Simple Circuitry for Cellular Telephone/Camera Flash Illumination

Application Note 95
March 2004
Simple Circuitry for Cellular Telephone/Camera
Flash Illumination
A Practical Guide for Successfully Implementing Flashlamps
Jim Williams and Albert Wu
INTRODUCTION
Next generation cellular telephones will include high quality photographic capability. Improved image sensors and
optics are readily utilized, but high quality “Flash” illumination requires special attention. Flash lighting is crucial
for obtaining good photographic performance and must
be quite carefully considered.
Their line source light output is hundreds of times greater
than point source LEDs, resulting in dense, easily diffused
light over a wide area. Additionally, the flashlamp color
temperature of 5500°K to 6000°K, quite close to natural
light, eliminates the color correction necessitated by a
white LED’s blue peaked output.
FLASH ILLUMINATION ALTERNATIVES
FLASHLAMP BASICS
Two practical choices exist for flash illumination—LEDs
(Light Emitting Diode) and flashlamps. Figure 1 compares
various performance categories for LED and flashlamp
approaches. LEDs feature continuous operating capability
and low density support circuitry among other advantages. Flashlamps, however, have some particularly
important characteristics for high quality photography.
Figure 2 shows a conceptual flashlamp. The cylindrical
glass envelope is filled with Xenon gas. Anode and cathode
electrodes directly contact the gas; the trigger electrode,
distributed along the lamp’s outer surface, does not. Gas
breakdown potential is in the multikilovolt range; once
breakdown occurs, lamp impedance drops to ≤1Ω. High
, LTC and LT are registered trademarks of Linear Technology Corporation.
PERFORMANCE CATEGORY
FLASHLAMP
LED
Light Output
High—Typically 10 to 400× Higher Than LEDs. Line Source
Output Makes Even Light Distribution Relatively Simple
Low. Point Source Output Makes Even Light Distribution
Somewhat Difficult
Illumination vs Time
Pulsed—Good for Sharp, Still Picture
Continuous—Good for Video
Color Temperature
5500°K to 6000°K—Very Close to Natural Light. No Color
Correction Necessary
8500°K—Blue Light Requires Color Correction
Solution Size
Typically 3.5mm × 8mm × 4mm for Optical Assembly.
27mm × 6mm × 5mm for Circuitry—Dominated by Flash
Capacitor (≈6.6mm Diameter; May be Remotely Mounted)
Typically 7mm × 7mm × 2.4mm for Optical Assembly,
7mm × 7mm × 5mm for Circuitry
Support Circuit Complexity
Moderate
Low
Charge Time
1 to 5 Seconds, Dependant Upon Flash Energy
None—Light Always Available
Operating Voltage
and Currents
Kilovolts to Trigger, 300V to Flash. ISUPPLY to Charge ≈ 100mA
to 300mA, Dependant Upon Flash Energy. Essentially Zero
Standby Current
Typically 3.4V to 4.2V at 30mA per LED Continuous—
100mA Peak. Essentially Zero Standby Current
Battery Power Consumption
200 to 800 Flashes per Battery Recharge, Dependent
Upon Flash Energy
≈120mW per LED (Continuous Light)
≈400mW per LED (Pulsed Light)
Partial Source: Perkin Elmer Optoelectronics
Figure 1. Performance Characteristics for LED and Flashlamp-Based Illumination. LEDs Feature Small Size,
No Charge Time and Continuous Operating Capability; Flashlamps are Much Brighter with Better Color Temperature
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Application Note 95
TRIGGER CONNECTION
AND CONDUCTIVE
MATERIAL ALONG LAMP
CATHODE
(NEGATIVE)
XENON GAS
INSIDE LAMP
ANODE
(POSITIVE)
AN95 F02
CLEAR GLASS
ENVELOPE
Figure 2. Flashlamp Consists of Xenon Gas-Filled Glass Cylinder
with Anode, Cathode and Trigger Electrodes. High Voltage
Trigger Ionizes Gas, Lowering Breakdown Potential to Permit
Light Producing Current Flow Between Anode and Cathode.
