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L6712A L6712

Figure 10. Soft Start.

INPUT CAPACITOR

The input capacitor is designed considering mainly the input RMS current that depends on the duty cycle

as reported in figure 11. Considering the two-phase topology, the input RMS current is highly reduced

comparing with a single-phase operation.

It can be observed that the input RMS value is one half of the single-phase equivalent input current in the

worst case condition that happens for D=0.25 and D=0.75.

The power dissipated by the input capacitance is then equal to:

Input capacitor is designed in order to sustain the ripple relative to the maximum load duty cycle. To reach

the RMS value needed and also to minimize components cost, the input capacitance is realized by more

than one physical capacitor. The equivalent RMS current is simply the sum of the single capacitor's RMS

current.

Input bulk capacitor must be equally divided between high-side drain mosfets and placed as close as pos-

sible to reduce switching noise above all during load transient. Ceramic capacitor can also introduce ben-

efits in high frequency noise de coupling, noise generated by parasitic components along power path.

Figure 11. Input RMS Current vs. Duty Cycle (D) and Driving Relationships.

PGOOD

t

t

t

t

Turn ON threshold

2048 Clock Cycles

Timing Diagram

Acquisition:

CH1=PGOOD; CH2=VOUT; CH3=REF_OUT

P

RMS

ESR

I

RMS

()

2

⋅

=

0.50

0.75

0.25

0.50

0.25

Single Phase

Dual Phase

I

rms

I

OUT

2

------------

2D

1

2D

–

()

⋅

if

D < 0.5

⋅

I

OUT

2

------------

2D

1

–

() 22D

–

()

⋅

if

D > 0.5

⋅















=