Multiphase Benefits

The main benefits are :

Minimization of the RMS current through the input capacitor therefore increasing of the efficiency and

reducing of the capacitor cost and size.

Minimization of ripple current through the output capacitor and ground path.

Fast load transient response.

Improved reliability /MTBF.

RMS current through the input capacitor are equivalent in Case 1) and Case 2). Even though the cir-

cuitry of Case 2) is simplifier than Case 1), Case 1) provides the opportunity to optimize this ripple cur-

rent.

Minimization of the RMS Current Through the Input Capacitor.

In Case 1) , Figure 3 shows the RMS current through the input capacitor, referred to the output current

(Iout), for various phase delays,

α , of the two PWM sections. This assumes a duty cycle of 0.5 and a

ripple current through the coil of 0.1

⋅ Iout.

For

α equal to a half period (180 degrees of phase delay) the RMS current is approximately zero. If the

two PWM signals are synchronized the RMS value is Irms = Iout/2. For example if Vout = 5V and

Iout = 7A the Output Power is 35W. If the Input capacitor has an ESR of 100mOhm the phase delay

allows a savings of 1.23W which corresponds to the 3.5% of the power delivered to the load.

Assuming the same duty cycle for the two sections, the RMS Current through the input filter for different

duty cycle, considering a phase delay of the second PWM signal equal to the Ton of the first section (

Case 2) ), is given approximately (the output current ripple can be negleted for this calculation) by the

following formula:

(















√









Iout

2

⋅ √

2

⋅ δ 



2

if

δ ≤ 0.5







Iout

2

⋅ √

2



2

− (iout⋅

if

δ > 0.5

Multiphase (1)

where

δ = duty cycle

time

α=0 °

α=180°

α=120°

α=90°

α=40 °

0

0

Iout/2

- Iout/2

[ A ]

Figure 5. RMS current through the input capacitor for a different phase delay,

α , with a duty

cycle of 0.5.

AN1126 APPLICATION NOTE

4/16