Distributed Trigger Connection Along Lamp Length Ensures
Complete Lamp Breakdown, Resulting in Optimal Illumination
current flow in the broken down gas produces intense
visible light. Practically, the large current necessary requires that the lamp be put into its low impedance state
before emitting light. The trigger electrode serves this
function. It transmits a high voltage pulse through the
glass envelope, ionizing the Xenon gas along the lamp
length. This ionization breaks down the gas, placing it into
a low impedance state. The low impedance permits large
current to flow between anode and cathode, producing
intense light. The energy involved is so high that current
flow and light output are limited to pulsed operation.
Continuous operation would quickly produce extreme
temperatures, damaging the lamp. When the current pulse
decays, lamp voltage drops to a low point and the lamp
reverts to its high impedance state, necessitating another
trigger event to initiate conduction.
SUPPORT CIRCUITRY
Figure 3 diagrams conceptual support circuitry for
flashlamp operation. The flashlamp is serviced by a trigger
circuit and a storage capacitor that generates the high
FLASH CAPACITOR
CHARGE CIRCUIT
CAPACITOR CHARGE/
SHUTDOWN
CAPACITOR STATUS
TRIGGER/FLASH
COMMAND
transient current. In operation, the flash capacitor is
typically charged to 300V. Initially, the capacitor cannot
discharge because the lamp is in its high impedance state.
A command applied to the trigger circuit results in a
multikilovolt trigger pulse at the lamp. The lamp breaks
down, allowing the capacitor to discharge1. Capacitor,
wiring and lamp impedances typically total a few ohms,
resulting in transient current flow in the 100A range. This
heavy current pulse produces the intense flash of light.
The ultimate limitation on flash repetition rate is the lamp’s
ability to safely dissipate heat. A secondary limitation is the
time required for the charging circuit to fully charge the
flash capacitor. The large capacitor charging towards a
high voltage combines with the charge circuit’s finite
output impedance, limiting how quickly charging can
occur. Charge times of 1 to 5 seconds are realizable,
depending upon available input power, capacitor value
and charge circuit characteristics.
The scheme shown discharges the capacitor in response
to a trigger command. It is sometimes desirable to effect
partial discharge, resulting in less intense light flashes.
Such operation permits “red-eye” reduction, where the
main flash is immediately preceded by one or more reduced intensity flashes2. Figure 4’s modifications provide
this operation. A driver and a high current switch have
been added to Figure 3. These components permit
Note 1. Strictly speaking, the capacitor does not fully discharge
because the lamp reverts to its high impedance state when the
potential across it decays to some low value, typically 50V.
Note 2. “Red-eye” in a photograph is caused by the human retina
reflecting the light flash with a distinct red color. It is eliminated by
causing the eye’s iris to constrict in response to a low intensity flash
immediately preceding the main flash.
VOUT TYPICALLY 300V
FLASH STORAGE
CAPACITOR
TRIGGER
CIRCUIT
MULTIKILOVOLT
TRIGGER
PULSE
FLASHLAMP
DISTRIBUTED
TRIGGER
ELECTRODE
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Figure 3. Conceptual Flashlamp Circuitry Includes Charge Circuit, Storage Capacitor, Trigger and Lamp.
Trigger Command Ionizes Lamp Gas, Allowing Capacitor Discharge Through Flashlamp. Capacitor Must be
Recharged Before Next Trigger Induced Lamp Flash Can Occur
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Application Note 95
FLASH CAPACITOR
CHARGE CIRCUIT
VOUT TYPICALLY 300V
FLASH STORAGE
CAPACITOR
CAPACITOR CHARGE/
SHUTDOWN
CAPACITOR STATUS
TRIGGER/FLASH
COMMAND
TRIGGER
CIRCUIT
MULTIKILOVOLT
TRIGGER
PULSE
FLASHLAMP
DISTRIBUTED
TRIGGER
ELECTRODE
HIGH CURRENT
SWITCH
DRIVER
AN95 F04
Figure 4. Driver/Power Switch Added to Figure 3 Permits Partial Capacitor Discharge, Resulting in Controllable Light Emission.
