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ISL6566 Fiches technique(PDF) 10 Page - Intersil Corporation

No de pièce ISL6566
Description  Three-Phase Buck PWM Controller with Integrated MOSFET Drivers for VRM9, VRM10, and AMD Hammer Applications
Download  28 Pages
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Fabricant  INTERSIL [Intersil Corporation]
Site Internet  http://www.intersil.com/cda/home
Logo INTERSIL - Intersil Corporation

ISL6566 Fiches technique(HTML) 10 Page - Intersil Corporation

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10
FN9178.3
July 25, 2005
Figures 22 and 23 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.
PWM Operation
The timing of each converter leg is set by the number of
active channels. The default channel setting for the ISL6566
is three. One switching cycle is defined as the time between
the internal PWM1 pulse termination signals. The pulse
termination signal is the internally generated clock signal
that triggers the falling edge of PWM1. The cycle time of the
pulse termination signal is the inverse of the switching
frequency set by the resistor between the FS pin and
ground. Each cycle begins when the clock signal commands
PWM1 to go low. The PWM1 transition signals the internal
channel-1 MOSFET driver to turn off the channel-1 upper
MOSFET and turn on the channel-1 synchronous MOSFET.
In the default channel configuration, the PWM2 pulse
terminates 1/3 of a cycle after the PWM1 pulse. The PWM3
pulse terminates 1/3 of a cycle after PWM2.
If PVCC3 is left open or connected to ground, two channel
operation is selected and the PWM2 pulse terminates 1/2 of
a cycle after the PWM1 pulse terminates. If both PVCC3 and
PVCC2 are left open or connected to ground, single channel
operation is selected.
Once a PWM pulse transitions low, it is held low for a
minimum of 1/3 cycle. This forced off time is required to
ensure an accurate current sample. Current sensing is
described in the next section. After the forced off time
expires, the PWM output is enabled. The PWM output state
is driven by the position of the error amplifier output signal,
VCOMP, minus the current correction signal relative to the
sawtooth ramp as illustrated in Figure 3. When the modified
VCOMP voltage crosses the sawtooth ramp, the PWM output
transitions high. The internal MOSFET driver detects the
change in state of the PWM signal and turns off the
synchronous MOSFET and turns on the upper MOSFET.
The PWM signal will remain high until the pulse termination
signal marks the beginning of the next cycle by triggering the
PWM signal low.
Channel-Current Balance
One important benefit of multi-phase operation is the thermal
advantage gained by distributing the dissipated heat over
multiple devices and greater area. By doing this the designer
avoids the complexity of driving parallel MOSFETs and the
expense of using expensive heat sinks and exotic magnetic
materials.
In order to realize the thermal advantage, it is important that
each channel in a multi-phase converter be controlled to
carry about the same amount of current at any load level. To
achieve this, the currents through each channel must be
sampled every switching cycle. The sampled currents, In,
from each active channel are summed together and divided
by the number of active channels. The resulting cycle
average current, IAVG, provides a measure of the total load-
current demand on the converter during each switching
cycle. Channel-current balance is achieved by comparing
the sampled current of each channel to the cycle average
current, and making the proper adjustment to each channel
pulse width based on the error. Intersil’s patented current-
balance method is illustrated in Figure 3, with error
correction for channel 1 represented. In the figure, the cycle
average current, IAVG, is compared with the channel 1
sample, I1, to create an error signal IER.
The filtered error signal modifies the pulse width
commanded by VCOMP to correct any unbalance and force
IER toward zero. The same method for error signal
correction is applied to each active channel.
Current Sampling
In order to realize proper current-balance, the currents in
each channel must be sampled every switching cycle. This
sampling occurs during the forced off-time, following a PWM
transition low. During this time the current-sense amplifier
uses the ISEN inputs to reproduce a signal proportional to
the inductor current, IL. This sensed current, ISEN, is simply
a scaled version of the inductor current. The sample window
opens exactly 1/6 of the switching period, tSW, after the
PWM transitions low. The sample window then stays open
the rest of the switching cycle until PWM transitions high
again, as illustrated in Figure 4.
The sampled current, at the end of the tSAMPLE, is
proportional to the inductor current and is held until the next
switching period sample. The sampled current is used only
for channel-current balance.
FIGURE 3. CHANNEL-1 PWM FUNCTION AND CURRENT-
BALANCE ADJUSTMENT
÷ N
IAVG
I3
I2
Σ
-
+
+
-
+
-
f(s)
PWM1
I1
VCOMP
SAWTOOTH SIGNAL
IER
NOTE: Channel 2 and 3 are optional.
FILTER
TO GATE
CONTROL
LOGIC
ISL6566
ISL6566


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