Photoflash Capacitor Charger Has Fast Efficient Charging
and Low Battery Drain – Design Note 303
Albert Wu
Figure 1a shows a typical LT3420 circuit that can charge
a 220μF photoflash capacitor to 320V in 3.5 seconds
from a 5V input. Figure 1b shows the charge time as a
function of battery voltage. In Figure 1a, the circuitry to
the right of C4 shows a typical method to generate the
light pulse once the photoflash capacitor is charged.
When the SCR is fired, the flying lead along the glass
envelope of the Xenon bulb reaches many kilovolts in
potential. This ionizes the gas inside the bulb forming a
low impedance path across the bulb. The energy stored
in the photoflash capacitor quickly flows through the
Xenon bulb, producing the burst of light needed for
flash photography.
Introduction
The LT®3420 is designed to charge large-valued capacitors—such as those used for the strobe flashes
of digital and film cameras—to high voltages. These
photoflash, or strobe, capacitors range in value from
a hundred microfarads to over a millifarad, with target
output voltages above 300V. Traditional strobe capacitor charging methods are either inefficient or require
software overhead. The LT3420 provides a compact,
simple to use and efficient charger solution that requires no software, saving space, battery life, design
time and cost.
LT3420 charger circuits typically achieve efficiencies
greater than 75%. The LT3420 includes important
features such as automatic capacitor charge refresh
and control/indicator pins which make it highly flexible
and easy to use. Its versatility allows it to be used in
applications that require a simple standalone photoflash
charger as well as applications where it is completely
controlled by a microprocessor (described below). No
voltage divider is needed on the high voltage output.
3,4
VBAT
FLYING
LEAD
1
R1
51.1k
3
6
RFB
VCC
SW
SEC
C4
220μF
7
330V
PHOTOFLASH
CAPACITOR
CT
10
C3
0.1μF
10
+
C2
4.7μF
LT3420
9
CHARGE
1
8
DONE
RREF
CHARGE
DONE
C1, C2:
C4:
T1:
D1:
D1
FLASH
GND
5
R2
2k
4.7F, X5R or X7R, 10V
RUBYCON 220μF PHOTOFLASH CAPACITOR
TDK SRW10EPC-U01H003 FLYBACK TRANSFORMER
GENERAL SEMICONDUCTOR GSD2004S SOT-23
DUAL DIODE. DIODES CONNECTED IN SERIES
6
COUT = 220μF
4
COUT = 100μF
BOLD LINES INDICATE
HIGH CURRENT PATHS
DN303 F01a
Figure 1a. 320V Photoflash Capacitor Charging Circuit
02/03/303_conv
VOUT CHARGED
FROM
50V TO 320V
8
TIME (s)
2
4
VCC
2.5V–10V
320V
8
5,6
C1
4.7μF
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
—DANGER HIGH VOLTAGE—
OPERATION BY HIGH VOLTAGE
TRAINED PERSONNEL ONLY
T1
1:12
VBAT
1.8V–10V
Features
The LT3420 includes an integrated 1.4A power switch
and utilizes a patent-pending control technique. Precise
control of the switching current is achieved by sensing
both the primary and secondary currents of transformers, a method which prolongs battery life. Figure 2 shows
2
0
2
4
6
VBAT (V)
8
DN303 F01b
Figure 1b. Charge Time
10
the relevant waveforms when the output has reached
300V in the circuit of Figure 1a. The peak primary current
is limited to 1.4A (typical), while the primary current
when the power switch turns on is 480mA (typical).
By operating the part in Continuous Conduction Mode
(CCM), charge time is minimized. The output voltage
is detected via the flyback waveform on the primary of
the transformer—VSW in Figure 2. The target output
voltage is controlled by two resistors, R1 and R2.
This flyback detection scheme removes the need for
a resistor divider string from the high voltage output
to ground, thus eliminating the associated power loss
found in many competing flash modules.
Once the target output voltage is reached, the device
enters a refresh mode where the quiescent current of
the device is reduced to 90μA (typical). The LT3420
has a user programmable refresh timer built in. The
value of C3 determines the time period after which
the part comes out of the refresh mode and recharges
the output to the target voltage. This process repeats
to maintain the output at the desired voltage. Figure 3
shows the different modes of the LT3420 from shutdown, to charging and finally refresh.
Interfacing to a Microcontroller
The LT3420 can be easily interfaced to the microcontroller found in digital cameras. The CHARGE and
DONE pins are the control and mode indicator pins,
respectively, for the part. By utilizing these pins, the
LT3420 can be selectively disabled and enabled at any
time. Figure 4 shows the LT3420 circuit being selectively
disabled when the CHARGE pin is driven low midway
through the charge cycle. This might be necessary during a sensitive operation in a digital camera. Once the
CHARGE pin is returned to the high state, the charging
continues from where it left off.
Conclusion
The LT3420 provides a highly efficient and integrated
standalone solution for charging photoflash capacitors.
Many important features are incorporated into the
device, including automatic refresh, tightly controlled
currents and an integrated power switch, thus reducing
external parts count. The LT3420 comes in a small,
low profile, MSOP-10 package, making a complete
solution that takes significantly less PC board space
than traditional methods.
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ISW
1A/DIV
ISEC
200mA/DIV
VSW
20V/DIV
DN303 F02
2μs/DIV
Figure 2. Switching Waveforms with
VOUT = 300V, VCC = VBAT = 3.3V
VOUT
100V/DIV
VCT
1V/DIV
IIN
1A/DIV
MODE SHUTDOWN
CHARGING
1s/DIV
REFRESH
DN303 F03
Figure 3. The Three Operating Modes
of the LT3420: Shutdown, Charging and
Refresh of the Photoflash Capacitor
VOUT
50V/DIV
VCHARGE
CHARGE
NO
CHARGE
5V/
DIV
0.5s/DIV
DN303 F04
Figure 4. Halting the Charge Cycle
at Any Time
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