Capability Allows Pulsed Low Level Light Before Main Flash, Minimizing “Red-Eye” Phenomena
stopping flash capacitor discharge by opening the lamp’s
conductive path. This arrangement allows the “trigger/
flash command” control line pulse width to set current
flow duration, and hence, flash energy. The low energy,
partial capacitor discharge allows rapid recharge, permitting several low intensity flashes in rapid succession immediately preceeding the main flash without lamp damage.
The flash capacitor charger (Figure 5) is basically a
transformer coupled step-up converter with some special
capabilities3. When the “charge” control line goes high,
the regulator clocks the power switch, allowing step-up
transformer T1 to produce high voltage pulses. These
pulses are rectified and filtered, producing the 300V DC
output. Conversion efficiency is about 80%. The circuit
regulates by stopping drive to the power switch when the
desired output is reached. It also pulls the “DONE” line
low, indicating that the capacitor is fully charged. Any
capacitor leakage-induced loss is compensated by intermittent power switch cycling. Normally, feedback would
be provided by resistively dividing down the output voltage. This approach is not acceptable because it would
require excessive switch cycling to offset the feedback
resistor’s constant power drain. While this action would
maintain regulation, it would also drain excessive power
from the primary source, presumably a battery. Regula-
+
VIN
DONE
REGULATED
VOUT —
TYPICALLY
300V
SWITCH
REGULATOR
CHARGE
FLASH CAPACITOR CHARGER CIRCUIT
CONSIDERATIONS
T1
VIN
LT3468
GND
AN95 F05
Figure 5. Flash Capacitor Charger Circuit Includes IC Regulator,
Step-Up Transformer, Rectifier and Capacitor. Regulator
Controls Capacitor Voltage by Monitoring T1’s Flyback Pulse,
Eliminating Conventional Feedback Resistor Divider’s Loss Path.
Control Pins Include Charge Command and Charging Complete
(“DONE”) Indication
tion is instead obtained by monitoring T1’s flyback pulse
characteristic, which reflects T1’s secondary amplitude.
The output voltage is set by T1’s turns ratio4. This feature
permits tight capacitor voltage regulation, necessary to
ensure consistent flash intensity without exceeding lamp
energy or capacitor voltage ratings. Also, flashlamp energy is conveniently determined by the capacitor value
without any other circuit alterations.
Note 3. Details on this device’s operation appear in Appendix A,
“A Monolithic Flash Capacitor Charger.”
Note 4. See Appendix A for recommended transformers.
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Application Note 95
DETAILED CIRCUIT DISCUSSION
primary. T2’s secondary delivers a high voltage trigger
pulse to the lamp, ionizing it to permit conduction. C1
discharges through the lamp, producing light.
BEFORE PROCEEDING ANY FURTHER, THE READER IS
WARNED THAT CAUTION MUST BE USED IN THE CONSTRUCTION, TESTING AND USE OF THIS CIRCUIT.
HIGH VOLTAGE, LETHAL POTENTIALS ARE PRESENT
IN THIS CIRCUIT. EXTREME CAUTION MUST BE USED
IN WORKING WITH, AND MAKING CONNECTIONS TO,
THIS CIRCUIT. REPEAT: THIS CIRCUIT CONTAINS DANGEROUS, HIGH VOLTAGE POTENTIALS. USE CAUTION.
