ISL6313 INTERSIL [Intersil Corporation], ISL6313 Datasheet - Page 30

• Manufacturer Part Number: ISL6313
• Description: Two-Phase Buck PWM Controller with Integrated MOSFET Drivers for Intel VR11 and AMD Applications
• Manufacturer: INTERSIL [Intersil Corporation]

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. 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
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 two-phase design, use Figure 25 to determine the
input-capacitor RMS current requirement set by the duty
cycle, maximum sustained output current (I
of the peak-to-peak inductor current (I
L
≤
1.25 N C
---------------------------- -
=
10
(
ΔI
100
⋅
[
500
10.61
10
FIGURE 24. R
)
2
50
⋅
⋅
–
ΔV
(
1.035
100
MAX
SWITCHING FREQUENCY (kHz)
⋅
T
–
log
vs SWITCHING FREQUENCY
(
ΔI ESR
(
f S
⋅
T
30
. Figure 24 and Equation 47
)
)
]
)
⋅
⎛
⎝
V
L,PP
IN
–
) to I
V
O
1000
O
), and the ratio
⎞
⎠
O
.
2000
(EQ. 46)
(EQ. 47)
T
.
ISL6313
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. Figure 26 provides the
same input RMS current information for single-phase
designs. 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 26. NORMALIZED INPUT-CAPACITOR RMS
FIGURE 25. NORMALIZED INPUT-CAPACITOR RMS
0.6
0.4
0.2
0.3
0.2
0.1
0
0
0
0
I
I
I
L(PP)
L(PP)
L(PP)
I
I
I
CURRENT FOR SINGLE-PHASE CONVERTER
CURRENT FOR 2-PHASE CONVERTER
L(PP)
L(PP)
L(PP)
= 0
= 0.5 I
= 0.75 I
0.2
0.2
= 0
= 0.5 I
= 0.75 I
O
O
DUTY CYCLE (V
DUTY CYCLE (V
O
0.4
0.4
O
0.6
0.6
IN
IN/
/V
V
O
O
)
)
0.8
0.8
March 5, 2007
FN6448.0
1.0
1.0