Technical
Article
A novel architecture for driving a supercap LED flash using a dual DC-DC converter
Peter Trattler
Technical
Article
A novel architecture for driving a supercap LED flash using a dual DC-DC converter
Peter Trattler
Users of mobile phone cameras, as well as digital still and digital video cameras, cannot always
choose perfect natural lighting for the pictures they wish to take. So the flash light is an essential
feature of these devices. For still pictures, a bright flash of light enables the use of a short exposure
time, and this produces sharp images unaffected by motion of the subject and ‘camera shake’ - the
blurring caused by minute, unconscious movements of the user’s body.
Manufacturers today normally integrate an LED flash into mobile phones. Unfortunately, the light
output from today’s LED flashes falls far short of the bright flash provided by a xenon bulb. So an
LED flash today typically requires long exposure times (sometimes longer than 100ms) and therefore does not capture images in dark conditions nearly as sharply as a digital still camera (DSC)
fitted with a xenon flash.
Nevertheless, an LED flash has some advantages over xenon tubes:
•
an LED system is much thinner
•
it is easier to handle in the assembly process
•
it can provide continuous light output for video recording.
LEDs are also better for macro (close-up) image capture, since the LED’s output can be dimmed,
whereas, for a xenon bulb, the duration of the light output can be controlled, but the intensity cannot. In macro photography, the use of a xenon bulb tends to produce over-exposed images.
In fact, improving the camera flash system is proving to be an important way in which mobile phone
manufacturers can gain a competitive edge, since more and more users are choosing to use their
smartphone instead of a separate DSC as their main or only camera. These users really care about
the picture quality their phone can provide, even in dark conditions. This has driven moves by
phone manufacturers to dramatically increase the resolution of the image sensor. Nokia’s Lumia
1020 (introduced mid-2013), for instance, features a 41Mpixel image sensor.
In dark conditions, however, there is a limit to the detail that even a very high-resolution sensor can
capture without assistance from a flash light. And since cameras were first integrated into phones,
manufacturers have been attempting to improve the light output from their flash LEDs. Over the
years, the drive current supplied to the flash LEDs has risen from, in the early days, a feeble
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100mA. Now, however, the LED power supply in even the best cameraphones is stuck at 2.5A
(peak), because of limitations in the way energy is supplied from the battery to the LEDs.
So phone and camera manufacturers have been looking for a technological breakthrough that will
enable LEDs to provide a high instantaneous light output that is closer in intensity to that of a xenon
bulb. This breakthrough has now been made: it is the world’s first chip to feature a dual DC-DC
converter, operating from two inputs at different voltages and producing a single voltage-regulated
output for an LED flash. This new power circuit enables the use of an auxiliary supercapacitor power source in combination with power from the device’s battery, producing sufficient energy to drive
dual LEDs emitting intense light.
This article describes why the space constraints in mobile phones and consumer digital cameras
call for such a solution, and how it is implemented in an LED flash driver system.
Wasted energy in supercapacitors
A battery is an inherently poor source of bursts of high current: a burst is required to produce a flash
of light, whether from LEDs or a xenon bulb. In mobile phones, there is in addition a physical limit to
the battery’s current rating. Batteries in mobile phones are designed to supply the peak current
drawn by the RF power amplifier (PA). In GSM mode – an old standard, but still supported by most
new phones – the PA can draw as much as 3A from the battery. Therefore batteries today are rated
for 3A peak current, but internal protection circuitry prevents them from delivering higher currents.
This 3A battery limit is sufficient to drive up to 2.5A through a flash LED. (A step-up converter is
required as the forward voltage of the LED is typically higher than the battery voltage.) This means
that for flash LED currents above 2.5A, the battery alone is not enough. (Phone and camera manufacturers’ roadmaps are planning to support LED currents of up to 8A.)
So space-constrained consumer designs such as mobile phones and DSCs today employ an auxiliary power source dedicated to the flash light: a dual supercapacitor (two supercapacitors in series
in a single package, both rated typically at 1000µF), a device which is capable of discharging quickly at high current. A typical application circuit is shown in Figure 1. Here, the battery charges the
supercap when the flash is triggered; the supercap then discharges the stored energy to the LEDs.
This means that the battery is not connected directly to the LEDs.
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Fig. 1: in conventional flash LED driver circuits, the phone or camera’s battery charges the supercap, and the supercap
supplies the flash LEDs
Unfortunately, a supercap has one great disadvantage as a power source for flash LEDs. As it discharges, its output voltage declines markedly. When fully charged, the output voltage of the device
is a typical 5.5V. The forward voltage of a flash LED is typically in a range from 3.8V to 4.2V. During
discharge, the supercap’s output falls below this forward voltage, at which point the current drawn
by the LED falls dramatically (see Figure 2). This results in the light dimming to a point at which it
becomes useless as a light source for a camera.