Figure 7 details the capacitor charging sequence. Trace A,
the “charge” input, goes high. This initiates T1 switching,
causing C1 to ramp up (trace B). When C1 arrives at the
regulation point, switching ceases and the resistively
pulled-up “DONE” line drops low (trace C), indicating C1’s
charged state. The “TRIGGER” command (trace D), resulting in C1’s discharge via the lamp-Q3 path, may occur any
time (in this case ≈600ms) after “DONE” goes low. Note
that this figure’s trigger command is lengthened for photographic clarity; it normally is 500µs to 1000µs in duration for a complete C1 discharge. Low level flash events,
such as for “red-eye” reduction, are facilitated by short
duration trigger input commands.
Figure 6 is a complete flashlamp circuit based on the
previous text discussion. The capacitor charging circuit,
similar to Figure 5, appears at the upper left. D2 has been
added to safely clamp T1-originated reverse transient
voltage events. Q1 and Q2 drive high current switch Q3.
The high voltage trigger pulse is formed by step-up
transformer T2. Assuming C1 is fully charged, when
Q1-Q2 turns Q3 on, C2 deposits current into T2’s
DANGER! Lethal Potentials Present — See Text
FLASH STORAGE
CAPACITOR
D1
+VIN
3V TO 6V
T1
4.7µF
5
4
8
1
+
C1
13µF
330V
R1
1M
C2
0.047µF
600V
TRIGGER
VIN
SW
D2
1
T2
A
T
FLASHLAMP
3
+VIN
LT3468-1 GND
2
CHARGE
CHARGE
DONE
1k
CAPACITOR
CHARGER
DONE
C
Q1
30Ω
100Ω
TRIGGER
Q2
20k
Q3
HIGH CURRENT
SWITCH
DRIVER
AN95 F06
C1: RUBYCON 330FW13AK6325
D1: TOSHIBA DUAL DIODE 1SS306,
CONNECT DIODES IN SERIES
D2: PANASONIC MA2Z720
Q1, Q2: SILICONIX Si1501DL DUAL
Q3: TOSHIBA GT5G131 IGBT
T1: TDK LDT565630T-002
T2: KIJIMA MUSEN KP-98
FLASHLAMP: PERKIN ELMER BGDC0007PKI5700
Figure 6. Complete Flashlamp Circuit Includes Capacitor Charging Components (Figure Left), Flash Capacitor C1, Trigger
(R1, C2, T2), Q1-Q2 Driver, Q3 Power Switch and Flashlamp. TRIGGER Command Simultaneously Biases Q3 and Ionizes
Lamp via T2. Resultant C1 Discharge Through Lamp Produces Light
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Application Note 95
A = 5V/DIV
B = 200V/DIV
A = 2kV/DIV
C = 5V/DIV
D = 5V/DIV
(INVERTED)
B = 20A/DIV
400ms/DIV
AN95 F07
Figure 7. Capacitor Charging Waveforms Include Charge Input
(Trace A), C1 (Trace B), DONE Output (Trace C) and TRIGGER
Input (Trace D). C1’s Charge Time Depends Upon Its Value and
Charge Circuit Output Impedance. TRIGGER Input, Widened for
Figure Clarity, May Occur any Time After DONE Goes Low
Figure 8 shows high speed detail of the high voltage trigger
pulse (trace A) and resultant flashlamp current (trace B).
Some amount of time is required for the lamp to ionize and
begin conduction after triggering. Here, 10µs after the
8kVP-P trigger pulse, flashlamp current begins its ascent
to nearly 100A. The current rises smoothly in 5µs to a well
defined peak before beginning its descent. The resultant
light produced (Figure 9) rises more slowly, peaking in
about 25µs before decaying. Slowing the oscilloscope
sweep permits capturing the entire current and light
events. Figure 10 shows that light output (trace B) follows
lamp current (trace A) profile, although current peaking is
more abrupt. Total event duration is ≈500µs with most
energy expended in the first 200µs. The leading edge’s
discontinuous presentation is due to oscilloscope chopped
display mode operation.
