DATASHEET

ISL6308
ESIGNS
NEW D
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MENDE LACEMENT P
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Data Sheet
D REP
N OT R
ME N D E
A
R E C OM
ISL6308
®
September 30, 2008
Three-Phase Buck PWM Controller with
High Current Integrated MOSFET Drivers
The ISL6308 is a three-phase PWM control IC with
integrated MOSFET drivers. It provides a precision voltage
regulation system for multiple applications including, but not
limited to, high current low voltage point-of-load converters,
embedded applications and other general purpose low
voltage medium to high current applications.The integration
of power MOSFET drivers into the controller IC marks a
departure from the separate PWM controller and driver
configuration of previous multi-phase product families. By
reducing the number of external parts, this integration allows
for a cost and space saving power management solution.
Output voltage can be programmed using the on-chip DAC
or an external precision reference. A 2-bit code programs
the DAC reference to one of 4 possible values (0.6V,
0.9V,1.2V and 1.5V). A unity gain, differential amplifier is
provided for remote voltage sensing, compensating for any
potential difference between remote and local grounds. The
output voltage can also be offset through the use of single
external resistor. An optional droop function is also
implemented and can be disabled for applications having
less stringent output voltage variation requirements or
experiencing less severe step loads.
A unique feature of the ISL6308 is the combined use of both
DCR and rDS(ON) current sensing. Load line voltage
positioning and overcurrent protection are accomplished
through continuous inductor DCR current sensing, while
rDS(ON) current sensing is used for accurate channel-current
balance. Using both methods of current sampling utilizes the
best advantages of each technique.
Protection features of this controller IC include a set of
sophisticated overvoltage and overcurrent protection.
Overvoltage results in the converter turning the lower
MOSFETs ON to clamp the rising output voltage and protect
the load. An OVP output is also provided to drive an optional
crowbar device. The overcurrent protection level is set
through a single external resistor. Other protection features
include protection against an open circuit on the remote
sensing inputs. Combined, these features provide advanced
protection for the output load.
1
FN9208.4
Features
• Integrated Multi-Phase Power Conversion
- 1, 2, or 3-Phase Operation
• Precision Output Voltage Regulation
- Differential Remote Voltage Sensing
- ±0.8% System Accuracy Over-Temperature
(for REF = 0.6V and 0.9V)
- ±0.5% System Accuracy Over-Temperature
(for REF = 1.2V and 1.5V)
- Usable for Output Voltages not Exceeding 2.3V
- Adjustable Reference-Voltage Offset
• Precision Channel Current Sharing
- Uses Loss-Less rDS(ON) Current Sampling
• Optional Load Line (Droop) Programming
- Uses Loss-Less Inductor DCR Current Sampling
• Variable Gate-Drive Bias - 5V to 12V
• Internal or External Reference Voltage Setting
- On-Chip Adjustable Fixed DAC Reference Voltage with
2-bit Logic Input Selects from Four Fixed Reference
Voltages (0.6V, 0.9V, 1.2V, 1.5V)
- Reference can be Changed Dynamically
- Can use an External Voltage Reference
• Overcurrent Protection
• Multi-tiered Overvoltage Protection
- OVP Pin to Drive Optional Crowbar Device
• Selectable Operation Frequency up to 1.5MHz Per Phase
• Digital Soft-Start
• Capable of Start-Up in a Pre-Biased Load
• Pb-Free (RoHS Compliant)
Applications
• High Current DDR/Chipset core voltage regulators
• High Current, Low voltage DC/DC converters
• High Current, Low voltage FPGA/ASIC DC/DC converters
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright Intersil Americas Inc. 2005, 2008. All Rights Reserved
All other trademarks mentioned are the property of their respective owners.
ISL6308
Ordering Information
PART
NUMBER
(Note)
ISL6308CRZ
PART
MARKING
TEMP. RANGE
(°C)
ISL6308 CRZ
0 to +70
PACKAGE
(Pb-Free)
40 Ld 6x6 QFN
PKG.
DWG. #
L40.6x6
ISL6308CRZ-T*
ISL6308 CRZ
0 to +70
40 Ld 6x6 QFN
L40.6x6
ISL6308IRZ
ISL6308 IRZ
-40 to +85
40 Ld 6x6 QFN
L40.6x6
ISL6308IRZ-T*
ISL6308 IRZ
-40 to +85
40 Ld 6x6 QFN
L40.6x6
ISL6308EVAL1Z
Evaluation Platform
* Please refer to TB347 for details on reel specifications.
