ISL6563 Intersil Corporation, ISL6563 Datasheet - Page 18

• Manufacturer Part Number: ISL6563
• Description: Two-Phase Multi-Phase Buck PWM Controller with Integrated MOSFET Drivers
• Manufacturer: Intersil Corporation

with lower ESR available in larger case sizes. However, the
equivalent series inductance (ESL) of these capacitors
increases with case size and can reduce the usefulness of the
capacitor to high slew-rate transient loading. Unfortunately,
ESL is not a specified parameter. Consult the capacitor
manufacturer and/or measure the capacitor’s impedance with
frequency to help select a suitable component.
Output Inductor Selection
One of the parameters limiting the converter’s response to a
load transient is the time required to change the inductor
current. In a multi-phase converter, small inductors reduce
the response time with less impact to the total output ripple
current (as compared to single-phase converters).
The output inductor of each power channel controls the
ripple current. The control IC is stable for channel ripple
current (peak-to-peak) up to twice the average current. A
single channel’s ripple current is approximated by:
The current from multiple channels tend to cancel each other
and reduce the total ripple current. The total output ripple
current can be determined using the curve in Figure 10; it
provides the total ripple current as a function of duty cycle
and number of active channels, normalized to the parameter
K
where L is the channel inductor value.
Find the intersection of the active channel curve and duty
cycle for your particular application. The resulting ripple
current multiplier from the y-axis is then multiplied by the
normalization factor, K
ripple current for the given application.
∆
I
K
L PP
I
NORM
NORM
,
TOTAL
=
1.0
0.8
0.6
0.4
0.2
FIGURE 10. RIPPLE CURRENT vs DUTY CYCLE
0
V
------------------------------- -
at zero duty cycle.
=
=
0
IN
F
------------------- -
L F
K
V
SW
–
⋅
NORM
OUT
V
SW
OUT
⋅
L
0.1
⋅
×
K
V
--------------- -
CM
V
NORM
OUT
IN
DUTY CYCLE (V
0.2
, to determine the total output
18
0.3
O
/V
IN
)
0.4
0.5
ISL6563
Input Capacitor Selection
The important parameters for the bulk input capacitors are
the voltage rating and the RMS current rating. For reliable
operation, select bulk input capacitors with voltage and
current ratings above the maximum input voltage and
largest RMS current required by the circuit. The capacitor
voltage rating should be at least 1.25 times greater than the
maximum input voltage. The input RMS current required for
a multi-phase converter can be approximated with the aid
of Figure 11.
As 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. Figure 11 can be used to determine the input-
capacitor RMS current function of duty cycle, maximum
sustained output current (I
peak inductor current (I
current, I
demonstrating the dramatic reduction in input capacitor RMS
current in a 2-phase DC/DC converter, as compared to a
single-phase regulator.
Use a mix of input bypass capacitors to control the input
voltage ripple. Use ceramic capacitance for the high
frequency decoupling and bulk capacitors to supply the
RMS current. Minimize the connection path inductance of
the high frequency decoupling ceramic capacitors (from
drain of upper MOSFET to source of lower MOSFET).
For bulk capacitance, several electrolytic or high-capacity MLC
capacitors may be needed. For surface mount designs, solid
tantalum capacitors can be used, but caution must be
exercised with regard to the capacitor surge current rating.
These capacitors must be capable of handling the surge-
current at power-up.
FIGURE 11. NORMALIZED INPUT RMS CURRENT VS. DUTY
0.3
0.2
0.1
0
0
O
. Figure 11 can also be used as a reference
CYCLE FOR A 2-PHASE CONVERTER
0.1
DUTY CYCLE (V
L,PP
I
0.2
O
I
L,PP
L,PP
) to the maximum sustained load
), and the ratio of the peak-to-
I
L,PP
= 0.75 x I
= 0.5 x I
= 0
0.3
O
O
O
/V
IN
)
0.4
0.5