5µs/DIV
AN95 F08
Figure 8. High Speed Detail of Trigger Pulse (Trace A) and
Resultant Flashlamp Current (Trace B). Current Approaches
100A After Trigger Pulse Ionizes Lamp
A = RELATIVE
LIGHT/DIV
5µs/DIV
AN95 F09
Figure 9. Smoothly Ascending Flashlamp
Light Output Peaks in 25µs
LAMP, LAYOUT, RFI AND RELATED ISSUES
Lamp Considerations
Several lamp related issues require attention. Lamp
triggering requirements must be thoroughly understood
and adhered to. If this is not done, incomplete or no lamp
flash may occur. Most trigger related problems involve
trigger transformer selection, drive and physical location
with respect to the lamp. Some lamp manufacturers
supply the trigger transformer, lamp and light diffuser as
a single, integrated assembly5. This obviously implies
trigger transformer approval by the lamp vendor, assuming it is driven properly. In cases where the lamp is
Note 5. See Reference 1.
A = 20A/DIV
B = RELATIVE
LIGHT/DIV
100µs/DIV
AN95 F10
Figure 10. Photograph Captures Entire Current (Trace A) and
Light (Trace B) Events. Light Output Follows Current Profile
Although Peaking is Less Defined. Leading Edges Dashed
Presentation Derives from Oscilloscope’s Chopped Display
Operation
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Application Note 95
triggered with a user-selected transformer and drive
scheme, it is essential to obtain lamp vendor approval
before going to production.
The lamp’s anode and cathode access the lamp’s main
discharge path. Electrode polarity must be respected or
severe lifetime degradation will occur. Similarly, lamp
energy dissipation restrictions must be respected or
lifetime will suffer. Severe lamp energy overdrive can
result in lamp cracking or disintegration. Energy is easily
and reliably controlled by selecting capacitor value and
charge voltage and restricting flash repetition rate. As
with triggering, lamp flash conditions promoted by the
user’s circuit require lamp manufacturer approval before
production.
Assuming proper triggering and flash energy, lamp lifetimes of ≈5000 flashes may be expected. Lifetime for
various lamp types differs from this figure, although all
are vendor specified. Lifetime is typically defined as the
point where lamp luminosity drops to 80% of its original
value.
Layout
The high voltages and currents mandate layout planning.
Referring back to Figure 6, C1’s discharge path is through
the lamp, Q3 and back to ground. The ≈100A peak current
means this discharge path must be maintained at low
impedance. Conduction paths between C1, the lamp and
Q3 should be short and well below 1Ω. Additionally, Q3’s
emitter and C1’s negative terminal should be directly
connected, the goal being a tight, highly conductive loop
between C1’s positive terminal, the lamp and Q3’s return
back to C1. Abrupt trace discontinuities and vias should be
avoided as the high current flow can cause conductor
erosion in local high resistivity regions. If vias must be
employed they should be filled, verified for low resistance
or used in multiples. Unavoidable capacitor ESR, lamp and
Q3 resistances typically total 1Ω to 2.5Ω, so total trace
resistance of 0.5Ω or less is adequate. Similarly, the high
current’s relatively slow risetime (see Figure 8) means
trace inductance does not have to be tightly controlled.
C1 is the largest component in the circuit; space considerations may make remotely mounting it desirable. This
can be facilitated with long traces or wires so long as
interconnect resistance is maintained within the limits
stated above.
Capacitor charger IC layout is similar to conventional
switching regulator practice. The electrical path formed
by the IC’s VIN pin, its bypass capacitor, the transformer
primary and the switch pin must be short and highly
conductive. The IC’s ground pin should directly return to
a low impedance, planar ground connection. The
transformer’s 300V output requires larger than minimum
spacing for all high voltage nodes to meet circuit board
breakdown requirements. Verify board material breakdown specifications and ensure that board washing procedures do not introduce conductive contaminants. T2’s
multikilovolt trigger winding must connect directly to the
lamp’s trigger electrode, preferably with less than 1/4" of
conductor. Adequate high voltage spacing must be employed. In general, what little conductor there is should
not contact the circuit board. Excessive T2 output length
can cause trigger pulse degradation or radio frequency
interference (RFI). Modular flashlamp-trigger transformer
assemblies are excellent choices in this regard.
