LTC3609 datasheet(14/26 Pages) LINER

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LTC3609

3609fb

applicaTions inForMaTion

Once the value for L is known, the type of inductor must

be selected. High efficiency converters generally cannot

afford the core loss found in low cost powdered iron cores.

A variety of inductors designed for high current, low volt-

age applications are available from manufacturers such as

Sumida, Panasonic, Coiltronics, Coilcraft and Toko.

CINandCOUTSelection

The input capacitance, CIN, is required to filter the square

wave current at the drain of the top MOSFET. Use a low ESR

capacitor sized to handle the maximum RMS current.

RMS

OUT MAX

OUT

≅

(

)

– 1

This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT(MAX)/2. This simple worst-case condition is

commonly used for design because even significant de-

viations do not offer much relief. Note that ripple current

ratings from capacitor manufacturers are often based on

only 2000 hours of life which makes it advisable to derate

the capacitor.

The selection of COUT is primarily determined by the

ESR required to minimize voltage ripple and load step

transients. The output ripple ∆VOUT is approximately

bounded by:

VOUT ≤ΔIL ESR+

8fCOUT













Since ∆IL increases with input voltage, the output ripple

is highest at maximum input voltage. Typically, once the

ESR requirement is satisfied, the capacitance is adequate

for filtering and has the necessary RMS current rating.

Multiple capacitors placed in parallel may be needed to

meet the ESR and RMS current handling requirements.

Dry tantalum, special polymer, aluminum electrolytic and

ceramic capacitors are all available in surface mount pack-

ages. Special polymer capacitors offer very low ESR but

have lower capacitance density than other types. Tantalum

capacitors have the highest capacitance density but it is

important to only use types that have been surge tested

foruseinswitchingpowersupplies.Aluminumelectrolytic

capacitors have significantly higher ESR, but can be used

in cost-sensitive applications providing that consideration

is given to ripple current ratings and long-term reliability.

Ceramic capacitors have excellent low ESR characteris-

tics but can have a high voltage coefficient and audible

piezoelectriceffects.ThehighQofceramiccapacitorswith

trace inductance can also lead to significant ringing. When

used as input capacitors, care must be taken to ensure

that ringing from inrush currents and switching does not

pose an overvoltage hazard to the power switches and

controller. To dampen input voltage transients, add a small

5µFto50µFaluminumelectrolyticcapacitorwithanESRin

the range of 0.5Ω to 2Ω. High performance through-hole

capacitors may also be used, but an additional ceramic

capacitor in parallel is recommended to reduce the effect

of their lead inductance.

TopMOSFETDriverSupply(CB,DB)

Anexternalbootstrapcapacitor,CB,connectedtotheBOOST

pin supplies the gate drive voltage for the topside MOSFET.

This capacitor is charged through diode DB from INTVCC

when the switch node is low. When the top MOSFET turns

on, the switch node rises to VIN and the BOOST pin rises

to approximately VIN + INTVCC. The boost capacitor needs

to store about 100 times the gate charge required by the

top MOSFET. In most applications an 0.1µF to 0.47µF, X5R

or X7R dielectric capacitor is adequate.

DiscontinuousModeOperationandFCBPin

The FCB pin determines whether the bottom MOSFET

remains on when current reverses in the inductor. Tying

this pin above its 0.6V threshold enables discontinuous

operation where the bottom MOSFET turns off when in-

ductor current reverses. The load current at which current

reverses and discontinuous operation begins depends on

the amplitude of the inductor ripple current and will vary

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