ISL6558EVAL1 Intersil, ISL6558EVAL1 Datasheet - Page 4

• Manufacturer Part Number: ISL6558EVAL1
• Description: EVAL BOARD W/LOAD TESTER ISL6
• Manufacturer: Intersil
• Series: Endura™r

Specifications of ISL6558EVAL1

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Power - Output: 150W
• Voltage - Output: 1.5V
• Current - Output: 100A
• Voltage - Input: 5V, 12V
• Regulator Topology: Buck
• Frequency - Switching: 500kHz
• Board Type: Fully Populated
• Utilized Ic / Part: HIP6601, ISL6558
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

DETERMINE NUMBER OF PHASES, SWITCHING
FREQUENCY, AND DUTY CYCLE
The first step in designing a multi-phase converter is to
determine the number of phases. This determination
depends heavily on the cost analysis, which in turn
depends on system constraints that differ from one
design to the next. Principally, the designer will be
concerned with whether components can be mounted on
both sides of the circuit board; whether through-hole
components are permitted on either side; and the total
board space available for power-supply circuitry.
Generally speaking, the most economical solution will be
for each phase to handle between 15A and 20A. All
surface mount designs will tend toward the lower end of
this current range; if through-hole MOSFETs can be used,
higher per-phase currents are possible. In cases where
board space is the limiting constraint, current can be
pushed as high as 30A per phase, but these designs
typically require heat sinks and forced air to cool the
MOSFETs. Paralleling MOSFETs in each leg is another way
to push per-phase currents even higher, but the power
and thermal stresses on each driver should be evaluated
carefully. In such a case, a 5V driver such as Intersil’s
ISL6609 could be considered. See “DRIVER LOSSES
CALCULATION” on page 8. In the reference design, all
four phases of the ISL6558 are used to deliver 100A of
total output current.
There are a number of variables to consider when
choosing the switching frequency for a particular
application. The size of the converter, the overall losses
of magnetics components, the switching losses of power
MOSFETs, the desired efficiency, the transient response,
and the maximum achievable duty cycle should all be
under consideration. It requires an iterative process,
monitoring changes of the above parameters, to obtain
an optimum switching frequency for a particular
application. Equations presented in this paper can be
used to develop a MathCAD worksheet that helps obtain
CASE 4
CASE 3
Irms3
where
Irms4
=
=
0
⎛
⎝
Ic
Ic
I Δ
2
2
Ic
I Δ
Id
+
•
Ia
(
=
=
Δ
------- -
=
12
D D
I
2
Ia
---------------- -
Ib Ia
Ic D
–
⎞
⎠
+
2
•
–
•
1 D
2
(
Ib
–
Ib
1 D
)
D
+
–
Δ
------- - D
12
I
)
2
•
4
Ia
I Δ
Ib
Application Note 1029
Ic
–
0
Id
a rough idea of the range of optimum frequencies and
efficiencies for a particular application. Note that the
higher the switching frequency, the higher the loop
bandwidth (typically 1/10 to 1/3 of the switching
frequency) potentially achieved, resulting in fewer output
capacitors to meet the same transient performance.
Equation 1 defines the duty cycle of each channel, and it
should be no greater than 75% (maximum duty cycle of
the ISL6558) at the minimum operational input line and
the maximum fully loaded output. The drops due to the
PCB resistances are included in the equation, and they
are very significant portions especially for high current
applications.
In Equation 1, Vo
is the input voltage, N is the number of active channels,
I
respectively. R
lower FETs, respectively. R
resistances of input and output inductors, respectively.
R
(including the connectors resistances), respectively.
OUTPUT FILTER DESIGN
The switching of each channel in a multi-phase converter
is timed to be symmetrically out of phase with each of
the other channels. In an N-phase converter, each
channel switches 1/N cycle after the previous channel
and 1/N cycle before the following channel. As a result,
the N-phase converter has a combined ripple frequency
N times the ripple frequency of any one phase. In
addition, the peak-to-peak amplitude of the combined
inductor currents is reduced in proportion to the number
of active phases. Increased ripple frequency and lower
current ripple amplitude mean that the designer can use
less per-channel inductance and lower total output
capacitance for any performance specification. Note that
the higher the inductor ripple current, the higher the
switching and conduction losses of each-channel’s bridge
MOSFETs. See “Lower MOSFET Power Calculation” on
page 7 and “Upper MOSFET Power Calculation” on
page 8.
where
IN
Bin
D
and Io are the input and output currents,
=
and R
Vo
---------------------------------------------------------------------------- -
V
+
V
2
(
2
Bo
R
+
=
Q2
(
R
are the input and output PCB resistances
V
Q1
Q2
+
IN
R
Vo
NL
and R
–
–
I
Lo
IN
(
R
R
=
is the output voltage at no load, V
+
Q1
=
Lin
R
Vo
)
Vo Io
------------------ -
η V
Q2
Bo
•
+
NL
•
Io
---- -
N
R
)
•
are the r
Lin
•
Bin
–
IN
Io
---- -
N
V
) I
DROOP
and R
•
IN
–
DS(ON)
Lo
⎛
⎝
•
Io
---- - I
N
--------------- -
Io
are the equivalent
–
Io
max
IN
’s of upper and
⎞
⎠
•
ESR
(EQ. 1)
July 31, 2009
IN
AN1029.3
IN