ISL6313B_14 Datasheet, PDF (10/33 Pages) Intersil Corporation – Two-Phase Buck PWM Controller with Integrated MOSFET Drivers for Intel VR11 and AMD Applications

ISL6313B
channel. Tie the ISEN+ pins to the V_CORE side of their
corresponding channel’s sense capacitor.
Tieing ISEN2- to VCC programs the part for single phase
operation.
UGATE1 and UGATE2
Connect these pins to the corresponding upper MOSFET
gates. These pins are used to control the upper MOSFETs
and are monitored for shoot-through prevention purposes.
BOOT1 and BOOT2
These pins provide the bias voltage for the corresponding
upper MOSFET drives. Connect these pins to appropriately
chosen external bootstrap capacitors. Internal bootstrap
diodes connected to the PVCC pin provides the necessary
bootstrap charge.
PHASE1 and PHASE2
Connect these pins to the sources of the corresponding
upper MOSFETs. These pins are the return path for the
upper MOSFET drives.
LGATE1 and LGATE2
These pins are used to control the lower MOSFETs. Connect
these pins to the corresponding lower MOSFETs’ gates.
SS
A resistor, R_SS, placed from SS to ground or VCC, will set
the soft-start ramp slope. Refer to Equations 20 and 21 for
proper resistor calculation.
The state of the SS pin also selects which of the available DAC
tables will be used to decode the VID inputs and puts the
controller into the corresponding mode of operation. For Intel
VR11 mode of operation the R_SS resistor should be tied to
ground. AMD compliance is selected if the R_SS resistor is tied
to VCC.
PGOOD
For Intel mode of operation, PGOOD indicates whether VSEN
is within specified overvoltage and undervoltage limits after a
fixed delay from the end of soft-start. If VSEN exceeds these
limits, an overcurrent event occurs, or if the part is disabled,
PGOOD is pulled low. PGOOD is always low prior to the end
of soft-start.
For AMD modes of operation, PGOOD will always be high as
long as VSEN is within the specified undervoltage/overvoltage
window and soft-start has ended. PGOOD only goes low if
VSEN is outside this window.
Operation
Multiphase Power Conversion
Microprocessor load current profiles have changed to the
point that using single-phase regulators is no longer a viable
solution. Designing a regulator that is cost-effective,
thermally sound, and efficient has become a challenge that
only multi-phase converters can accomplish. The ISL6313B
controller helps simplify implementation by integrating vital
functions and requiring minimal external components. The
block diagram on page 3 provides a top level view of multi-
phase power conversion using the ISL6313B controller.
I_L1 + I_L2 + I_L3, 7A/DIV
I_L3, 7A/DIV
PWM3, 5V/DIV
I_L2, 7A/DIV
I_L1, 7A/DIV
PWM2, 5V/DIV
PWM1, 5V/DIV
1µs/DIV
FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS
FOR 3-PHASE CONVERTER
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 1 and 2).
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 (I_L1, I_L2, and I_L3)
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.
To understand the reduction of ripple current amplitude in the
multi-phase circuit, examine Equation 1 representing an
individual channel peak-to-peak inductor current.
I_{PP =} ⁽_---V----^I_--^N_-----^–_{-L----V-⋅---}^O_-f-S---^U_--⋅--^T_--V-⁾_---I-^â‹_-N---V----^O_----^U_-----^T_-
(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.
10
FN6809.0
November 6, 2008