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ISL6334 Datasheet

  • ISL6334

  • Intersil Corporation [VR11.1, 4-Phase PWM Controller with L...

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ISL6334, ISL6334A
bulk capacitors allows them to supply the increased current
with less output voltage deviation.
The ESR of the bulk capacitors also creates the majority of
the output-voltage ripple. As the bulk capacitors sink and
source the inductor AC ripple current (see 鈥淚nterleaving鈥?on
page 12 and Equation 2), a voltage develops across the
bulk-capacitor ESR equal to I
C,PP
(ESR). Thus, once the
output capacitors are selected, the maximum allowable
ripple voltage, V
PP(MAX)
, determines the lower limit on the
inductance, as shown in Equation 37.
鈳?/div>
V
鈥?/div>
N V
鈳?/div>
OUT
鈳?/div>
V
OUT
鈳?/div>
IN
L
鈮?(
ESR
)
-----------------------------------------------------------
-
f
S
V
IN
V
PP
(
MAX
)
(EQ. 37)
0.3
INPUT-CAPACITOR CURRENT (I
RMS
/I
O
)
0.2
0.1
I
L,PP
= 0
I
L,PP
= 0.5 I
O
I
L,PP
= 0.75 I
O
0
0
0.2
0.4
0.6
DUTY CYCLE (V
O
/V
IN
)
0.8
1.0
INPUT-CAPACITOR CURRENT (I
RMS/
I
O
)
Since the capacitors are supplying a decreasing portion of
the load current while the regulator recovers from the
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
螖V
MAX
. This places an upper limit on inductance.
Equation 38 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 39
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.
2NCV
O
L
鈮?/div>
--------------------
螖V
MAX
鈥?/div>
螖I (
ESR
)
-
( 螖I )
2
(
1.25
)
NC
-
L
鈮?/div>
-------------------------
螖V
MAX
鈥?/div>
螖I (
ESR
) 鈳?/div>
V
IN
鈥?/div>
V
O
鈳?/div>
鈳?/div>
鈳?/div>
( 螖I )
2
(EQ. 38)
FIGURE 18. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 2-PHASE CONVERTER
0.3
I
L,PP
= 0
I
L,PP
= 0.25 I
O
I
L,PP
= 0.5 I
O
I
L,PP
= 0.75 I
O
0.2
0.1
0
0
0.2
0.4
0.6
DUTY CYCLE (V
O/
V
IN
)
0.8
1.0
FIGURE 19. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 3-PHASE CONVERTER
(EQ. 39)
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 鈥淢OSFETs鈥?on page 25, 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 鈥淥utput Filter
Design鈥?on page 27. Choose the lowest switching frequency
that allows the regulator to meet the transient-response
requirements.
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
O
), and the ratio of the
per-phase peak-to-peak inductor current (I
L,PP
) to I
O
. 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 1.25 times greater
than the maximum input voltage.
Figures 19 and 20 provide the same input RMS current
information for three and four 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.
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.
28
FN6482.0
February 26, 2008

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