LTC3129 datasheet(20/30 Pages) LINER

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LTC3129

3129fc

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When powered through long leads or from a power

source with significant resistance, a larger value bulk input

capacitor may be required and is generally recommended.

In such applications, a 47µF to 100µF low-ESR electrolytic

capacitor in parallel with a 1µF ceramic capacitor generally

yields a high performance, low cost solution.

Note that applications using the MPPC feature should

use a minimum CIN of 22µF. Larger values can be used

without limitation.

Recommended Input and Output Capacitor Types

The capacitors used to filter the input and output of the

LTC3129 must have low ESR and must be rated to handle

the AC currents generated by the switching converter.

This is important to maintain proper functioning of the

IC and to reduce output voltage ripple. There are many

capacitor types that are well suited to these applications

including multilayer ceramic, low ESR tantalum, OS-CON

and POSCAP technologies. In addition, there are certain

types of electrolytic capacitors such as solid aluminum

organic polymer capacitors that are designed for low

ESR and high AC currents and these are also well suited

to some LTC3129 applications. The choice of capacitor

technology is primarily dictated by a trade-off between

size, leakage current and cost. In backup power applica-

tions, the input or output capacitor might be a super or

ultra capacitor with a capacitance value measuring in the

Farad range. The selection criteria in these applications

are generally similar except that voltage ripple is generally

not a concern. Some capacitors exhibit a high DC leak-

age current which may preclude their consideration for

applications that require a very low quiescent current in

Burst Mode operation. Note that ultra capacitors may have

a rather high ESR, therefore a 4.7µF (minimum) ceramic

capacitor is recommended in parallel, close to the IC pins.

Ceramic capacitors are often utilized in switching con-

verter applications due to their small size, low ESR and

low leakage currents. However, many ceramic capacitors

intended for power applications experience a significant

loss in capacitance from their rated value as the DC bias

voltage on the capacitor increases. It is not uncommon for

a small surface mount capacitor to lose more than 50%

of its rated capacitance when operated at even half of its

maximum rated voltage. This effect is generally reduced

as the case size is increased for the same nominal value

capacitor. As a result, it is often necessary to use a larger

value capacitance or a higher voltage rated capacitor than

wouldordinarilyberequiredtoactuallyrealizetheintended

capacitanceattheoperatingvoltageoftheapplication.X5R

and X7R dielectric types are recommended as they exhibit

the best performance over the wide operating range and

temperature of the LTC3129. To verify that the intended

capacitance is achieved in the application circuit, be sure

to consult the capacitor vendor's curve of capacitance

versus DC bias voltage.

Using the Programmable RUN Function to Operate

from Extremely Weak Input Sources

Another application of the programmable RUN pin is that

it can be used to operate the converter in a hiccup mode

from extremely low current sources. This allows operation

from sources that can only generate microamps of output

current,andwouldbefartooweaktosustainnormalsteady-

stateoperation,evenwiththeuseoftheMPPCpin.Because

the LTC3129 draws only 1.9µA typical from VIN until it is

enabled, the RUN pin can be programmed to keep the IC

disabled until VIN reaches the programmed voltage level.

Inthismanner,theinputsourcecantrickle-chargeaninput

storage capacitor, even if it can only supply microamps of

current, until VIN reaches the turn-on threshold set by the

RUN pin divider. The converter will then be enabled using

the stored charge in the input capacitor, until Vin drops

below the turn-off threshold, at which point the converter

will turn off and the process will repeat.

This approach allows the converter to run from weak

sources such as thin-film solar cells using indoor lighting.

Although the converter will be operating in bursts, it is

enough to charge an output capacitor to power low duty

cycle loads, such as wireless sensor applications, or to

trickle charge a battery. In addition, note that the input

voltage will be cycling (with a small ripple as set by the

RUN hysteresis) about a fixed voltage, as determined by

the divider. This allows the high impedance source to

operate at the programmed optimal voltage for maximum

power transfer.

applicaTions inForMaTion

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