isl6312a Intersil Corporation, isl6312a Datasheet - Page 31

• Manufacturer Part Number: isl6312a
• Description: Four-phase Buck Pwm Controller With Integrated Mosfet Drivers For Intel Vr10, Vr11, And Amd Applications
• Manufacturer: Intersil Corporation

dictates the selection of L because duty cycles are usually
less than 50%. Nevertheless, both inequalities should be
evaluated, and L should be selected based on the lower of
the two results. In each equation, L is the per-channel
inductance, C is the total output capacitance, and N is the
number of active channels.
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 MOSFETs, 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 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.
L
L
≤
≤
2 N C V
---------------------------------
1.25 N C
---------------------------- -
=
10 00
⋅
10
100
(
(
10
ΔI
ΔI
⋅
⋅
[
10.61
FIGURE 21. R
)
10
)
2
2
⋅
⋅
O
⋅
–
⋅
ΔV
(
1.035
ΔV
MAX
MAX
SWITCHING FREQUENCY (kHz)
⋅
T
100
–
log
vs SWITCHING FREQUENCY
–
(
ΔI ESR
(
(
f S
ΔI ESR
⋅
T
31
. Figure 21 and Equation 49
)
⋅
)
]
)
)
⋅
⎛
⎝
1000
V
IN
–
V
O
⎞
⎠
10 000
(EQ. 47)
(EQ. 48)
(EQ. 49)
T
.
ISL6312A
For a four-phase design, use Figure 22 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
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 23 and 24 provide the same input RMS current
information for three-phase and two-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
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 23. NORMALIZED INPUT-CAPACITOR RMS
FIGURE 22. NORMALIZED INPUT-CAPACITOR RMS CURRENT
0.3
0.2
0.1
0.3
0.2
0.1
0
0
0
0
I
I
L
L
(P-P) = 0
(P-P) = 0.25 I
I
I
L(P-P)
L(P-P)
vs DUTY CYCLE FOR 4-PHASE CONVERTER
CURRENT FOR 3-PHASE CONVERTER
0.2
= 0
= 0.25 I
0.2
O
O
DUTY CYCLE (V
DUTY CYCLE (V
0.4
0.4
I
I
L
L
(P-P) = 0.5 I
(P-P) = 0.75 I
I
I
L(P-P)
L(P-P)
0.6
O/
0.6
IN/
L,PP
V
= 0.5 I
= 0.75 I
IN
V
O
)
O
)
O
) to I
O
O
), and the ratio
O
0.8
0.8
O
. Select a
August 1, 2007
FN9290.3
1.0
1.0