ISL6336 Datasheet, PDF (29/31 Pages) Intersil Corporation – 6-Phase PWM Controller with Light Load Efficiency Enhancement and Current Monitoring

ISL6336, ISL6336A
and small output-voltage ripple as outlined in “Output Filter
Design” on page 28. 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_T (see “Typical Application - 5-
Phase Buck Converter with DCR Sensing and Integrated
TCOMP” on page 5 and “Typical Application - 4-Phase Buck
Converter with coupled inductors” on page 6). Equation 3 is
provided to assist in selecting the correct value for 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 that is related to duty cycle and the number of
active phases.
0.3
0.2
0.1
I_L(P-P) = 0
I_L(P-P) = 0.5 I_O
I_L(P-P) = 0.75 I_O
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_O/V_IN)
FIGURE 21. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 2-PHASE CONVERTER
0.3
I_L(P-P) = 0
I_L(P-P) = 0.25 I_O
I_L(P-P) = 0.5 I_O
I_L(P-P) = 0.75 I_O
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_O/V_IN)
FIGURE 22. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 3-PHASE CONVERTER
0.3
I_L(P-P) = 0
I_L(P-P) = 0.25 I_O
0.2
I_L(P-P) = 0.5 I_O
I_L(P-P) = 0.75 I_O
0.1
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_O/V_IN)
FIGURE 23. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 4-PHASE CONVERTER
For a two phase design, use Figure 21 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(P-P)) 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 1.25x greater than the
maximum input voltage.
Figures 22 and 23 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. They result from the high
current slew rates produced by the upper MOSFETs turning
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. More than one of these low ESL capacitors
may be needed.
MULTIPHASE RMS IMPROVEMENT
Figure 24 is provided as a reference to demonstrate the
dramatic reductions in input-capacitor RMS current upon the
implementation of the multiphase topology. For example,
compare the input RMS current requirements of a two-phase
converter versus that of a single phase. Assume both
converters have a duty cycle of 0.25, maximum sustained
output current of 40A, and a ratio of I_L(P-P) to I_O of 0.5. The
single phase converter would require 17.3A_RMS current
capacity while the two-phase converter would only require
10.9A_RMS. The advantages become even more pronounced
when output current is increased and additional phases are
added to keep the component cost down relative to the
single phase approach.
29
FN6504.1
May 28, 2009