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LTC3109

3109fa

APPLICATIONS INFORMATION

Note that there must be enough energy available from the

input voltage source for VOUT to recharge the capacitor

during the interval between load pulses (as discussed in

Design Example 1). Reducing the duty cycle of the load

pulse will allow operation with less input energy.

The VSTORE capacitor may be of very large value (thou-

sands of microfarads or even Farads), to provide energy

storage at times when the input voltage is lost. Note that

this capacitor can charge all the way to the VAUX clamp

voltage of 5.25V typical (regardless of the settings for

VOUT), so be sure that the holdup capacitor has a work-

ing voltage rating of at least 5.5V at the temperature that

it will be used.

The VSTORE input is not designed to provide high pulse

load currents to VOUT. The current path from VSTORE to

VOUT is limited to about 26mA max.

The VSTORE capacitor can be sized using the following

formula:

CSTORE ≥

7μA

+IQ +ILDO + IPULSE •tPULSE •f

()

()•tSTORE

5.25 – VOUT

where 7μA is the quiescent current of the LTC3109, IQ is

the load on VOUT in between pulses, ILDO is the load on

the LDO between pulses, IPULSE is the total load during the

pulse, tPULSE is the duration of the pulse, f is the frequency

of the pulses, tSTORE is the total storage time required

and VOUT is the output voltage required. Note that for a

programmed output voltage of 5V, the VSTORE capacitor

cannot provide any beneﬁcial storage time to VOUT.

To minimize losses and capacitor charge time, all capaci-

tors used for VOUT and VSTORE should be low leakage.

See Table 6 for recommended storage capacitors.

Table 6. Recommended Storage Capacitors

VENDOR

PART NUMBER/SERIES

AVX

www.avx.com

BestCap Series

TAJ and TPS Series Tantalum

Cap-XX

www.cap-xx.com

GZ Series

Cooper/Bussman

www.bussmann.com/3/PowerStor.html

KR Series

P Series

Vishay/Sprague

www.vishay.com/capacitors

Tantamount 592D

595D Tantalum

Note that storage capacitors requiring voltage balancing

resistors are not recommended due to the steady-state

current draw of the resistors.

PCB LAYOUT GUIDELINES

Due to the rather low switching frequency of the resonant

converter and the low power levels involved, PCB layout

is not as critical as with many other DC/DC converters.

There are however, a number of things to consider.

Due to the very low input voltages the circuit operates from,

the connections to VIN, the primary of the transformers

and the SW, VIN and GND pins of the LTC3109 should be

designed to minimize voltage drop from stray resistance,

and able to carry currents as high as 500mA. Any small

voltage drop in the primary winding conduction path will

lower efﬁciency and increase start-up voltage and capaci-

tor charge time.

Also, due to the low charge currents available at the out-

puts of the LTC3109, any sources of leakage current on

the output voltage pins must be minimized. An example

board layout is shown in Figure 4.

Figure 4. Example Component Placement for 2-Layer PC Board

(QFN Package). Note That VSTORE and VOUT Capacitor Sizes

are Application Dependent

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