DN1009 - Tiny, Efficient High Power LED Camera Flash Solutions for Cell Phone Applications

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Tiny, Efficient High Power LED Camera Flash Solutions for
Cell Phone Applications – Design Note 1009
Keith Szolusha
Introduction
Cell phones and PDAs are approaching the limit for small
form factors, but their feature sets are growing with new
functions such as built-in cameras and even video cameras.
The initial in-phone cameras were basic, low resolution devices with little in the way of usable flash capability. However, cell phone camera resolutions have improved and
sophisticated flash technology is expected to keep pace.
There are two flash technologies used in digital still cameras. The first uses a Xenon flash lamp to provide a very
short, high lumen flash with excellent spectral characteristics. LTC offers Xenon photoflash solutions that are
simple, compact and efficient (see Design Note 345).
The second flash technology uses LEDs. LED solutions hold
the upper hand when simplicity and space is at a premium
and they are the only choice when ‘torch’ or video-mode
lighting is required. LED flash units fit into spaces where
even the smallest Xenon photoflash cannot. Until recently,
LEDs could not compete with Xenon solutions in light
output and spectral performance, relegating them to the
realm of low resolution (sub 1 megapixel) cameras. However, recent (>10×) performance improvements have made
LEDs an attractive solution for the now popular 1 megapixel
and higher cell phone cameras.
Linear Technology offers many simple and extremely
compact solutions for driving the latest high-light-output LEDs, including Lumileds Luxeon Flash LEDs. These
LEDs are up to 12 times brighter than conventional LEDs
(http:/www.lumileds.com/solutions/solution.cfm?id=9).
Constant Current LED Drivers
Switcher-based DC/DC converters are small and efficient,
so they are prime candidates for LED driver solutions. The
problem with a traditional switcher is that it is designed to
produce a regulated voltage for varying load currents. LEDs
must be driven with constant current to generate predictable light output. Using a typical switcher for constant
current involves adding an op amp and additional circuitry,
which are undesirable in space-constrained applications.
Four new devices solve this problem. The LTC®3216 charge
pump, the LTC3453 buck-boost, and the LT®1618 and
LT3479 boost regulators all incorporate the required circuitry to generate a constant current output while maintaining high efficiency operation.
, LTC and LT are registered trademarks of Linear Technology Corporation.
4.7µH
DO3314-472
3.3mm × 3.3mm × 1.4mm
VIN
2.7V to 4.2V
2.2µF
4.7µF
2.2µF
VIN PVIN SW1
VC
0.1µF
EN1
EN2 (FLASH)
EN2
ILED
LED1-4
2.9V TO 4.4V
C1– C2+
VIN
C2–
D1: LUMILEDS LXCL-PWF1
CPO
D1
2.2µF
4.7µF
D1: LUMILEDS LXCL-PWF1
LTC3453
ISET1
3.48k
1%
C1+
D1
LTC3216
EN1 (TORCH)
8.25k
1%
SW2 VOUT
2.2µF
ISET2
GND
EN1
EN2
0
1
0
1
0
0
1
1
ILED
0 (SHUTDOWN)
150mA (TORCH)
350mA
500mA (FLASH)
DN1009 F01
EN1 (TORCH)
EN1
EN2 (FLASH)
EN2
ISET1
ILED
ISET2
19.1k
1%
6.34k
1%
GND
ILED
EN1 EN2 ILED
0
0 0 (SHUTDOWN)
1
0 200mA (TORCH)
0
1 600mA
1
1 800mA (FLASH)
DN1009 F02
Figure 1. High Performance LED Flash Driver Yields Greater
than 90% Efficiency. The LTC3453 Buck-Boost Delivers
500mA to the LXCL-PWF1 LED Flash with Only Six Tiny
External Components
10/04/1009
Figure 2. This LXCL-PWF1 Luxeon Flash Driver is
Extremely Small. It Requires Only an LTC3216 Low Noise
High Efficiency Charge Pump in a Small DFN Package,
Along with Four 0603 Capacitors and Two 0402 Resistors
Buck-Boost for LXCL-PWF1
Although constant-current solutions do not directly regulate or monitor the output voltage of the LED driver circuit,
the level of the output voltage relative to the input voltage
is a key determinant of the type of part required. A buck
or boost regulator does not have the ability, when converted to constant-current, to properly regulate the LED
current if the forward voltage of the LED is not respectively
below or above the input voltage. The wide input voltage
range of a lithium-ion battery requires that a converter be
able to both step-up and step-down the voltage when the
forward voltage of the LED is within the range of the
battery’s discharge profile in order to achieve steady state
regulation of the LED current.
