LTC1266CS datasheet(11/20 Pages) LINER

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LTC1266

LTC1266-3.3/LTC1266-5

APPLICATIO S I FOR ATIO

From the duty cycles, the required RDS(ON) for each

MOSFET can be derived:

TS RDS(ON) =

VIN × PT

VOUT × IMAX2 × (1 + δT)

BS RDS(ON) =

VIN × PB

(VIN – VOUT) × IMAX2 × (1 + δB)

where PT and PB are the allowable power dissipations and

δTandδBarethetemperaturedependenciesofRDS(ON).PT

and PB will be determined by efficiency and/or thermal

requirements(seeEfficiencyConsiderations).ForaMOSFET,

(1 +

δ) is generally given in the form of a normalized

RDS(ON) vs temperature curve, but δPCH = 0.007/°C and

δNCH = 0.005/°C can be used as an approximation for low

voltage MOSFETs.

The minimum input voltage determines whether standard

threshold or logic-level threshold MOSFETs must be used.

For VIN > 8V, standard threshold MOSFETs (VGS(TH) < 4V)

may be used. If VIN is expected to drop below 8V, logic-

level threshold MOSFETs (VGS(TH) < 2.5V) are strongly

recommended. The LTC1266 series Power VIN must al-

ways be less than the absolute maximum VGS ratings for

the MOSFETs.

The Schottky diode D1 shown in Figure 1 only conducts

during the deadtime between the conduction of the two

power MOSFETs. D1’s sole purpose in life is to prevent the

body diode of the bottom-side MOSFET from turning on

and storing charge during the deadtime, which could cost

as much as 1% in efficiency (although there are no other

harmful effects if D1 is omitted). Therefore, D1 should be

selected for a forward voltage of less than 0.7V when

conducting IMAX.

CIN and COUT Selection

In continuous mode, the current through the topside

MOSFET is a square wave of duty cycle VOUT/VIN. To

prevent large voltage transients, a low ESR (Effective

Series Resistance) input capacitor sized for the maximum

RMS current must be used. The maximum RMS capacitor

current is given by:

CIN Required IRMS ≈ IMAX

[VOUT(VIN – VOUT)]1/2

VIN

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

IRMS = IOUT/2. This simple worst-case condition is com-

monly used for design because even significant devia-

tions do not offer much relief. Note that capacitor

manufacturer’s ripple current ratings are often based on

only 2000 hours of life. This makes it advisable to further

derate the capacitor, or to choose a capacitor rated at a

higher temperature than required. Always consult the

manufacturer if there is any question. An additional 0.1

µF

to 1

µF ceramic capacitor is also required on Power VIN

(Pin 2) for high frequency decoupling.

The selection of COUT is driven by the required ESR. The

ESR of COUT must be less than twice the value of RSENSE

for proper operation of the LTC1266 series:

COUT Required ESR < 2RSENSE

Optimum efficiency is obtained by making the ESR equal

to RSENSE. As the ESR is increased up to 2RSENSE, the

efficiency degrades by less than 1%. If the ESR is greater

than 2RSENSE, the voltage ripple on the output capacitor

will prematurely trigger Burst Modeoperation, resulting in

disruption of continuous mode and an efficiency hit which

can be several percent. If Burst Mode operation is dis-

abled, the ESR requirement can be relaxed and is limited

only by the allowable output voltage ripple.

Manufacturers such as Nichicon and United Chemicon

should be considered for high performance capacitors.

The OS-CON semiconductor dielectric capacitor available

from Sanyo has the lowest ESR/size ratio of any aluminum

electrolytic at a somewhat higher price. Once the ESR

requirement for COUT has been met, the RMS current

rating generally far exceeds the IRIPPLE(P-P) requirement.

In surface mount applications multiple capacitors may

have to be paralleled to meet the capacitance, ESR or RMS

current handling requirements of the application. An

excellent choice is the AVX TPS series of surface mount

tantalums.

At low supply voltages, a minimum capacitance at COUT

is needed to prevent an abnormal low frequency oper-

ating mode (see Figure 4). When COUT is made too

small, the output ripple at low frequencies will be large

enough to trip the voltage comparator. This causes

Burst Modeoperation to be activated when the LTC1266

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