isl6312a Intersil Corporation, isl6312a Datasheet - Page 10

• Manufacturer Part Number: isl6312a
• Description: Four-phase Buck Pwm Controller With Integrated Mosfet Drivers For Intel Vr10, Vr11, And Amd Applications
• Manufacturer: Intersil Corporation

LGATE1, LGATE2 and LGATE3
These pins are used to control the lower MOSFETs. Connect
these pins to the corresponding lower MOSFETs’ gates.
PWM4
Pulse-width modulation output. Connect this pin to the PWM
input pin of an Intersil driver IC if 4-phase operation is
desired.
EN_PH4
This pin has two functions. First, a resistor divider connected
to this pin will provide a POR power up synch between the
on-chip and external driver. The resistor divider should be
designed so that when the POR-trip point of the external
driver is reached the voltage on this pin should be 1.21V.
The second function of this pin is disabling PWM4 for
3-phase operation. This can be accomplished by connecting
this pin to a +5V supply.
SS
A resistor, placed from SS to ground, will set the soft-start
ramp slope for the Intel DAC modes of operation. Refer to
Equations 18 and 19 for proper resistor calculation.
For AMD modes of operation, the soft-start ramp frequency
is preset, so this pin can be left unconnected.
OVPSEL
This pin selects the OVP trip point during normal operation.
Leaving it unconnected or tieing it to ground selects the
default setting of VDAC+175mV for Intel Modes of operation
and VDAC+250mV for AMD modes of operation. Connecting
this pin to VCC will select an OVP trip setting of VID+350mV
for all modes of operation.
DRSEL
This pin selects the adaptive dead time scheme the internal
drivers will use. If driving MOSFETs, tie this pin to ground to
select the PHASE detect scheme or to a +5V supply through
a 50kΩ resistor to select the LGATE detect scheme.
PGOOD
During normal operation PGOOD indicates whether the
output voltage is within specified overvoltage and
undervoltage limits. If the output voltage exceeds these limits
or a reset event occurs (such as an overcurrent event),
PGOOD is pulled low. PGOOD is always low prior to the end
of soft-start.
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 multiphase converters can accomplish. The ISL6312A
10
ISL6312A
ISL6312A
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
multiphase power conversion using the ISL6312A controller.
Interleaving
The switching of each channel in a multiphase 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 (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.
To understand the reduction of ripple current amplitude in the
multiphase circuit, examine the equation representing an
individual channel peak-to-peak inductor current.
In Equation 1, V
voltages respectively, L is the single-channel inductor value,
and f
I
(
P P
FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS
–
S
)
is the switching frequency.
=
(
--------------------------------------------------------- -
PWM1, 5V/DIV
V
IN
IL1 + IL2 + IL3, 7A/DIV
FOR 3-PHASE CONVERTER
–
L f
V
⋅
IN
IL1, 7A/DIV
OUT
S
and V
⋅
V
) V
IN
⋅
PWM3, 5V/DIV
OUT
OUT
1μs/DIV
are the input and output
IL3, 7A/DIV
PWM2, 5V/DIV
IL2, 7A/DIV
August 1, 2007
FN9290.3
(EQ. 1)