LM2647MTC datasheet(20/25 Pages) NSC

20 / 25 page

Application Information (Continued)

The V-I crossover losses (exist only in upper FET) are:

Pswon_u = 1/2

• VIN • Io • f • ton_u

Pswon_u = 464mW

Pswoff_u = 1/2

• VIN • Io • f • toff_u

Pswoff_u = 232mW

There is another loss term associated with charging C

OSS

every cycle, then dumping it into the FET before the next

charge cycle. This applies to both upper and lower FETs.

Pcoss_u = 1/2

• C

OSS_u • VIN

2 • f

Pcoss_l = 1/2

• C

OSS_l • VIN

2 • f

From the datasheets of the chosen FETs, ’Coss’ are respec-

tively about:

OSS_u = 250pF

OSS_l = 500pF

Pcoss_u = 15mW

Pcoss_I = 30mW

Summing up,

Psw_u = 464+132+15=611mW

Psw_I = 30mW

Controller Losses

In addition to the losses in the FETs, there is another loss

term associated with the switching, and this is dissipated in

the controller. The LM2647 has to pump in current pulses at

each transition to turn-ON or turn-OFF the FETs. Several

simplified or more complicated equations exist for calculat-

ing this, but this is most easily deduced by simply turning to

the measured consumption (see Electrical Characteristics

table). The current into the V5 pin is I

Q_V5 and reflects the

driver consumption. This can be as high as 1.5mA (mea-

sured at 300kHz). Let us also include the current into the

control sections (VDD pin), which can be as high as 4mA.

The total controller consumption is therefore

IC =(IQ_V5 +IQ_VDD)x5

IC = 28mW

Inductor Losses

The DC resistance (‘DCR’) of the chosen inductor is typically

is 26m

Ω. The DC loss is therefore DCR*Io2. The core losses

typically add 10% more to this. Therefore our estimate of

total inductor loss is

Pind=1.1 x (DCR x Io

Pind = 257mW

Capacitor Losses

The output capacitor of a typical buck regulator has very low

ripple current going through it. So its loss term can be

ignored. The input capacitor however provides the sharp

pulses of current for the Switch, and therefore the RMS

current through it can be fairly high. But the dissipation can

still be negligible if the ESR is very low. This is the situation

if the input capacitors are monolithic ceramic capacitors as in

the Evaluation board (if Tantalum or Aluminum electrolytic

capacitors are used at the input, their dissipation must be

accounted for here). The final efficiency/loss terms are pro-

vided in Table 1.

20056323

FIGURE 13. Crossover (turn-on or turn-off)

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