ISL6559 Intersil Corporation, ISL6559 Datasheet - Page 18

• Manufacturer Part Number: ISL6559
• Description: Multi-Phase PWM Controller
• Manufacturer: Intersil Corporation

Input Supply Voltage Selection
The VCC input of the ISL6559 can be connected to either a
+5V supply directly or through a current limiting resistor to a
+12V supply. An integrated 5.8V shunt regulator maintains
the voltage on the VCC pin when a +12V supply is used. A
300Ω resistor is suggested for limiting the current into the
VCC pin to approximately 20mA.
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 as outlined in Output Filter Design. 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
Application on page 3). Figure 15 and Equation 29 are
provided to assist in the selecting the correct value for R
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.
R
T
1000
100
=
10
10
10
FIGURE 14. R
[
11.09 1.13
–
SWITCHING FREQUENCY (kHz)
log
( )
T
f
100
S
vs SWITCHING FREQUENCY
]
T
18
(see the figure Typical
1000
(EQ. 29)
10000
T
.
ISL6559
For a two phase design, use Figure 15 to determine the
input-capacitor RMS current requirement given the duty
cycle, maximum sustained output current (I
of the combined peak-to-peak inductor current (I
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 16 and 17 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 described above.
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
FIGURE 15. NORMALIZED INPUT-CAPACITOR RMS
FIGURE 16. NORMALIZED INPUT-CAPACITOR RMS
0.3
0.2
0.1
0.3
0.2
0.1
0
0
0
0
I
I
C,PP
C,PP
I
I
I
C,PP
C,PP
C,PP
= 0
= 0.25 I
CURRENT VS DUTY CYCLE FOR 2-PHASE
CONVERTER
CURRENT VS DUTY CYCLE FOR 3-PHASE
CONVERTER
= 0
= 0.5 I
= 0.75 I
0.2
0.2
O
O
O
DUTY CYCLE (V
DUTY CYCLE (V
0.4
0.4
I
I
C,PP
C,PP
= 0.5 I
= 0.75 I
0.6
0.6
IN
IN
/ V
/ V
O
O
O
)
)
O
O
), and the ratio
0.8
0.8
C,PP
) to I
1.0
1.0
O
.