A demonstration layout for Figure 6 appears in Figure 11.
The topside component layer is shown. Power and ground
are distributed on internal layers. LT3468 layout is typical
of switching regulator practice previously described, although wide trace spacing accomodates T1’s 300V output. The ≈100A pulsed current flows in a tight, low
resistance loop from C1’s postive terminal, through the
lamp, into Q3 and back to C1. In this case lamp connections are made with wires, although modular flashlamptrigger transformers allow trace-based connections6.
Note 6. See Reference 1.
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Application Note 95
+
C1
1
D1
1
Q3
CATHODE
ANODE
1M
D2
1
•
1
1k VIN
T2
TRIGGER
TRANSFORMER
100Ω
T1
FLYBACK
TRANSFORMER
LT3468-1
Q1
0.047µF
600V
Q2
20k
SECONDARY
1
VIN
1
30Ω
•
4.7µF
PRIMARY
AN95 F11
= RETURNED TO INTERNAL GROUND PLANE
VIN = RETURNED TO INTERNAL VIN PLANE
Figure 11. Magnified Demonstration Layout for Figure 6. High Current Flows in Tight Loop from C1 Positive Terminal, Through Lamp,
Into Q3 and Back to C1. Lamp Connections are Wires, Not Traces. Wide T1 Secondary Spacing Accommodates 300V Output
Radio Frequency Interference
The flash circuit’s pulsed high voltages and currents make
RFI a concern. The capacitor’s high energy discharge is
actually far less offensive than might be supposed. Figure
12 shows the discharge’s 90A current peak confined to a
70kHz bandwidth by its 5µs risetime. This means there is
little harmonic energy at radio frequencies, easing this
concern. Conversely, Figure 13’s T2 high voltage output
has a 250ns risetime (BW ≈ 1.5MHz), qualifying it as a
potential RFI source. Fortunately, the energy involved and
A = 20A/DIV
the exposed path length (see layout comments) are small,
making interference management possible.
The simplest interference management involves placing
radiating components away from sensitive circuit nodes
or employing shielding. Another option takes advantage of
the predictable time when the flash circuit operates. Sensitive circuitry within the telephone can be blanked during
flash events, which typically last well under 1ms.
Note: This application note was derived from a manuscript originally
prepared for publication in EDN magazine.
1000V/DIV
5µs/DIV
AN95 F12
Figure 12. 90A Current Peak is Confined Within 70kHz
Bandwidth by 5µs Risetime, Minimizing Noise Concerns
2µs/DIV
AN95 F13
Figure 13. Trigger Pulses High Amplitude and Fast Risetime
Promote RFI, but Energy and Path Exposure are Small,
Simplifying Radiation Management
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Application Note 95
REFERENCES
1. Perkin Elmer, “Flashtubes.”
2. Perkin Elmer, “Everything You Always Wanted to
Know About Flashtubes.”
3. Linear Technology Corporation, “LT®3468/LT3468-1/
LT3468-2 Data Sheet.”
4. Wu, Albert, “Photoflash Capacitor Chargers Fit Into
Tight Spots,” Linear Technology, Vol. XIII, No. 4,
December, 2003.
5. Rubycon Corporation. Catalog 2004, “Type FW
Photoflash Capacitor,” Page 187.
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Application Note 95
APPENDIX A
A MONOLITHIC FLASH CAPACITOR CHARGER
The LT3468/LT3468-1/LT3468-2 charge photoflash capacitors quickly and efficiently. Operation is understood
by referring to Figure A1. When the CHARGE pin is driven
high, a one shot sets both SR latches in the correct state.
