MAX782CSLASHD datasheet(19/32 Pages) MAXIM

19 / 32 page

Triple-Output Power-Supply

Controller for Notebook Computers

______________________________________________________________________________________

__________Applications Information

Efficiency Considerations

Achieving outstanding efficiency over a wide range of

loads is a result of balanced design rather than brute-

force overkill, particularly with regard to selecting the

power MOSFETs. Generally, the best approach is to

design for two loading conditions, light load and heavy

load (corresponding to suspend and run modes in the

host computer), at some nominal battery voltage (such

as 1.2V/cell for NiCd or NiMH). Efficiency improves as

the input voltage is reduced, as long as the high-side

switch saturation voltage is low relative to the input volt-

age. If there is a choice, use the lowest-voltage battery

pack possible, but with at least six cells.

Heavy-Load Efficiency

Losses due to parasitic resistances in the switches,

coil, and sense resistor dominate at high load-current

levels. Under heavy loads, the MAX782 operates in the

continuous-conduction mode, where there is a large

DC offset to the inductor current plus a small sawtooth

AC component (see the

+3.3V Inductor section). This

DC current is exactly equal to the load current – a fact

that makes it easy to estimate resistive losses through

the assumption that total inductor current is equal to

this DC offset current.

The major loss mechanisms under heavy loads are, in

usual order of importance:

I2R losses

gate-charge losses

diode-conduction losses

transition losses

capacitor-ESR losses

losses due to the operating supply current of the IC.

Inductor core losses are fairly low at heavy loads

because the inductor current’s AC component is small.

Therefore, they are not accounted for in this analysis.

Efficiency = POUT/PIN x 100% = POUT/(POUT +

PDTOTAL) x 100%

PDTOTAL = PD(I2R) + PDGATE + PDDIODE + PDTRAN +

PDCAP + PDIC

PD(I2R) = resistive loss = (ILOAD2) x (RCOIL + rDS(ON) +

RCS)

where RCOIL is the DC resistance of the coil, rDS(ON) is

the drain-source on resistance of the MOSFET, and

RCS is the current-sense resistor value. Note that the

rDS(ON) term assumes that identical MOSFETs are

employed for both the synchronous rectifier and high-

side switch, because they time-share the inductor cur-

rent. If the MOSFETs are not identical, losses can be

estimated by averaging the two individual rDS(ON)

terms according to duty factor.

PDGATE = gate driver loss = qG x f x VL

where VL is the MAX782’s logic supply voltage (nomi-

nally 5V) and qG is sum of the gate charge for low-

side and high-side switches. Note that gate charge

losses are dissipated in the IC, not the MOSFETs,

and therefore contribute to package temperature rise.

For matched MOSFETs, qG is simply twice the gate

charge of a single MOSFET (a data sheet specifica-

tion). If the +5V buck SMPS is turned off, replace VL

in this equation with VIN.

PDIODE = diode conduction losses = ILOAD x VD x tD x f

where tD is the diode’s conduction time (typically

110ns), VD is the forward voltage of the Schottky diode,

and f is the switching frequency.

VIN2 x CRSS x ILOAD x f

PDTRAN = transition loss = ———————————

IDRIVE

Company

USA Phone

Central Semi

Coiltronics

IRC

Murata-Erie

Nihon

Siliconix

Sprague

Sumida

TDK

Transpower Tech.

Factory FAX

[country code]

Table 4. Surface-Mount Components

(516) 435-1110

(407) 241-7876

(512) 992-7900

(404) 736-1300

(805) 867-2555

(408) 988-8000

(603) 224-1961

(708) 956-0666

(708) 803-6100

(702) 831-0140

[ 1] (516) 435-1824

[ 1] (407) 241-9339

[ 1] (213) 772-9028

[ 1] 404 736-3030

[81] 3-3494-7414

[ 1] (408) 727-5414

[ 1] (603) 224-1430

[81] 3-3607-5144

[81] 3-3278-5358

[ 1] 702 831-3521

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