ISL6323A Datasheet, PDF (12/35 Pages) Intersil Corporation – Monolithic Dual PWM Hybrid Controller Powering AMD SVI Split-Plane and PVI Uniplane Processors

ISL6323A
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. 3)
Another benefit of interleaving is to reduce input ripple
current. Input capacitance is determined in part by the
maximum input ripple current. Multi-phase 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 1.5V to a 36A 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.
INPUT-CAPACITOR CURRENT, 10A/DIV
CHANNEL 3
INPUT CURRENT
10A/DIV
CHANNEL 2
INPUT CURRENT
10A/DIV
CHANNEL 1
INPUT CURRENT
10A/DIV
1µs/DIV
FIGURE 2. CHANNEL INPUT CURRENTS AND INPUT-
CAPACITOR RMS CURRENT FOR 3-PHASE
CONVERTER
Figures 25, 26 and 27 in the section entitled “Input Capacitor
Selection” on page 32 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.
Active Pulse Positioning Modulated PWM
Operation
The ISL6323A uses a proprietary Active Pulse Positioning
(APP) modulation scheme to control the internal PWM
signals that command each channel’s driver to turn their
upper and lower MOSFETs on and off. The time interval in
which a PWM signal can occur is generated by an internal
clock, whose cycle time is the inverse of the switching
frequency set by the resistor between the FS pin and
ground. The advantage of Intersil’s proprietary Active Pulse
Positioning (APP) modulator is that the PWM signal has the
ability to turn on at any point during this PWM time interval,
and turn off immediately after the PWM signal has
transitioned high. This is important because it allows the
controller to quickly respond to output voltage drops
associated with current load spikes, while avoiding the ring
back affects associated with other modulation schemes.
The PWM output state is driven by the position of the error
amplifier output signal, V_COMP, minus the current correction
signal relative to the proprietary modulator ramp waveform
as illustrated in Figure 3. At the beginning of each PWM time
interval, this modified V_COMP signal is compared to the
internal modulator waveform. As long as the modified
V_COMP voltage is lower then the modulator waveform
voltage, the PWM signal is commanded low. The internal
MOSFET driver detects the low state of the PWM signal and
turns off the upper MOSFET and turns on the lower
synchronous MOSFET. When the modified V_COMP voltage
crosses the modulator ramp, the PWM output transitions
high, turning off the synchronous MOSFET and turning on
the upper MOSFET. The PWM signal will remain high until
the modified V_COMP voltage crosses the modulator ramp
again. When this occurs the PWM signal will transition low
again.
During each PWM time interval the PWM signal can only
transition high once. Once PWM transitions high it can not
transition high again until the beginning of the next PWM
time interval. This prevents the occurrence of double PWM
pulses occurring during a single period.
To further improve the transient response, ISL6323A also
implements Intersil’s proprietary Adaptive Phase Alignment
(APA) technique, which turns on all phases together under
transient events with large step current. With both APP and
APA control, ISL6323A can achieve excellent transient
performance and reduce the demand on the output
capacitors.
Adaptive Phase Alignment (APA)
To further improve the transient response, the ISL6323A
also implements Intersil’s proprietary Adaptive Phase
Alignment (APA) technique, which turns on all of the
channels together at the same time during large current step
transient events. As Figure 3 shows, the APA circuitry works
by monitoring the voltage on the APA pin and comparing it to
a filtered copy of the voltage on the COMP pin. The voltage
on the APA pin is a copy of the COMP pin voltage that has
been negatively offset. If the APA pin exceeds the filtered
COMP pin voltage an APA event occurs and all of the
channels are forced on.
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FN6878.0