isl6334d Intersil Corporation, isl6334d Datasheet - Page 25

• Manufacturer Part Number: isl6334d
• Description: Vr11.1, 4-phase Pwm Controller With Phase Dropping, Droop Disabled And Load Current Monitoring Features
• Manufacturer: Intersil Corporation

Switching Frequency Selection
There are a number of variables to consider when choosing
the switching frequency, as there are considerable effects on
the upper MOSFET loss calculation. These effects are
outlined in “MOSFETs” on page 22, and they establish the
upper limit for the switching frequency. The lower limit is
established by the requirement for fast transient response
and small output-voltage ripple as outlined in “Output Filter
Design” on page 24. Choose the lowest switching frequency
that allows the regulator to meet the transient-response
requirements.
Input Capacitor Selection
The input capacitors are responsible for sourcing the AC
component of the input current flowing into the upper
MOSFETs. Their RMS current capacity must be sufficient to
handle the AC component of the current drawn by the upper
MOSFETs, which is related to duty cycle and the number of
active phases.
FIGURE 18. NORMALIZED INPUT-CAPACITOR RMS CURRENT
FIGURE 19. NORMALIZED INPUT-CAPACITOR RMS CURRENT
0.3
0.2
0.1
0.3
0.2
0.1
0
0
0
0
I
I
L,PP
L,PP
I
I
I
L,PP
L,PP
L,PP
vs DUTY CYCLE FOR 2-PHASE CONVERTER
vs DUTY CYCLE FOR 3-PHASE CONVERTER
= 0
= 0.25 I
= 0
= 0.5 I
= 0.75 I
0.2
0.2
O
O
O
DUTY CYCLE (V
DUTY CYCLE (V
0.4
0.4
25
I
I
L,PP
L,PP
= 0.5 I
= 0.75 I
0.6
0.6
O
O/
/V
V
IN
O
IN
O
)
)
0.8
0.8
1.0
1.0
ISL6334D
For a 2-phase design, use Figure 18 to determine the input
capacitor RMS current requirement given the duty cycle,
maximum sustained output current (I
per-phase peak-to-peak inductor current (I
a bulk capacitor with a ripple current rating which will
minimize the total number of input capacitors required to
support the RMS current calculated. The voltage rating of
the capacitors should also be at least 1.25 times greater
than the maximum input voltage.
Figures 19 and 20 provide the same input RMS current
information for 3- and 4-phase designs respectively. Use the
same approach to selecting the bulk capacitor type and
number, as previously described.
Low capacitance, high-frequency ceramic capacitors are
needed in addition to the bulk capacitors to suppress leading
and falling edge voltage spikes. The result from the high
current slew rates produced by the upper MOSFETs turn on
and off. Select low ESL ceramic capacitors and place one as
close as possible to each upper MOSFET drain to minimize
board parasitic impedances and maximize suppression.
FIGURE 21. NORMALIZED INPUT-CAPACITOR RMS
FIGURE 20. NORMALIZED INPUT-CAPACITOR RMS CURRENT
0.3
0.2
0.1
0.6
0.4
0.2
0
0
0
0
I
I
L,PP
L,PP
vs DUTY CYCLE FOR 4-PHASE CONVERTER
CURRENT vs DUTY CYCLE FOR SINGLE-PHASE
CONVERTER
= 0
= 0.25 I
I
I
I
L,PP
L,PP
L,PP
0.2
0.2
= 0
= 0.5 I
= 0.75 I
O
DUTY CYCLE (V
DUTY CYCLE (V
O
O
0.4
0.4
I
I
L,PP
L,PP
= 0.5 I
= 0.75 I
0.6
0.6
O/
O/
O
V
V
), and the ratio of the
IN
O
IN
O
)
)
L,PP
0.8
0.8
) to I
October 29, 2008
O
. Select
1.0
1.0
FN6802.0