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LTC3108-1

31081fa

at times when the input power may be lost. Note that this

capacitor can charge all the way to 5.25V (regardless of

the settings for VOUT), so ensure that the holdup capacitor

has a working voltage rating of at least 5.5V at the tem-

perature for which it will be used. The VSTORE capacitor

can be sized using the following:

CSTORE ≥

6μA

+IQ +ILDO +(IBURST •t •f)

[] •TSTORE

5.25

− VOUT

Where 6μA is the quiescent current of the LTC3108-1, IQ

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

the LDO between bursts, IBURST is the total load during

the burst, t is the duration of the burst, f is the frequency

of the bursts, TSTORE is the storage time required and

VOUT is the output voltage required.

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/Bussmann

www.bussmann.com/3/PowerStor.html

KR Series

P Series

Vishay/Sprague

www.vishay.com/capacitors

Tantamount 592D

595D Tantalum

150CRZ/153CRV Aluminum

013 RLC (Low Leakage)

Storage capacitors requiring voltage balancing are not

recommended due to the current draw of the balancing

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 voltage the circuit may operate

from, the connections to VIN,theprimaryofthetransformer

and the SW and GND pins of the LTC3108-1 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 capacitor charge time.

Also, due to the low charge currents available at the outputs

of the LTC3108-1, any sources of leakage current on the

output voltage pins must be minimized. An example board

layout is shown in Figure 3.

APPLICATIONS INFORMATION

Figure 3. Example Component Placement for

Two-Layer PC Board (DFN Package)

31081 FO3

VOUT2

VOUT

VIN

VIAS TO GROUND PLANE

VLDO

PGOOD

GND

VOUT2_EN

VS1

VS2

VOUT

VOUT2

VLDO

PGD

VAUX

VSTORE

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