ISL6326B Datasheet, PDF (11/30 Pages) Intersil Corporation – 4-Phase PWM Controller with 8-Bit DAC Code Capable of Precision DCR Differential Current Sensing

ISL6326B
To understand the reduction of ripple current amplitude in the
multiphase circuit, examine the equation representing an
individual channel’s peak-to-peak inductor current.
I_{PP =}
(_{---V----I--N-----}–_{-----V----O-----U----T---})_{----V----O----U-----T-}
L f_S V_IN
(EQ. 1)
In Equation 1, V_IN and V_OUT are the input and output
voltages respectively, L is the single-channel inductor value,
and f_S is the switching frequency.
INPUT-CAPACITOR CURRENT, 10A/DIV
CHANNEL 1
INPUT CURRENT
10A/DIV
CHANNEL 2
INPUT CURRENT
10A/DIV
CHANNEL 3
INPUT CURRENT
10A/DIV
1µs/DIV
FIGURE 2. CHANNEL INPUT CURRENTS AND INPUT-
CAPACITOR RMS CURRENT FOR 3-PHASE
CONVERTER
The output capacitors conduct the ripple component of the
inductor current. In the case of multiphase converters, the
capacitor current is the sum of the ripple currents from each
of the individual channels. Compare Equation 1 to the
expression for the peak-to-peak current after the summation
of N symmetrically phase-shifted inductor currents in
Equation 2. Peak-to-peak ripple current decreases by an
amount proportional to the number of channels. Output-
voltage ripple is a function of capacitance, capacitor
equivalent series resistance (ESR), and inductor ripple
current. Reducing the inductor ripple current allows the
designer to use fewer or less costly output capacitors.
I_{C, PP}=
(_{---V----I--N-----}–_{-----N------V----O-----U----T---})_{----V----O----U-----T-}
L f_S V_IN
(EQ. 2)
Another benefit of interleaving is to reduce input ripple
current. Input capacitance is determined in part by the
maximum input ripple current. Multiphase topologies can
improve overall system cost and size by lowering input ripple
current and allowing the designer to reduce the cost of input
capacitance. The example in Figure 2 illustrates input
currents from a three-phase converter combining to reduce
the total input ripple current.
The converter depicted in Figure 2 delivers 36A to a 1.5V load
from a 12V input. The RMS input capacitor current is 5.9A.
Compare this to a single-phase converter also stepping down
12V to 1.5V at 36A. The single-phase converter has 11.9A
RMS input capacitor current. The single-phase converter
must use an input capacitor bank with twice the RMS current
capacity as the equivalent three-phase converter.
Figures 17, 18 and 19 in the section entitled Input Capacitor
Selection can be used to determine the input-capacitor RMS
current based on load current, duty cycle, and the number of
channels. They are provided as aids in determining the
optimal input capacitor solution. Figure 20 shows the single
phase input-capacitor RMS current for comparison.
PWM Modulation Scheme
The ISL6326B adopts Intersil's proprietary Active Pulse
Positioning (APP) modulation scheme to improve transient
performance. APP control is a unique dual-edge PWM
modulation scheme with both PWM leading and trailing
edges being independently moved to give the best response
to transient loads. The PWM frequency, however, is constant
and set by the external resistor between the FS pin and
GND. To further improve the transient response, the
ISL6326B also implements Intersil's proprietary Adaptive
Phase Alignment (APA) technique. APA, with sufficiently
large load step currents, can turn on all phases together.
With both APP and APA control, ISL6326B can achieve
excellent transient performance and reduce the demand on
the output capacitors.
Under steady state conditions the operation of the ISL6326B
PWM modulator appears to be that of a conventional trailing
edge modulator. Conventional analysis and design methods
can therefore be used for steady state and small signal
operation.
PWM Operation
The timing of each channel is set by the number of active
channels. The default channel setting for the ISL6326B is
four. The switching cycle is defined as the time between
PWM pulse termination signals of each channel. The cycle
time of the pulse signal is the inverse of the switching
frequency set by the resistor between the FS pin and
ground. The PWM signals command the MOSFET driver to
turn on/off the channel MOSFETs.
For 4-channel operation, the channel firing order is 4-3-2-1:
PWM3 pulse happens 1/4 of a cycle after PWM4, PWM2
output follows another 1/4 of a cycle after PWM3, and
PWM1 delays another 1/4 of a cycle after PWM2. For
3-channel operation, the channel firing order is 3-2-1.
Connecting PWM4 to VCC selects three channel operation
and the pulse times are spaced in 1/3 cycle increments. If
PWM3 is connected to VCC, two channel operation is
selected and the PWM2 pulse happens 1/2 of a cycle after
PWM pulse.
Switching Frequency
Switching frequency is determined by the selection of the
frequency-setting resistor, R_T, which is connected from FS
pin to GND (see the figures labelled Typical Applications on
11
FN9286.0
April 21, 2006