Dec 2005 Photoflash Capacitor Chargers Keep Up with Shrinking Cameras

DESIGN FEATURES
Photoflash Capacitor Chargers
Keep Up with Shrinking Cameras
by Mike Negrete
Introduction
Camera-phones have come a long way
since the first generation of integrated
cameras offered low-resolution CMOS
images through the eye of a plastic
lens. Now PDAs and high-end cell
phones include high quality cameras
with 2 megapixel resolutions and glass
optics. Since these devices are carried
by most users at all times, size is of the
utmost importance. LED flashes were
introduced in early model cell phone
cameras, but they cannot produce
enough light and lack the spectral
quality required for higher-end cameras. Although xenon flashes are an
optimal source of light for photography, they required substantially more
board space than LED flashes until
10
COUT = 50µF
CHARGE TIME (SECONDS)
9
8
7
6
LT3484-1
5
LT3484-2
4
3
2
LT3484-0
1
0
2
3
4
5
VIN (V)
6
7
8
Figure 2. Charge time for the LT3484
VBAT
1.8V TO 8V
T1
1:10.2
C1
4.7µF
D1
1
4
2
5
320V
+
4, 5
SW
6
VIN
2.5V TO 8V
C2
0.1µF
DONE
CHARGE
3
VBAT
VIN
R1
100k
1
2
LT3484-0
GND
COUT
PHOTOFLASH
CAPACITOR
7
DONE
CHARGE
C1: 4.7µF, X5R OR X7R, 10V
T1: KIJIMA MUSEN PART# SBL-5.6-1, LPRI = 10µH, N = 10.2
D1: VISHAY GSD2004S DUAL DIODE CONNECTED IN SERIES
R1: PULL UP RESISTOR NEEDED IF DONE PIN USED
Figure 1. Compact, 320V photoflash capacitor charging circuit needs no external Schottky diode
the LT3468 allowed xenon flashes
to fit into the spaces of cell phones
and PDAs. The LT3484 and LT3485
photoflash capacitor chargers improve
upon the LT3468.
The LT3484 and LT3485 are based
on the LT3468’s patented control
scheme, providing well controlled
battery current, fast charge times
and high efficiency. Both series of
parts use the same tiny, low-profile
transformers as the LT3468. Available in a 6-Lead 2mm × 3mm DFN,
the LT3484 reduces the board space
significantly with its smaller package
and total solution size compared to the
LT3468. The LT3484 has also added
an additional pin, VBAT, to allow it to
operate from two alkaline cells. For
xenon photoflash applications with
an IGBT, the LT3485 decreases the
solution size further with the same
photoflash functionality as the LT3484
and an integrated IGBT driver in its
10-Lead 3mm × 3mm DFN package.
The LT3485 also features an output
voltage monitor pin.
Overview
A typical application circuit for the
LT3484 is shown in Figure 1. With a
high level of integration inside the part,
Table 1. Photoflash capacitor charger features
LT3484-0
LT3484-1
LT3484-2
LT3485-0
LT3485-1
LT3485-2
LT3485-3
Peak SW Current (A)
1.4
0.7
1.0
1.4
0.7
1.0
2.0
Average Input Current (mA)
(VIN = 3.6V, VOUT = 225V)
500
250
400
500
250
400
750
Charge Time Coefficient Kijima (τ)
0.65
0.30
0.50
0.75
0.34
0.51
NA
Charge Time Coefficient TDK (τ)
0.62
0.32
0.51
0.73
0.37
0.51
1.10
Minimum Battery Voltage(V)
1.8
1.8
Integrated IGBT Drive + VOUT Monitor
No
Yes
External Schottky Diode Required
No
No
Package
2mm × 3mm DFN 6L
3mm × 3mm DFN 10L
Linear Technology Magazine • December 2005
9
DESIGN FEATURES
DANGER HIGH VOLTAGE — OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY
1:10.2
320V
SEE TABLE 2
1
4.7µF
2
•
•
5
SW
VBAT
DONE
1M
150µF
PHOTOFLASH
CAPACITOR
TRIGGER T
1
LT3485-0
VIN
0.22µF
IGBTPWR
IGBTIN
5
A
2.2µF
600V
CHARGE
VCC
5V
6
4
GND
VMONT
2
FLASHLAMP
3
TO
MICRO
C
CHARGE TIME (SECONDS)
VBAT
2 AA OR
1 TO 2 Li-Ion
LT3485-1
4
LT3485-2
3
2
1
LT3485-0
IGBT
IGBTOUT
0
Figure 3. Compact, 320V photoflash capacitor charging circuit with integrated IGBT drive
the application circuit only requires a
tiny, low-profile transformer, a high
voltage diode, and an input bypass
capacitor to charge any size output
capacitor to 320V. Despite requiring
only 70mm2 of valuable board space,
the patented control scheme with its
high power, integrated low resistance
NPN power switch produces fast charge
times shown in Figure 2. There are
three versions of the LT3484 depending on charge time and input current
requirements. The LT3484-0 has the
highest input current at 500mA, while
the LT3484-1 has the lowest average
input current at 225mA. The LT3484-2
has an input current at 375mA.