VF
LED
3.7V
Fig. 2: because the output voltage of the supercap falls below the forward voltage of the flash LEDs as it discharges, the
effective flash duration is abbreviated
The flash light, then, stops drawing a high current from the supercap before it is completely discharged. This is unfortunate in a space-constrained environment such as a mobile phone: even
though supercaps have benefited from recent improvements to their density, they are still big devices, with a large footprint and high profile. It is not desirable for the phone designer simply to overPage 4 / 8
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specify the supercap, so that the first part of its discharge alone – when the output voltage is >4.0V
– is sufficient to drive the flash LEDs. There simply is not room in sleek consumer-device form factors for an over-specified supercap.
Today’s mobile phones, then, feature LED flash circuits that are under-powered and – for all the
handset manufacturers’ claims about the number of megapixels their cameras boast – produce lowquality images in dark conditions.
New DC-DC architecture uses battery supply efficiently and almost fully drains supercap
Now a novel dual DC-DC converter architecture developed by ams enables system designers to
specify a supercap with a much smaller nominal capacity than in today’s conventional LED flash
power systems, since:
a) the system uses nearly all the supercap’s nominal capacity
b) the supercap’s input to the LEDs is supplemented by the input from the battery
This dual DC-DC converter architecture provides these benefits by combining the supercap and
battery inputs (see Figure 3). This difficult task of combining inputs at two different and varying voltages into a single output is implemented in a new LED driver IC from ams, the AS3630.
Fig. 3: the ams dual DC-DC converter architecture combines inputs from supercap and battery into a single output
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The concept of an integrated dual-converter IC is not in itself new: multi-phase DC-DC converters
exist today, but the dual inputs are at the same voltage as each other.
In the flash LED application, the input from the battery may be at a different voltage from the input
from the supercap. Moreover, both input voltages vary – the battery’s according to its state of
charge, and the supercap’s, as discussed, across its discharge cycle.
In the AS3630, one converter operates from VBAT to VLED, and the other from VSUPERCAP to
VLED. Given that they have different and varying input voltages, the difficulty comes in ensuring
that the converters’ two separate control loops do not interfere with each other: both are contending
for a single output capacitor. How can the device provide a stable, regulated output in these circumstances?
The answer developed by ams is to simplify the control requirement. The first converter, from VBAT
to VLED, operates on a current-limit-controlled basis, and is configured to provide a supply at the
programmable current limit, which could be up to the battery’s current limit: as above, a typical 3A in
a mobile phone. An additional control loop can then be applied to the input from the supercap as
necessary in order to ensure that the combined input to the LEDs remains at a level above their
forward voltage.
Demonstration circuits show that current throughput of up to 8A is possible using the AS3630 with a
supercap and a typical mobile phone battery.
Another advantage of the supplemented power supply enabled by the AS3630 is that it can supply
twin flash LEDs configured in series, whereas conventional flash LED power supplies can only support a parallel configuration. In a parallel configuration, forward voltage mismatch is dangerous,
causing excessive heat generation in the driver circuit. As a result, system manufacturers or their
LED suppliers have to devote production resources to binning LEDs in matched pairs. The series
configuration supported by the AS3630 eliminates the need for matching, saving both time and cost
in the production process.
Instant-on torch mode
The direct connection of the battery to the LEDs also provides for one other useful feature that is
superior to that of conventional LED power circuits. As described above, in these conventional designs power to the LEDs only comes from the supercap. The supercap need to be charged by the
battery immediately before being discharged. This charging process takes 2-3s. For image-capture
operations, this delay is normally acceptable.
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But in torch mode, the user might prefer the light to operate immediately. The AS3630 provides for
this, enabling the flash LEDs in torch mode to be supplied by the battery only (see Figure 4). In this
mode, the supercap does not supply the LEDs, and so the user does not have to wait for it to be
charged before the torch light will operate.
Fig. 4: flash LEDs can operate from the battery power supply alone in torch mode
Conclusion
A patent pending circuit design from ams has, for the first time, combined dual DC-DC converters
operating from different input voltages for an LED flash in a single chip. This innovative power circuit enables the inputs from a supercap and a battery to be combined in order to maintain a minimum input voltage above the forward voltage of the LEDs they supply.
As a result, smartphones can dramatically improve the performance of their flash LED system while
maintaining their thin form factor. Since the circuit makes full use of the power storage capacity of
the supercap, the system designer can specify the smallest possible supercap, an important benefit
in consumer devices that have very thin and light form factors.
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For further information
ams AG
Tel: +43 (0) 3136 500
info@ams.com
www.ams.com
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