LT3751EUFD datasheet(15/28 Pages) LINER

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LT3751

3751f

APPLICATIONS INFORMATION

The LT3751 charger controller can be optimized for either

capacitor charging only or low noise regulation applica-

tions. Several equations are provided to aid in the design

process.

Safety Warning

Large capacitors charged to high voltage can deliver a lethal

amount of energy if handled improperly. It is particularly

important to observe appropriate safety measures when

designing the LT3751 into applications. First, create a dis-

charge circuit that allows the designer to safely discharge

the output capacitor. Second, adequately space high voltage

nodes from adjacent traces to satisfy printed circuit board

voltage breakdown requirements.

Selecting Operating Mode

Tie the FB pin to GND to operate the LT3751 as a capacitor

charger. In this mode, the LT3751 charges the output at

peak primary current in boundary mode operation. This

constitutes maximum power delivery and yields the fast-

est charge times. Power delivery is halted once the output

reaches the desired output voltage set by the RVOUT and

RBG pins.

Tie a resistor divider from the FB pin to VOUT and GND

to operate the LT3751 as a low noise voltage regulator

(refer to Low Noise regulation section for proper design

procedures). The LT3751 operates as a voltage regulator

using both peak current and duty cycle modulation to vary

output current during different loading conditions.

Selecting Component Parameters

Most designs start with the initial selection of VTRANS,

VOUT, COUT, and either charge time, tCHARGE, (capacitor

charger) or POUT,MAX (regulator). These design inputs

are then used to select the transformer ratio, N, the peak

primary current, IPK, and the primary inductance, LPRI.

Figure 7 can be used as a rough guide for maximum power

output for a given VTRANS and IPK.

Selecting Transformer Turns Ratio

The transformer ratio, N, should be selected based on

the input and output voltages. Smaller N values equate

to faster charge times and larger available output power.

Note that drastically reducing N below the VOUT/VTRANS

ratio will increase the ﬂyback voltage on the drain of the

NMOS and increase the current through the output diode.

A good choice is to select N equal to VOUT/VTRANS.

≤

VOUT

VTRANS

Choosing Capacitor Charger IPK

When operating the LT3751 as capacitor charger, choose

IPK based on the required capacitor charge time, tCHARGE,

and the initial design inputs.

IPK =

2•N • VTRANS + VOUT

()•C

OUT •VOUT

Efficiency • VTRANS •tCHARGE − td

()

Theconverterefﬁciencyvariesovertheoutputvoltagerange.

The IPK equation is based on the average efﬁciency over the

entire charging period. Several factors can cause the charge

time to increase. Efﬁciency is the most dominant factor and

is mainly affected by the transformer winding resistance,

core losses, leakage inductance, and transistor RDS. Most

applications have overall efﬁciencies above 70%.

Figure 7. Maximum Power Output

PEAK PRIMARY CURRENT (A)

3751 F07

100

P = 20 WATTS

P = 50 WATTS

P = 100 WATTS

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