A typical application circuit for
the LT3485 is shown in Figure 3. In
addition to the photoflash capacitor
charging circuitry, the LT3485 integrates an IGBT drive and a voltage
output monitor. The integrated IGBT
drive saves valuable board space and
cost by eliminating several external
components. The voltage output monitor provides a solution to monitor the
output voltage without resorting to a
resistor divider on the output, which
would drain the output capacitor.
Along with identical current level versions of the LT3484, the LT3485 series
features a high input current part, the
LT3485-3, at 750mA. Typical charge
times are shown in Figure 4.
Operation
Figure 5 shows a block diagram for
the LT3484 and LT3485, which have
identical operation except for the IGBT
drive and voltage output monitor in the
LT3485—highlighted in the diagram. A
low-to-high transition on the CHARGE
pin initiates the part. An edge triggered
one-shot triggered by the CHARGE pin
2
4
3
LT3485-3
5
VIN (V)
6
7
8
Figure 4. Charge time for the LT3485
puts the various latches inside the
part into the proper state.
The part begins charging by turning
on the power NPN transistor Q1. With
Q1 on, the current in the primary of
the flyback transformer increases.
When it reaches the current limit,
Q1 is turned off and the secondary
of the transformer delivers current to
the photoflash capacitor via diode D1.
During this time, the voltage on the
SW pin is proportional to the output
voltage. Since the SW pin is higher than
VBAT by an amount roughly equal to
(VOUT + 2 • VD)/N, the output of the discontinuous conduction (DCM) mode
comparator is high. In this equation,
VOUT is the photoflash capacitor voltage, VD is the rectifying diode forward
drop, and N is the turns ratio of the
transformer.
Once the current in the secondary
of the transformer decays to zero,
Table 2. Pre-designed transformers — typical specifications unless otherwise noted
For Use
With
Transformer
Name
Size
(W × L × H)
mm
LPRI
(µH)
LPRILeakage
(nH)
N
RPRI
(MΩ)
RSEC (Ω)
Vendor
LT3484/5-0
LT3484/5-2
LT3484/5-1
SBL-5.6-1
SBL-5.6-1
SBL-5.6S-1
5.6 × 8.5 × 4.0
5.6 × 8.5 × 4.0
5.6 × 8.5 × 3.0
10
10
24
200 Max
200 Max
400 Max
10.2
10.2
10.2
103
103
305
26
26
55
Kijima Musen
Hong Kong Office
852-2489-8266 (ph)
[email protected] (email)
LT3484/5-0
LT3484/5-1
LT3484/5-2
LT3485-3
LDT565630T-001
LDT565630T-002
LDT565630T-003
LDT565630T-041
5.8 × 5.8 × 3.0 6
5.8 × 5.8 × 3.0 14.5
5.8 × 5.8 × 3.0 10.5
5.8 × 5.8 × 3.0 4.7
200 Max
500 Max
550 Max
150 Max
10.4
10.2
10.2
10.4
100 Max
240 Max
210 Max
90 Max
10 Max
16.5 Max
14 Max
16.4 Max
TDK
Chicago Sales Office
(847) 803-6100 (ph)
www.components.tdk.com
LT3485-0
LT3485-1
LT3485-1
LT3485-3
T-15-089
T-15-089
T-15-083
T-17-109A
6.4 × 7.7 × 4.0
6.4 × 7.7 × 4.0
8.0 × 8.9 × 2.0
6.5 × 7.9 × 4.0
400 Max
400 Max
500 Max
300 Max
10.2
10.2
10.2
10.2
211 Max 27 Max
211 Max 27 Max
675 Max 35 Max
78 Max 18.61 Max
10
12
12
20
5.9
Tokyo Coil Engineering
Japan Office
0426-56-6262 (ph)
www.tokyo-coil.co.jp
Linear Technology Magazine • December 2005
DESIGN FEATURES
PRIMARY
C1
TO VIN
C2
DONE
9
10
3
Q3
SAMPLE
AND HOLD
CHARGE
TO VIN
IGBTIN
R
2
R2
60k
CHIP
POWER
Q2
ENABLE
R3
4k
R1
2.5k
R4
120k
DCM
COMPARATOR
ONESHOT