The LTC3453 buck-boost LED driver (Figure 1) does this
very efficiently with its four internal switches. This tiny
solution only requires one, small and low profile inductor
(3.3mm × 3.3mm × 1.4mm) and provides 90% LED
efficiency. The even smaller LTC3216 charge-pump LED
driver (Figure 2) takes a different approach to the same
problem. By operating in 1×, 1.5× or 2× mode, it provides
constant current to the PWF1 LED with either a step-up or
step-down in voltage. The external components are only
four small ceramic capacitors and two smaller resistors.
The LTC3453 can provide up to 500mA LED flash current
and the LTC3216 can provide 800mA LED flash current to
the PWF1.
The LT1618 constant-current, constant-voltage converter
(Figure 3) has a unique current sense amplifier with the
ability to sample current from the input or output. By
LUMILEDS
LXCL-PWF1
0.10Ω
1%
ILED
VIN
3.3V TO 4.2V
returning the LED from the output to the input, this boost
converter is given buck-boost capability with up to 500mA
LED flash current. The LT1618 solution provides effective
performance for cost sensitive applications.
LXCL-PWF2 Boost Driver
To provide significantly more light output, the Lumileds
LXCL-PWF2 Luxeon Flash LED stacks two PWF1s in series on a single die. This effectively provides twice the light
of the PWF1 at the same LED current, but doubles the forward voltage.
The higher forward voltage of the LXCL-PWF2 is always
above the battery range, so a step-up (boost) solution is
sufficient. The LT3479 can provide up to 700mA current
flash to the PWF2 (Figure 4). The LT1618 can flash 350mA
through the PWF2 when configured as a boost only. With
the ability to externally reduce the error amplifier’s reference voltage, the LT3479 drives the LED current directly
through a sense resistor tied to the feedback pin without
compromising efficiency.
An output disconnect MOSFET is required to turn off the
LED. Boost regulators inherently provide a direct path from
the input to the output even when the low side switch is
shutdown. Since LEDs emit light even with very small
forward current, a complete disconnect in the path to
ground is required. By placing the FET in series with the
LED, instead of between FB and GND, the LED current and
feedback voltage are not compromised, maintaining excellent regulation.
3µH
CDRH5D28-3R0
GND
PMEG2010EA
SD25 10µH
10nF
SS
L
VIN
EN1
(TORCH)
SW
SHDN
VS
ISP
866k
1%
LT1618
IADJ
1µF
GND
EN2
(FLASH)
2.2M
ISN
FB
VC
4.7µF
10V
VIN
2.7V TO 4.2V
LT3479
VIN
0.1µF
EN1 EN2 ILED
0
0 0 (SHUTDOWN)
0
1 0 (SHUTDOWN)
1
0 250mA (TORCH)
1
1 500mA (FLASH)
DN1009 F03
Figure 3. The LT1618 Constant-Current, Constant-Voltage
Boost Regulator can Be Configured to Give a Low Parts
Count Buck-Boost Solution for LXCL-PWF1 Luxeon Flash
at 500mA
VC
0.150Ω
1%
FBP
RT
2.2µF
ILED
2.2nF
10k
FBN
VREF
SHDN
124k
1%
LUMILEDS
LXCL-PWF2
10µF
10V
PMEG2010EA
SW
GND
15k
100k
1%
1.27k
1%
10k
1%
EN1
EN2
0
0
1
1
0
1
0
1
ILED
0 (SHUTDOWN)
0 (SHUTDOWN)
700mA (FLASH)
100mA (TORCH)
EN2
EN1
DN1009 F04
Figure 4. The LT3479 Boost Regulator Delivers 700mA
Constant-Current to the LXCL-PWF2 High Voltage Luxeon
Flash with 85% Efficiency and Output Disconnect
Data Sheet Download
http://www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900, Ext. 2759
dn1009 LT 1004 • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2004
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