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LT3420-1_15 Datasheet, PDF (13/20 Pages) Linear Technology – Photoflash Capacitor Chargers with Automatic Refresh | |||
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LT3420/LT3420-1
APPLICATIO S I FOR ATIO
CAPACITOR SELECTION
The VBAT and VCC decoupling capacitors should be multi-
layer ceramic type with X5R or X7R dielectric. This insures
adequate decoupling across wide ambient temperature
ranges. A good quality ceramic capacitor is also recom-
mended for the timing capacitor on the CT pin. Avoid Y5V
or Z5U dielectrics.
Selectively Disabling the LT3420/LT3420-1
The LT3420/LT3420-1 can be disabled at any time, even
during the charge phase. This may be useful when a digital
camera enters a sensitive data acquisition phase. Figure 8
illustrates this feature. Midway through the charge cycle,
the CHARGE pin is brought low, which disables the part.
After the sensitive data operation is complete, the CHARGE
pin is brought high and the charging operation continues.
Measuring Efï¬ciency
Measuring the efï¬ciency of a circuit designed to charge
large capacitive loads is a difï¬cult issue, particularly with
photoï¬ash capacitors. The ideal way to measure the
efï¬ciency of a capacitor charging circuit would be to ï¬nd
the energy delivered to the output capacitor (0.5 ⢠C ⢠V2)
and divide it by the total input energy. This method does
not work well here because photoï¬ash capacitors are far
from ideal. Among other things, they have relatively high
leakage currents, large amounts of dielectric absorption,
and signiï¬cant voltage coefï¬cients. A much more accu-
rate, and easier, method is to measure the efï¬ciency as a
function of the output voltage. In place of the photoï¬ash
capacitor, use a smaller, high quality capacitor, reducing
errors associated with the non-ideal photoï¬ash capacitor.
Using an adjustable load, the output voltage can be set
anywhere between ground and the maximum output
voltage. The efï¬ciency is measured as the output power
(VOUT ⢠IOUT) divided by the input power (VIN ⢠IIN). This
method also provides a good means to compare various
charging circuits since it removes the variability of the
photoï¬ash capacitor from the measurement. The total
efï¬ciency of the circuit, charging an ideal capacitor, would
be the time average of the given efï¬ciency curve, over time
as VOUT changes.
Adjustable Input Current
With many types of modern batteries, the maximum
allowable current that can be drawn from the battery is
limited. This is generally accomplished by active circuitry
or a polyfuse. Different parts of a digital camera may
require high currents during certain phases of operation
and very little at other times. A photoï¬ash charging circuit
should be able to adapt to these varying currents by
drawing more current when the rest of the camera is
drawing less, and vice-versa. This helps to reduce the
charge time of the photoï¬ash capacitor, while avoiding the
VOUT
50V/DIV
CHARGE
VCHARGE
NO
CHARGE
0.5s/DIV
5V/
DIV
3420 F08
Figure 8. Halting the Charge Cycle at Any Time
3420fb
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