Power NPN, Q1, turns on and current begins ramping up
in T1’s primary. Comparator A1 monitors switch current
and when peak current reaches 1.4A (LT3468), 1A(LT34682) or 0.7A (LT3468-1), Q1 is turned off. Since T1 is utilized
as a flyback transformer, the flyback pulse on the SW pin
causes A3’s output to be high. SW pin voltage must be at
least 36mV above VIN for this to happen.
During this phase, current is delivered to the photoflash
capacitor via T1’s secondary and D1. As the secondary
current decreases to zero, the SW pin voltage begins to
collapse. When the SW pin voltage drops to 36mV above
VIN or lower, A3’s output goes low. This fires a one shot
which turns Q1 back on. This cycle continues, delivering
power to the output.
Output voltage detection is accomplished via R2, R1, Q2
and comparator A2. Resistors R1 and R2 are sized so that
when the SW voltage is 31.5V above VIN, A2’s output goes
high, resetting the master latch. This disables Q1, halting
D1
T1
TO BATTERY
VOUT
PRIMARY
C1
SECONDARY
D2
3
DONE
5
VIN
+
SW
1
R2
60k
COUT
PHOTOFLASH
CAPACITOR
Q3
+
ONESHOT
A3
–
+
–
36mV
Q2
Q1
ENABLE
MASTER
LATCH
Q
S
R1
2.5k
Q
R
DRIVER
R
S
Q1
Q
+
A2
–
+
1.25V
REFERENCE
A1
CHARGE
4
VOUT COMPARATOR
ONESHOT
20mV
–
RSENSE
+–
2
GND
CHIP ENABLE
LT3468: RSENSE = 0.015Ω
LT3468-2: RSENSE = 0.022Ω
LT3468-1: RSENSE = 0.03Ω
AN95 FA1
Figure A1. LT3468 Block Diagram. Charge Pin Controls Power Switching to T1. Photoflash Capacitor Voltage
is Regulated by Monitoring T1’s Flyback Pulse, Eliminating Conventional Feedback Resistors Loss Path
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Application Note 95
power delivery. Q3 is turned on, pulling the DONE pin low,
indicating the part has finished charging. Power delivery
can only be restarted by toggling the CHARGE pin.
The CHARGE pin gives the user full control of the part.
Charging can be halted at any time by bringing the
CHARGE pin low. Only when the final output voltage is
reached will the DONE pin go low. Figure A2 shows these
various modes in action. When CHARGE is first brought
high, charging commences. When CHARGE is brought
LT3468-2
VIN = 3.6V
VOUT COUT = 50µF
100V/DIV
VDONE
5V/DIV
VCHARGE
5V/DIV
Figure A2. Halting the Charging Cycle with the CHARGE Pin
LT3468 Charge Times
LT3468-1 Charge Times
10
TA = 25°C
LT3468-2 Charge Times
10
TA = 25°C
9
9
8
8
8
7
7
7
6
5
COUT = 100µF
4
3
2
CHARGE TIME (s)
9
CHARGE TIME (s)
CHARGE TIME (s)
10
The only difference between the three LT3468 versions is
the peak current level. The LT3468 offers the fastest
charge time. The LT3468-1 has the lowest peak current
capability, and is designed for applications requiring limited battery drain. Due to the lower peak current, the
LT3468-1 can use a physically smaller transformer. The
LT3468-2 has a current limit between the LT3468 and the
LT3468-1. Comparative plots of the three versions charge
time, efficiency and output voltage tolerance appear in
Figures A3, A4 and A5.
Standard off-the-shelf transformers, available for all
LT3468 versions, are available and detailed in Figure A6.
For transformer design considerations, as well as other
supplemental information, see the LT3468 data sheet.
AN95 FA2
1s/DIV
low during charging, the part shuts down and VOUT no
longer rises. When CHARGE is brought high again, charging resumes. When the target VOUT voltage is reached, the
DONE pin goes low and charging stops. Finally, the CHARGE
pin is brought low again, the part enters shutdown and the
DONE pin goes high.
