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LTC3109

3109fa

APPLICATIONS INFORMATION

Using a 1:100 primary-secondary ratio yields start-up

voltages as low as 30mV. Other factors that affect per-

formance are the resistance of the transformer windings

and the inductance of the windings. Higher DC resistance

will result in lower efﬁciency and higher start-up volt-

ages. The secondary winding inductance will determine

the resonant frequency of the oscillator, according to the

formula below.

Freq

2•

π •LSEC •C

where LSEC is the inductance of one of the secondary

windings and C is the load capacitance on the second-

ary winding. This is comprised of the input capacitance

at pin C2A or C2B, typically 70pF each, in parallel with

the transformer secondary winding’s shunt capacitance.

The recommended resonant frequency is in the range of

10kHz to 100kHz. Note that loading will also affect the

resonant frequency. See Table 5 for some recommended

transformers.

Table 5. Recommended Transformers

VENDOR

TYPICAL START-

UP VOLTAGE

PART NUMBER

Coilcraft

www.coilcraft.com

25mV

35mV

85mV

LPR6235-752SML (1:100 ratio)

LPR6235-123QML (1:50 ratio)

LPR6235-253PML (1:20 ratio)

Würth

www.we-online

25mV

35mV

85mV

S11100034 (1:100 Ratio)

S11100033 (1:50 Ratio)

S11100032 (1:20 Ratio)

USING EXTERNAL CHARGE PUMP RECTIFIERS

The synchronous rectiﬁers in the LTC3109 have been

optimized for low frequency, low current operation, typical

of low input voltage applications. For applications where

the resonant oscillator frequency exceeds 100kHz, or a

transformer turns ratio of less than 1:20 is used, or the

C1A and C1B capacitor values are greater than 68nF, the

use of external charge pump rectiﬁers (1N4148 or 1N914

or equivalent) is recommended. See the Typical Application

circuits for an example. Avoid the use of Schottky recti-

ﬁers, as their low forward voltage increases the minimum

start-up voltage.

C1 CAPACITOR

The charge pump capacitor that is connected from each

transformer’s secondary winding to the corresponding

C1A and C1B pins has an effect on converter input resis-

tance and maximum output current capability. Generally

a minimum value of 1nF is recommended when operating

from very low input voltages using a transformer with

a ratio of 1:100. Capacitor values of 2.2nF to 10nF will

provide higher output current at higher input voltages,

however larger capacitor values can compromise perfor-

mance when operating at low input voltage or with high

resistance sources. For higher input voltages and lower

turns ratios, the value of the C1 capacitor can be increased

for higher output current capability. Refer to the Typical

Applications examples for the recommended value for a

given turns ratio.

C2 CAPACITOR

The C2 capacitors connect pins C2A and C2B to their

respective transformer secondary windings. For most

applications a capacitor value of 470pF is recommended.

Smaller capacitor values tend to raise the minimum

start-up voltage, and larger capacitor values can lower

efﬁciency.

Note that the C1 and C2 capacitors must have a voltage

rating greater than the maximum input voltage times the

transformer turns ratio.

VOUT AND VSTORE CAPACITOR

For pulsed load applications, the VOUT capacitor should

be sized to provide the necessary current when the load

is pulsed on. The capacitor value required will be dictated

by the load current (ILOAD), the duration of the load pulse

(tPULSE), and the amount of VOUT voltage droop the ap-

plication can tolerate (ΔVOUT). The capacitor must be

rated for whatever voltage has been selected for VOUT by

VS1 and VS2:

COUT(μF) ≥

ILOAD(mA)•tPULSE(ms)

ΔVOUT (V)

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