L6919E

16/33

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 dual-phase topology, the input RMS current is highly reduced comparing

with a single phase operation.

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

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

high RMS value needed by the CPU power supply application 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 possible

Timing Diagram

Acquisition:

(CH1=LGATEx; CH2=VCC; CH3=VOUT;

PGOOD

t

t

t

t

Turn ON threshold

2048 Clock Cycles

0.50

0.75

0.25

0.50

0.25

Single Phase

Dual Phase











>

−

⋅

⋅

<

−

⋅

⋅

=

0.5

D

if

D)

2

(2

1)

-

(2D

2

OUT

I

5

.

0

D

if

D)

2

(1

2D

2

OUT

I

rms

I

P

RM S

ESR

I

RM S

()

2

⋅

=