ISL6323EVAL1Z Intersil, ISL6323EVAL1Z Datasheet - Page 31

• Manufacturer Part Number: ISL6323EVAL1Z
• Description: EVAL BOARD 1 FOR ISL6323
• Manufacturer: Intersil

Specifications of ISL6323EVAL1Z

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

transient, the capacitor voltage becomes slightly depleted.
The output inductors must be capable of assuming the entire
load current before the output voltage decreases more than
ΔV
Equation 56 gives the upper limit on L for the cases when
the trailing edge of the current transient causes a greater
output-voltage deviation than the leading edge. Equation 57
addresses the leading edge. Normally, the trailing edge
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” on page 24, 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 30. 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
L
L
≤
≤
MAX
2 N C V
---------------------------------
1.25 N C
---------------------------- -
=
⋅
10
(
(
ΔI
. This places an upper limit on inductance.
ΔI
⋅
⋅
[
100
10.61
FIGURE 24. R
)
1k
10
)
2
2
60k
⋅
⋅
O
⋅
–
⋅
ΔV
(
100k
1.035
ΔV
MAX
MAX
SWITCHING FREQUENCY (Hz)
⋅
T
–
log
vs SWITCHING FREQUENCY
–
(
ΔI ESR
(
(
f S
ΔI ESR
⋅
T
31
. Figure 24 and Equation 58
)
⋅
)
]
)
)
⋅
⎛
⎝
V
IN
–
V
O
⎞
⎠
1M
(EQ. 56)
(EQ. 57)
(EQ. 58)
2M
T
.
ISL6323
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.
For a 4-phase design, use Figure 25 to determine the input-
capacitor RMS current requirement set by the duty cycle,
maximum sustained output current (I
peak-to-peak inductor current (I
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 26
and 27 provide the same input RMS current information for
3-phase and 2-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 25. NORMALIZED INPUT-CAPACITOR RMS CURRENT
0.3
0.2
0.1
0
0
I
I
L(P-P)
L(P-P)
= 0
= 0.25 I
vs DUTY CYCLE FOR 4-PHASE CONVERTER
0.2
O
DUTY CYCLE (V
0.4
I
I
L(P-P)
L(P-P)
L(P-P)
= 0.5 I
= 0.75 I
0.6
O/
O
) to I
V
), and the ratio of the
IN
O
)
O
O
. Select a bulk
0.8
October 21, 2008
FN9278.4
1.0