LT3420 datasheet(13/20 Pages) LINER

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LT3420/LT3420-1

3420fa

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 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 • V2)

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

(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

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 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 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

Figure 8. Halting the Charge Cycle at Any Time

VOUT

50V/DIV

CHARGE

0.5s/DIV

3420 F08

5V/

DIV

VCHARGE

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