ISL6557A Datasheet, PDF (8/19 Pages) Intersil Corporation – Multi-Phase PWM Controller for Core-Voltage Regulation

ISL6557A
commands the channel-1 PWM output to go low. This
signals the channel-1 MOSFET driver to turn off the
channel-1 upper MOSFET and turn on the channel-1
synchronous MOSFET. If two-channel operation is selected,
the PWM2 pulse terminates 1/2 of a cycle later. If three
channels are selected the PWM2 pulse terminates 1/3 of a
cycle after PWM1, and the PWM3 output will follow after
another 1/3 of a cycle. When four channels are selected, the
pulse-termination times are spaced in 1/4 cycle increments.
Once a channel’s PWM pulse terminates, it remains low for a
minimum of 1/4 cycle. This forced off time is required to
assure an accurate current sample as described in Current
Sensing. Following the 1/4-cycle forced off time, the
controller enables the PWM output. Once enabled, the PWM
output transitions high when the sawtooth signal crosses the
adjusted error-amplifier output signal, V_COMP as illustrated
in Figures 1 and 5. This is the signal for the MOSFET driver
to turn off the synchronous MOSFET and turn on the upper
MOSFET. The output will remain high until the clock signals
the beginning of the next cycle by commanding the PWM
pulse to terminate.
CURRENT SENSING
Intersil multi-phase controllers sense current by sampling the
voltage across the lower MOSFET during its conduction
interval. MOSFET r_DS(ON) sensing is a no-added-cost
method to sense current for load-line regulation, channel-
current balance, module current sharing, and overcurrent
protection. If desired, an independent current-sense resistor
in series with the lower-MOSFET source can serve as a
sense element in place of the MOSFET r_DS(ON).
V_IN
I_n
ISEN
=
IL
-^r--^D-----^S----⁽---^O-----^N-----⁾
RISEN
CHANNEL N
UPPER MOSFET
SAMPLE
&
HOLD
-
+
I_L
ISEN(n)
R_ISEN
-
IL rDS_(ON)
+
CHANNEL N
LOWER MOSFET
ISL6557A INTERNAL CIRCUIT EXTERNAL CIRCUIT
FIGURE 4. INTERNAL AND EXTERNAL CURRENT-SENSING
CIRCUITRY
The ISEN input for each channel uses a ground-referenced
amplifier to reproduce a signal proportional to the channel
current (Figure 4). After sufficient settling time, the sensed
current is sampled, and the sample is used for current
balance, load-line regulation and overcurrent protection. The
ISL6557A samples channel current once each cycle.
Figure 4 shows how the sampled current, I_n, is created from
the channel current I_L. The circuitry in Figure 4 represents
the current measurement and sampling circuitry for channel
n in an N-channel converter. This circuitry is repeated for
each channel in the converter but may not be active in
channels 3 and 4 depending on the particular
implementation (see PWM Operation).
CHANNEL-CURRENT BALANCE
Another 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 multiple parallel
MOSFETs and the expense of using expensive heat sinks
and exotic magnetic materials.
In order to fully realize the thermal advantage, it is important
that each channel in a multi-phase converter be controlled to
deliver about the same current at any load level. Intersil
multi-phase controllers guarantee current balance by
comparing each channel’s current to the average current
delivered by all channels and making an appropriate
adjustment to each channel’s pulse width based on the error.
Intersil’s patented current-balance method is illustrated in
Figure 5 where the average of the 2, 3, or 4 sampled channel
currents combines with the channel 1 sample, I₁, to create
an error signal I_ER. The filtered error signal modifies the
pulse width commanded by V_COMP to correct any
unbalance and force I_ER toward zero.
In some circumstances, it may be necessary to deliberately
design some channel-current unbalance into the system. In
a highly compact design, one or two channels may be able to
cool more effectively than the other(s) due to nearby air flow
or heat sinking components. The other channel(s) may have
more difficulty cooling with comparatively less air flow and
heat sinking. The hotter channels may also be located close
to other heat-generating components tending to drive their
temperature even higher. In these cases, a proper selection
of the current sense resistors (R_ISEN in Figure 4) introduces
channel current unbalance into the system. Increasing the
value of R_ISEN in the cooler channels and decreasing it in
the hotter channels moves all channels into thermal balance
at the expense of current balance.
OVERCURRENT PROTECTION
The average current, I_AVG in Figure 5, is continually
compared with a constant 75µA reference current. If the
average current at any time exceeds the reference current,
the comparator triggers the converter to shut down. All PWM
signals are placed in a high-impedance state which signals
the drivers to turn off both upper and lower MOSFETs. The
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