NOTE: These Intersil Pb-free plastic packaged products employ special Pb-free material sets, molding compounds/die attach materials, and 100%
matte tin plate plus anneal (e3 termination finish, which is RoHS compliant and compatible with both SnPb and Pb-free soldering operations).
Intersil Pb-free products are MSL classified at Pb-free peak reflow temperatures that meet or exceed the Pb-free requirements of IPC/JEDEC J
STD-020.
Pinout
REF0
REF1
OVP
ENLL
FS
PGOOD
LGATE1
PVCC1
ISEN1
UGATE1
ISL6308
(40 LD 6x6 QFN)
TOP VIEW
40
39
38
37
36
35
34
33
32
31
3PH
1
30
BOOT1
2PH
2
29
PHASE1
DAC
3
28
PHASE2
REF
4
27
UGATE2
OFST
5
26
BOOT2
VCC
6
25
ISEN2
COMP
7
24
PVCC2
FB
8
23
LGATE2
VDIFF
9
22
PHASE3
RGND
10
21
BOOT3
2
11
12
13
14
15
16
17
18
19
20
VSEN
OCSET
ICOMP
DROOP
ISUM
IREF
LGATE3
PVCC3
ISEN3
UGATE3
41
GND
FN9208.4
September 30, 2008
ISL6308
Block Diagram
ICOMP
DROOP
OCSET
ISEN AMP
PGOOD
OVP
100µA
ENLL
0.66V
ISUM
POWER-ON
OC
IREF
VCC
RESET
PVCC1
RGND
VSEN
BOOT1
+1V
UGATE1
SOFT-START
AND
x1
x1
GATE
CONTROL
LOGIC
FAULT LOGIC
SHOOTTHROUGH
PROTECTION
PHASE1
VDIFF
LGATE1
UVP
0.2V
FS
OVP
PVCC2
CLOCK AND
SAWTOOTH
GENERATOR
BOOT2
OVP
UGATE2
∑
PWM1
GATE
CONTROL
LOGIC
+150mV
x 0.82
∑
SHOOTTHROUGH
PROTECTION
PHASE2
PWM2
LGATE2
REF1
DAC
∑
REF0
PWM3
2PH
CHANNEL
DETECT
3PH
DAC
PVCC3
BOOT3
CHANNEL
CURRENT
BALANCE
REF
E/A
FB
1
N
∑
COMP
OFST
UGATE3
GATE
CONTROL
LOGIC
SHOOTTHROUGH
PROTECTION
OFFSET
PHASE3
LGATE3
CHANNEL
CURRENT
SENSE
ISEN1
3
ISEN2
ISEN3
GND
FN9208.4
September 30, 2008
ISL6308
Typical Application - ISL6308
+12V
VDIFF
FB
COMP
PVCC1
BOOT1
VSEN
+5V
RGND
UGATE1
3PH
2PH
PHASE1
ISEN1
VCC
LGATE1
OFST
+12V
FS
PVCC2
BOOT2
DAC
UGATE2
ISL6308
PHASE2
REF
ISEN2
LOAD
LGATE2
REF1
REF0
+12V
+12V
OVP
PGOOD
PVCC3
GND
BOOT3
UGATE3
PHASE3
ENLL
ISEN3
IREF
DROOP
OCSET
LGATE3
ICOMP
4
ISUM
FN9208.4
September 30, 2008
ISL6308
Absolute Maximum Ratings
Thermal Information
Supply Voltage, VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +6V
Supply Voltage, PVCC . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +15V
Absolute Boot Voltage, VBOOT . . . . . . . . GND - 0.3V to GND + 36V
Phase Voltage, VPHASE . . . . . . . . GND - 0.3V to 15V (PVCC = 12)
GND - 8V (<400ns, 20µJ) to 24V (<200ns, VBOOT-PHASE = 12V)
Upper Gate Voltage, VUGATE . . . . VPHASE - 0.3V to VBOOT + 0.3V
VPHASE - 3.5V (<100ns Pulse Width, 2µJ) to VBOOT + 0.3V
Lower Gate Voltage, VLGATE. . . . . . . . GND - 0.3V to PVCC + 0.3V
GND - 5V (<100ns Pulse Width, 2µJ) to PVCC+ 0.3V
Input, Output, or I/O Voltage . . . . . . . . . GND - 0.3V to VCC + 0.3V
Thermal Resistance (Typical Notes 1, 2)
θJA (°C/W)
θJC (°C/W)
