LTC3108 datasheet(13/20 Pages) LINER

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LTC3108

3108p

COMPONENT SELECTION

Step-Up Transformer

The step-up transformer turns ratio will determine how

low the input voltage can be for the converter to start.

Using a 1:100 ratio can yield start-up voltages as low as

20mV. Other factors that affect performance are the DC

resistance of the transformer windings and the inductance

of the windings. Higher DC resistance will result in lower

efﬁciency. The secondary winding inductance will deter-

mine the resonant frequency of the oscillator, according

to the following formula.

Frequency

2 •

•

(sec)•

Where L is the inductance of the transformer secondary

winding and C is the load capacitance on the secondary

winding. This is comprised of the input capacitance at pin

C2, typically 30pF, in parallel with the transformer secondary

winding’s shunt capacitance. The recommended resonant

frequency is in the range of 20kHz to 200kHz. See Table 4

for some recommended transformers.

Table 4. Recommended Transformers

VENDOR

PART NUMBER

Coilcraft

www.coilcraft.com

LPR6235-752SML (1:100 Ratio)

LPR6235-253PML (1:20 Ratio)

LPR6235-123QML (1:50 Ratio)

C1 Capacitor

The charge pump capacitor that is connected from the

transformer’s secondary winding to the C1 pin has an ef-

fect on converter input resistance and maximum output

current capability. Generally, a minimum value of 1nF is

recommended when operating from very low input volt-

ages using a transformer with a ratio of 1:100. Too large

a capacitor value can compromise performance 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

schematic examples for the recommended value for a

given 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, the duration of the load pulse, and

the amount of voltage droop the circuit can tolerate. The

capacitor must be rated for whatever voltage has been

selected for VOUT by VS1 and VS2.

CmF

ImA

OUT

LOAD

PULSE

OUT

()

(

) •

(sec)

≥

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 (to be discussed

in the next example). 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 holdup

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:

µA I

t f

TSTORE

STORE

LDO

BURST

≥

⎡⎣

⎤⎦

(• • ) •

..25

− V

OUT

Where 6μA is the quiescent current of the LTC3108, 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. Note that for a programmed

output voltage of 5V, the VSTORE capacitor cannot provide

any beneﬁcial storage time.

APPLICATIONS INFORMATION

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