has also allowed ST to
implement extra functionality in the STCF04. It is the most
feature-rich flash/torch controller on the market today,
supporting eight user-selectable flash power levels, 12
torch power levels, control of four or more individual
power LEDs and an input for a light sensor to control the
flash duration, in addition to typical protection and
safety features.
A complete flash module built using the STCF04 occupies
around 60mm2 of pc-board space. Although larger than other
LED modules of lower power, the result remains
significantly smaller and simpler than a xenon circuit
while satisfying the demand for very high-energy flash
illumination.
Since ST began sampling the STCF04, two major partners -
one a supercapacitor producer and the other a supplier of
flash modules for leading smartphone platforms - have begun
using the device to showcase the best capabilities of their
latest products.
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