+
–
+
+
–
60mV
A2
IGBT
DRIVER
POWER
VOUT
COMPARATOR
–
1.25V
REFERENCE
7
DRIVER
R
IGBT
DRIVER
COUT
PHOTOFLASH
CAPACITOR
A3
LT3485 ONLY
ONESHOT
1
8
Q
S
VOUT
SECONDARY
SW
4, 5
VMONT
Q
D1
T1
TO BATTERY
S
Q
Q1
20Ω
+
ONESHOT
20k
RSENSE
A1
– +–
GND
11
20mV
6
LT3485 ONLY
LT3485-3: RSENSE = 0.010Ω
LT3485-0: RSENSE = 0.015Ω
LT3485-2: RSENSE = 0.022Ω
LT3485-1: RSENSE = 0.030Ω
TO GATE OF IGBT
Figure 5. Block diagram for the LT3484 and the LT3485
the voltage on the SW pin collapses
to VBAT, or lower. As a result, the
output of the DCM comparator goes
low, which triggers the one-shot. This
leads to Q1 turning on again and the
cycle repeats.
Output voltage detection is accomplished via comparator A2. When the
SW pin is 31.5V higher than VBAT on
any cycle, the output of A2 goes high.
This resets the master latch and the
part stops delivering power to the
photoflash capacitor. Power delivery
can only restart by taking the CHARGE
pin low and then high.
Note that the flux in the flyback
transformer is brought to zero on each
switching cycle. This is generally referred to as boundary mode operation
since the transformer is operated in
between continuous conduction mode
and discontinuous conduction mode
(CCM and DCM respectively). When
the CHARGE pin is forced low at any
time, the LT3484/LT3485 ceases
power delivery and goes into shutdown
mode, thus reducing quiescent current
to less than 1µA. Figure 6 shows some
typical waveforms for the LT3484 and
LT3485.
Voltage Output Monitor
VSW
10V/DIV
IPRI
1A/DIV
VIN = 3.6V
VOUT = 300V
1µs/DIV
Figure 6. A LT3485 switching
waveform at 300V output
Linear Technology Magazine • December 2005
Camera manufacturers continue to
try to differentiate their product with
novel features such as strobe shots
and sequential shots. These new features rely on fast capacitor charging
to be done in the time between shots.
If the capacitor is not fully charged,
is the voltage high enough to produce
a flash? The LT3485 addresses this
problem by including a 1V full-scale
output, VMONT, proportional to the
capacitor voltage. This output can
easily be read by a microcontroller
with an ADC.
Figure 7 shows the measured
output of VMONT. Because of the high
speed nature of the circuit and the
high dV/dt of the switch pin, there
is a small amount of ripple on the
VMONT output, which can be reduced
by adding a 0.1µF capacitor to the
output or by using the ADC to sample
the VMONT output multiple times and
taking the average.
CHARGE
2V/DIV
VOUT
100V/DIV
VMONT
200mV/DIV
100ms/DIV
Figure 7. Voltage output monitor
waveform during charging
11
DESIGN FEATURES
IGBT Drive
Most camera flashes are capable of
redeye reduction and light-feedback
flashing. These features quench, or
stop, the flash before the capacitor
drains completely. This added level
of control requires a high current,
high voltage Insulated Gate Bipolar
Transistor (IGBT). An IGBT has the
advantage of a BJT’s high voltage
and high current capabilities but
does not need base current since it
has a MOSFET gate as the input. The
tradeoff for these two advantages is
speed. Since a flash is on the order
of milliseconds, speed is not an issue
in this application and an IGBT fits
perfectly for the role.
