isl6333 Intersil Corporation, isl6333 Datasheet - Page 37

• Manufacturer Part Number: isl6333
• Description: Three-phase Buck Pwm Controller With Integrated Mosfet Drivers And Light Load Efficiency Enhancements For Intel Vr11.1 Applications
• Manufacturer: Intersil Corporation

Switching Frequency
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 the “MOSFETs General Design Guide” on
page 31 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.
Choose the lowest switching frequency that allows the
regulator to meet the transient-response requirements.
Switching frequency is determined by the selection of the
frequency-setting resistor, R
are provided to assist in selecting the correct value for R
R T
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
enough 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.
For a three-phase design, use Figure 27 to determine the
input-capacitor RMS current requirement set by the duty
cycle, maximum sustained output current (I
of the peak-to-peak inductor current (I
Select 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 1.25x greater than the
maximum input voltage. Figures 28 and 29 provide the same
input RMS current information for two-phase and single-phase
designs respectively. Use the same approach for selecting
the bulk capacitor type and number.
Low capacitance, high-frequency ceramic capacitors are
needed in addition to the input bulk capacitors to suppress
leading and falling edge voltage spikes. The spikes result from
the high current slew rate produced by the upper MOSFET
=
10
500
100
[
10.61
10
FIGURE 26. R
50k
–
(
1.035
100k
SWITCHING FREQUENCY (Hz)
⋅
T
log
vs SWITCHING FREQUENCY
(
f S
T
37
. Figure 26 and Equation 46
)
)
]
ISL6333, ISL6333A, ISL6333B, ISL6333C
L,(P-P)
O
1M
) to I
), and the ratio
O
.
2M
(EQ. 46)
T
.
turn on and off. Select low ESL ceramic capacitors and place
one as close as possible to each upper MOSFET drain to
minimize board parasitics and maximize suppression.
FIGURE 29. NORMALIZED INPUT-CAPACITOR RMS
FIGURE 27. NORMALIZED INPUT-CAPACITOR RMS
FIGURE 28. NORMALIZED INPUT-CAPACITOR RMS
0.6
0.4
0.2
0.3
0.2
0.1
0.3
0.2
0.1
0
0
0
0
0
0
I
I
I
I
I
L(P-P)
L(P-P)
L(P-P)
L(P-P)
L(P-P)
I
I
I
CURRENT FOR SINGLE-PHASE CONVERTER
CURRENT FOR 3-PHASE CONVERTER
CURRENT FOR 2-PHASE CONVERTER
L(P-P)
L(P-P)
L(P-P)
= 0
= 0.25 I
= 0
= 0.5 I
= 0.75 I
0.2
0.2
0.2
= 0
= 0.5 I
= 0.75 I
O
O
O
DUTY CYCLE (V
DUTY CYCLE (V
DUTY CYCLE (V
O
O
0.4
0.4
0.4
I
I
L(P-P)
L(P-P)
0.6
0.6
0.6
IN
IN/
IN/
= 0.5 I
= 0.75 I
/V
V
V
O
O
O
)
)
)
O
O
0.8
0.8
0.8
April 10, 2008
FN6520.0
1.0
1.0
1.0