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LTC3780

3780fe

APPLICATIONS INFORMATION

Figure 11 is a basic LTC3780 application circuit. External

component selection is driven by the load requirement,

and begins with the selection of RSENSE and the inductor

value. Next, the power MOSFETs are selected. Finally, CIN

and COUT are selected. This circuit can be conﬁgured for

operation up to an input voltage of 36V.

Selection of Operation Frequency

The LTC3780 uses a constant frequency architecture and

has an internal voltage controlled oscillator. The switching

frequency is determined by the internal oscillator capacitor.

This internal capacitor is charged by a ﬁxed current plus

an additional current that is proportional to the voltage

applied to the PLLFLTR pin. The frequency of this oscillator

can be varied over a 2-to-1 range. The PLLFLTR pin can

be grounded to lower the frequency to 200kHz or tied to

2.4V to yield approximately 400kHz. When PLLIN is left

open, the PLLFLTR pin goes low, forcing the oscillator to

minimum frequency.

A graph for the voltage applied to the PLLFLTR pin vs

frequency is given in Figure 7. As the operating frequency

is increased the gate charge losses will be higher, reducing

efﬁciency. The maximum switching frequency is approxi-

mately 400kHz.

Inductor Selection

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use of

smaller inductor and capacitor values. The inductor value

has a direct effect on ripple current. The inductor current

ripple

∆IL is typically set to 20% to 40% of the maximum

inductor current at boost mode VIN(MIN). For a given ripple

the inductance terms in continuous mode are as follows:

BOOST

IN MIN

OUT

IN MIN

OUT

()

(

100

MAX

OUT

BUCK

OUT

IN MAX

Ripple V

)

()

OUT MAX

IN MAX

Ripple V

()

100

where:

f is operating frequency, Hz

% Ripple is allowable inductor current ripple, %

VIN(MIN) is minimum input voltage, V

VIN(MAX) is maximum input voltage, V

VOUT is output voltage, V

IOUT(MAX) is maximum output load current

For high efﬁciency, choose an inductor with low core loss,

such as ferrite and molypermalloy (from Magnetics, Inc.).

Also, the inductor should have low DC resistance to reduce

the I2R losses, and must be able to handle the peak inductor

current without saturating. To minimize radiated noise,

use a toroid, pot core or shielded bobbin inductor.

RSENSE Selection and Maximum Output Current

RSENSE is chosen based on the required output current.

The current comparator threshold sets the peak of the

inductor current in boost mode and the maximum inductor

valley current in buck mode. In boost mode, the maximum

average load current at VIN(MIN) is:

OUT MAX BOOST

SENSE

)

(

⎛

⎝⎜

⎞

⎠⎟

160

MIN

OUT

)

PLLFLTR PIN VOLTAGE (V)

150

200

250

2.5

450

3780 F07

100

0.5

1.5

300

350

400

Figure 7. Frequency vs PLLFLTR Pin Voltage

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