ISL6308CRZ Intersil, ISL6308CRZ Datasheet - Page 9

• Manufacturer Part Number: ISL6308CRZ
• Description: IC CTRLR PWM 3PHASE BUCK 40-QFN
• Manufacturer: Intersil

Specifications of ISL6308CRZ

• Pwm Type: Voltage Mode
• Number Of Outputs: 1
• Frequency - Max: 275kHz
• Duty Cycle: 66.6%
• Voltage - Supply: 4.75 V ~ 5.25 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: 0°C ~ 70°C
• Package / Case: 40-VFQFN, 40-VFQFPN
• Frequency-max: 275kHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

PHASE1, PHASE2, and PHASE3 (Pins 29, 28, 22)
Connect these pins to the sources of the upper MOSFETs.
These pins are the return path for the upper MOSFETs’
drives.
LGATE1, LGATE2, and LGATE3 (Pins 34, 23, 17)
These pins are used to control the lower MOSFETs and are
monitored for shoot-through prevention purposes. Connect
these pins to the lower MOSFETs’ gates. Do not use external
series gate resistors as this might lead to shoot-through.
PGOOD (Pin 35)
PGOOD is used as an indication of the end of soft-start. It is
an open-drain logic output that is low impedance until the
soft-start is completed and VOUT is equal to the VID setting.
Once in 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 becomes high impedance again. The potential at
this pin should not exceed that of the potential at VCC pin by
more than a typical forward diode drop at any time.
OVP (Pin 38)
Overvoltage protection pin. This pin pulls to VCC when an
overvoltage condition is detected. Connect this pin to the
gate of an SCR or MOSFET tied across V
prevent damage to a load device.
Operation
Multi-Phase Power Conversion
Modern low voltage DC/DC converter 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 ISL6308 controller helps simplify implementation by
integrating vital functions and requiring minimal output
components. The “Block Diagram” on page 3 provides a top
level view of multi-phase power conversion using the ISL6308
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 one third cycle after the previous channel and one
third 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
9
IN
and ground to
ISL6308
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
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 one third 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, which represents
an individual channel peak-to-peak inductor current.
In Equation 1, V
voltages respectively, L is the single-channel inductor value,
and F
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 1 to the
expression for the peak-to-peak current after the summation
of N symmetrically phase-shifted inductor currents in
Equation 2. Peak-to-peak ripple current decreases by an
amount proportional to the number of channels. Output
voltage ripple is a function of capacitance, capacitor
equivalent series resistance (ESR), and inductor ripple
current. Reducing the inductor ripple current allows the
designer to use fewer or less costly output capacitors.
I
I
PP
C PP
,
FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS
=
SW
=
(
--------------------------------------------------------- -
V
(
------------------------------------------------------------------- -
IN
PWM1, 5V/DIV
V
is the switching frequency.
L F
IN
–
⋅
I
V
–
L1
FOR 3-PHASE CONVERTER
L F
OUT
SW
N V
⋅
+ I
I
IN
⋅
L1
L2
⋅
SW
) V
, 7A/DIV
V
and V
OUT
⋅
+ I
IN
⋅
L3
OUT
V
PWM3, 5V/DIV
) V
, 7A/DIV
IN
⋅
OUT
OUT
I
L3
are the input and output
, 7A/DIV
PWM2, 5V/DIV
L1
I
, I
L2
L2
, 7A/DIV
September 30, 2008
, and I
L3
FN9208.4
(EQ. 1)
(EQ. 2)
)