AN1126 datasheet(8/16 Pages) STMICROELECTRONICS

8 / 16 page

The gained power

∆P% as a function

of input voltage, output voltage, output

current and

input capacitor ESR is

shown in figure 12.

Figure 12 shows the measured effi-

ciency with the L4973D board , with

Vin = 12V, Vout = 5.1V, fsw = 200kHz,

using a input capacitor 470

µF/50V

ROE with an ESR = 85m

Ω. Using the

multiphase application with a phase

delay,

α, equal to half period, case1) ,

there is a gained efficiency of 2%

compared to the synchronous applica-

tion. So it is possible to maintain high

efficiency values using low cost and

size capacitor.

Conclusions

To sum up in application in which the duty

cycle is between 0.2 and 0.8 there is a big

advantage using the multiphase PWMs, in

terms of dissipated power on the input ca-

pacitor compared with the added circuitry to

achieve it. The more the output current is

the more this advantage increases.

Applications with

δ = 0.5, using a half period

of multiphase phase delay, gives the best

benefit because the RMS current through

the input capacitor is approximately zero.

Minimization

the

Ripple

Current

Through the Output Capacitor.

Figure 13 shows the current ripple through

the output filter for different phase delay,

α,

of the two PWMs considering a duty cycle of

0.5.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

∆P%(δ)

Figure 11.

∆P% vs. duty cycle.

141618

Iout [ A ]

50m

Ω

75m

Ω

ESR=100m

Ω

Vcc=12V

∆ P%=(Psaved/Po) · 100

ESR=60m

Ω

Vcc=8V

Vcc=12V

Vcc=24V

∆ P%=(Psaved/Po) · 100

02468

Iout [ A ]

∆P%

Figure 12.

∆P%

345

Io [A]

Multiphase

α=180

Vo = 5.1V

fsw = 200kHz

Vcc = 12V

synchronous

Cin = 470

µF/50V ROE EKE

ESR = 85mOhm

Figure 13. Efficiency vs. Output Current.

⋅ IO

VIN

⋅ IIN

⋅ 100 [%]

AN1126 APPLICATION NOTE

8/16

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