ISL6323B Datasheet, PDF (11/35 Pages) Intersil Corporation – Monolithic Dual PWM Hybrid Controller Powering AMD SVI Split-Plane and PVI Uniplane Processors

ISL6323B
ISEN_NB-, ISEN_NB+
These pins are used for differentially sensing the North
Bridge output current. The sensed current is used for
protection and load line regulation if droop is enabled.
Connect ISEN_NB- to the node between the RC sense
element surrounding the inductor. Tie the ISEN_NB+ pin to
the VNB side of the sense capacitor.
UGATE_NB
Connect this pin to the corresponding upper MOSFET gate.
This pin provides the PWM-controlled gate drive for the
upper MOSFET and is monitored for shoot-through
prevention purposes.
BOOT_NB
This pin provides the bias voltage for the corresponding
upper MOSFET drive. Connect this pin to appropriately-
chosen external bootstrap capacitor. The internal bootstrap
diode connected to the PVCC_NB pin provides the
necessary bootstrap charge.
PHASE_NB
Connect this pin to the source of the corresponding upper
MOSFET. This pin is the return path for the upper MOSFET
drive. This pin is used to monitor the voltage drop across the
upper MOSFET for overcurrent protection.
LGATE_NB
Connect this pin to the corresponding MOSFET’s gate. This
pin provides the PWM-controlled gate drive for the lower
MOSFET. This pin is also monitored by the adaptive
shoot-through protection circuitry to determine when the
lower MOSFET has turned off.
Operation
The ISL6323B utilizes a multi-phase architecture to provide
a low cost, space saving power conversion solution for the
processor core voltage. The controller also implements a
simple single phase architecture to provide the Northbridge
voltage on the same chip.
Multi-phase Power Conversion
Microprocessor load current profiles have changed to the
point that the advantages of multi-phase power conversion
are impossible to ignore. The technical challenges
associated with producing a single-phase converter that is
both cost-effective and thermally viable have forced a
change to the cost-saving approach of multi-phase. The
ISL6323B controller helps simplify implementation by
integrating vital functions and requiring minimal external
components. The “Controller Block Diagram” on page 3
provides a top level view of the multi-phase power
conversion using the ISL6323B controller.
Interleaving
The switching of each channel in a multi-phase converter is
timed to be symmetrically out of phase with each of the other
channels. In a 3-phase converter, each channel switches 1/3
cycle after the previous channel and 1/3 cycle before the
following channel. As a result, the three-phase converter has
a combined ripple frequency three times greater than the
ripple frequency of any one phase. In addition, the peak-to-
peak amplitude of the combined inductor currents is reduced
in proportion to the number of phases (Equations 2 and 3).
Increased ripple frequency and lower ripple amplitude mean
that the designer can use less per-channel inductance and
lower total output capacitance for any performance
specification.
Figure 1 illustrates the multiplicative effect on output ripple
frequency. The three channel currents (IL1, IL2, and IL3)
combine to form the AC ripple current and the DC load
current. The ripple component has three times the ripple
frequency of each individual channel current. Each PWM
pulse is terminated 1/3 of a cycle after the PWM pulse of the
previous phase. The peak-to-peak current for each phase is
about 7A, and the DC components of the inductor currents
combine to feed the load.
IL1 + IL2 + IL3, 7A/DIV
IL3, 7A/DIV
PWM3, 5V/DIV
IL2, 7A/DIV
IL1, 7A/DIV
PWM2, 5V/DIV
PWM1, 5V/DIV
1μs/DIV
FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS
FOR 3-PHASE CONVERTER
To understand the reduction of ripple current amplitude in the
multi-phase circuit, examine Equation 2, which represents
an individual channel peak-to-peak inductor current.
I_{P-P =}
(_{---V----I--N-----}–_{-----V----O-----U----T---})_{----V----O----U-----T-}
L f_S V_IN
(EQ. 2)
In Equation 2, 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.
The output capacitors conduct the ripple component of the
inductor current. In the case of multi-phase converters, the
capacitor current is the sum of the ripple currents from each
of the individual channels. Compare Equation 2 to the
expression for the peak-to-peak current after the summation
of N symmetrically phase-shifted inductor currents in
Equation 3. Peak-to-peak ripple current decreases by an
amount proportional to the number of channels.
Output-voltage ripple is a function of capacitance, capacitor
11
FN6879.0
March 23, 2009