LT3420/LT3420-1
13
3420fb
CAPACITOR SELECTION
The V
BAT
and V
CC
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 C
T
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 Efficiency
Measuring the efficiency of a circuit designed to charge
large capacitive loads is a difficult issue, particularly with
photoflash capacitors. The ideal way to measure the
efficiency of a capacitor charging circuit would be to find
the energy delivered to the output capacitor (0.5 • C • V
2
)
and divide it by the total input energy. This method does
not work well here because photoflash capacitors are far
from ideal. Among other things, they have relatively high
leakage currents, large amounts of dielectric absorption,
and significant voltage coefficients. A much more accu-
rate, and easier, method is to measure the efficiency as a
function of the output voltage. In place of the photoflash
capacitor, use a smaller, high quality capacitor, reducing
errors associated with the non-ideal photoflash capacitor.
Using an adjustable load, the output voltage can be set
anywhere between ground and the maximum output
voltage. The efficiency is measured as the output power
(V
OUT
• I
OUT
) divided by the input power (V
IN
• I
IN
). This
method also provides a good means to compare various
charging circuits since it removes the variability of the
photoflash capacitor from the measurement. The total
efficiency of the circuit, charging an ideal capacitor, would
be the time average of the given efficiency curve, over time
as V
OUT
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 photoflash 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 photoflash capacitor, while avoiding the
APPLICATIO S I FOR ATIO
WUUU
Figure 8. Halting the Charge Cycle at Any Time
V
OUT
50V/DIV
CHARGE
NO
CHARGE
0.5s/DIV
3420 F08
5V/
DIV
V
CHARGE