Like a MOSFET, the gate acts like
a capacitor. The IGBT driver’s job is
to charge and discharge the gate. The
IGBT driver does not need to be fast,
and actually a fast driver can potentially destroy the device. The IGBT
turns on when the IGBTIN pin is above
1.5V and turns off when the IGBTIN
pin is below 0.3V. When the input is
high, the driver draws a small amount
of current to hold the gate high with a
PNP. When the input is low, the driver
has zero quiescent current. During
transitions the driver is capable of
delivering 150mA of current.
The speed of the driver needs to be
carefully controlled or the IGBT may
be destroyed. The IGBT driver does not
need to pull up the gate fast because
of the inherently slow nature of the
IGBT. A rise time of 2µs is sufficient
to charge the gate of the IGBT and
create a trigger pulse. With slower
rise times, the trigger circuitry does
not have a fast enough edge to create
the required 4kV pulse. The fall time
of the IGBT drive is critical to the safe
operation of the IGBT. The IGBT gate
is a network of resistors and capacitors. When the gate terminal is pulled
low too quickly, the capacitance closest to the terminal goes low but the
capacitance further from the terminal
remains high, causing a small portion
of the IGBT device to handle the full
100A of current which quickly destroys
the device. The pull down circuitry
therefore needs to be slower than the
internal RC time constant in the gate
of the IGBT. To slow down the driver,
a 20Ω series resistor is integrated into
the LT3485.
Which Part to Use
The LT3484 and LT3485 families of
photoflash capacitor chargers suit
about any photoflash need. The basic
photoflash functionality in each part
is identical and both parts are capable
of operating from 2AA cells. The integrated IGBT drive and voltage output
monitor differentiate the LT3485 from
the LT3484, along with its higher current capabilities. The LT3484 is the
smallest solution available if quenching the bulb is not needed. When
using an IGBT to trigger the flash, the
LT3485 offers valuable board space
savings over the LT3484 by eliminating
several external components. Table 1
shows the major functional differences
between these seven parts.
Once the decision is made on the
integrated IGBT driver, choosing a
current option is a matter of balancing the inherent trade-off between
input current and charge time. For
a given photoflash capacitor size, the
device which results in the highest
input current offers the fastest charge
time. The limit on how much current
the photoflash charger can draw is
usually set by the battery technology used, and how much load they
LT6555/56, continued from page 22
Demonstration
Circuits Available
The LT6555 and LT6556 have Demo
Boards available that make evaluation
of these parts a simple plug-and-play
operation. To evaluate the LT6555 ask
for DC858A (SSOP-24 package) or
DC892A-A (QFN package). To evalu12
ate the LT6556 ask for DC892A-B (in
QFN package). All three of these demo
circuits have high-quality 75Ω BNC
connections for best performance
and illustrate high-frequency layout
practices that are important to obtaining the best performance from these
super-fast amplifiers.
can handle. The LT3485-3 offers the
fastest charge times of the chargers
discussed here.
The following equation predicts the
charge times (T) in seconds for the
seven parts:
T=
(
COUT • VOUT(FINAL)2 – VOUT(INIT)2
τ • VIN
)
where COUT is the value of the photoflash capacitor in Farads, VOUT(FINAL)
is the target output voltage, VOUT(INIT)
is the initial output voltage, VIN is the
battery voltage to which the flyback
transformer is connected, and τ is
the charge time coefficient listed in
Table 1.
The charge time coefficients for
each part are different depending on
the transformer due to differences in
efficiency and average input current.
The charge time coefficients are given
for Kijima Musen and TDK transformers, with part numbers and typical
specifications for these transformers
listed in Table 2.
Conclusion
The LT3484 and LT3485 provide
simple, efficient capacitor charging
solutions for digital still cameras
and integrated digital cameras in cell
phones. The high level of integration
reduces the amount of external components while also producing tightly
controlled output voltage and average
input current distributions. The three
current limits in the LT3484 family
and the four current limits in the
LT3485 family allow for flexibility in the
trade-off between input current and
charge time. The LT3485 saves even
more space for some applications by
integrating an IGBT driver and voltage
output monitor.
For further information on any
of the devices mentioned in
this issue of Linear Technology,
visit www.linear.com, use the
reader service card or call the LTC
literature service number:
1-800-4-LINEAR
Linear Technology Magazine • December 2005
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