6
5
COUT = 50µF
4
3
2
COUT = 50µF
1
3
4
5
6
VIN (V)
7
8
9
5
COUT = 100µF
4
3
COUT = 50µF
1
0
2
6
2
COUT = 20µF
1
0
TA = 25°C
0
2
3
4
5
6
VIN (V)
7
8
9
2
3
4
6
5
VIN (V)
7
8
9
AN95 FA3
Figure A3. Typical LT3438 Charge Times. Charge Time Varies with IC Version, Capacitor Size and Input Voltage
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Application Note 95
LT3468 Efficiency
90
LT3468-1 Efficiency
90
TA = 25°C
TA = 25°C
80
80
EFFICIENCY (%)
VIN = 2.8V
VIN = 3.6V
60
VIN = 2.8V
70
EFFICIENCY (%)
80
70
VIN = 3.6V
60
50
50
100
150
200
VOUT (V)
250
VIN = 2.8V
70
VIN = 3.6V
60
40
50
300
VIN = 4.2V
50
40
40
50
TA = 25°C
VIN = 4.2V
VIN = 4.2V
EFFICIENCY (%)
LT3468-2 Efficiency
90
100
150
200
VOUT (V)
250
50
300
100
200
150
VOUT (V)
250
300
AN95 FA4
Figure A4. Efficiency for the Three LT3468 Versions Varies with Input and Output Voltages
LT3468 Output Voltage
LT3468-1 Output Voltage
324
LT3468-2 Output Voltage
319
324
TA = –40°C
323
318
323
TA = –40°C
317
322
TA = 25°C
TA = 85°C
321
TA = 85°C
320
320
319
319
318
TA = 25°C
TA = 25°C
VOUT (V)
321
VOUT (V)
VOUT (V)
322
3
4
5
VIN (V)
6
7
8
315
TA = –40°C
314
313
318
2
TA = 85°C
316
312
2
3
4
5
VIN (V)
6
7
8
2
4
3
5
VIN (V)
6
7
8
AN95 FA5
Figure A5. Typical Output Voltage Tolerance for the Three LT3468 Versions.
Tight Voltage Tolerance Prevents Overcharging Capacitor, Controls Flash Energy
FOR USE WITH
LT3468/LT3468-2
LT3468-1
LT3468
LT3468-1
LT3468-2
LT3468/LT3468-1
LT3468-1
TRANSFORMER NAME
SIZE
(W × L × H) mm
LPRI
(µH)
LPRI-LEAKAGE
(nH)
N
RPRI
(mΩ)
RSEC
(Ω)
SBL-5.6-1
SBL-5.6S-1
5.6 × 8.5 × 4.0
5.6 × 8.5 × 3.0
10
24
200 Max
400 Max
10.2
10.2
103
305
26
55
LDT565630T-001
LDT565630T-002
LDT565630T-003
5.8 × 5.8 × 3.0
5.8 × 5.8 × 3.0
5.8 × 5.8 × 3.0
6
14.5
10.5
200 Max
500 Max
550 Max
10.4
10.2
10.2
100 Max 10 Max
240 Max 16.5 Max
210 Max 14 Max
TDK
Chicago Sales Office
(847) 803-6100 (ph)
www.components.tdk.com
T-15-089
T-15-083
6.4 × 7.7 × 4.0
8.0 × 8.9 × 2.0
12
20
400 Max
500 Max
10.2
10.2
211 Max 27 Max
675 Max 35 Max
Tokyo Coil Engineering
Japan Office
0426-56-6336 (ph)
www.tokyo-coil.co.jp
VENDOR
Kijima Musen
Hong Kong Office
852-2489-8266 (ph)
[email protected] (email)
Figure A6. Standard Transformers Available for LT3468 Circuits. Note Small Size Despite High Output Voltage
an95f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
AN95-11
Application Note 95
an95f
AN95-12
Linear Technology Corporation
LT/TP 0304 1K • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2004