40 Ld QFN Package . . . . . . . . . . . . . .
32
3.5
Maximum Junction Temperature . . . . . . . . . . . . . . . . . . . . . . +150°C
Maximum Storage Temperature Range . . . . . . . . . .-65°C to +150°C
Pb-Free Reflow Profile. . . . . . . . . . . . . . . . . . . . . . . . .see link below
http://www.intersil.com/pbfree/Pb-FreeReflow.asp
Recommended Operating Conditions
VCC Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .+5V ±5%
PVCC Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . +5V to 12V ±5%
Ambient Temperature (ISL6308CRZ) . . . . . . . . . . . . . 0°C to +70°C
Ambient Temperature (ISL6308IRZ) . . . . . . . . . . . . .-40°C to +85°C
CAUTION: Do not operate at or near the maximum ratings listed for extended periods of time. Exposure to such conditions may adversely impact product reliability and
result in failures not covered by warranty.
NOTES:
1. θJA is measured in free air with the component mounted on a high effective thermal conductivity test board with “direct attach” features. See
Tech Brief TB379.
2. For θJC, the “case temp” location is the center of the exposed metal pad on the package underside.
Electrical Specifications
Recommended Operating Conditions, Unless Otherwise Specified. Parameters with MIN and/or MAX limits are
100% tested at +25°C, unless otherwise specified. Temperature limits established by characterization and are
not production tested.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
12
15
18
mA
BIAS SUPPLY AND INTERNAL OSCILLATOR
Input Bias Supply Current
IVCC; ENLL = high
Gate Drive Bias Current
IPVCC; ENLL = high; all gate outputs open, Fsw = 250kHz
0.4
0.8
1.2
mA
VCC POR (Power-On Reset) Threshold
VCC Rising
4.25
4.38
4.47
V
VCC Falling
3.75
3.88
4.00
V
PVCC POR (Power-On Reset) Threshold
PVCC Rising
4.25
4.38
4.47
V
PVCC Falling
3.75
3.88
4.00
V
Oscillator Frequency
FSW
225
250
275
kHz
Oscillator Ramp Amplitude (Note 3)
VP-P
-
1.50
-
V
-
66.6
-
%
0.595
0.630
0.670
V
Maximum Duty Cycle (Note 3)
CONTROL THRESHOLDS
ENLL Rising Threshold
ENLL Hysteresis
54
61
68
mV
ENLL Pull-up current
ENLL = 1V
-0.5
-1.4
-3.0
µA
COMP Shutdown Threshold
COMP Falling
0.1
0.25
0.4
V
System Accuracy (DAC = 0.6V, 0.9V)
DROOP connected to IREF
-0.8
-
0.8
%
System Accuracy (DAC = 1.2V, 1.50V)
DROOP connected to IREF
-0.5
-
0.5
%
-
-
0.4
V
REFERENCE AND DAC
DAC Input Low Voltage (REF0, REF1)
DAC Input High Voltage (REF0, REF1)
0.8
-
-
V
Minimum External Reference (Note 4)
REF pin input
-
0.6
-
V
Maximum External Reference (Note 4)
REF pin input
-
1.75
-
V
OFS Sink Current Accuracy (Negative Offset)
ROFS = 30kΩ from OFS to VCC
47.5
50.0
52.5
µA
OFS Source Current Accuracy (Positive Offset) ROFS = 10kΩ from OFS to GND
47.5
50.0
52.5
µA
5
FN9208.4
September 30, 2008
ISL6308
Electrical Specifications
Recommended Operating Conditions, Unless Otherwise Specified. Parameters with MIN and/or MAX limits are
100% tested at +25°C, unless otherwise specified. Temperature limits established by characterization and are
not production tested. (Continued)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
RL = 10k to ground
-
96
-
dB
ERROR AMPLIFIER
DC Gain (Note 3)
Gain-Bandwidth Product (Note 3)
CL = 100pF, RL = 10k to ground
-
20
-
MHz
Slew Rate (Note 3)
CL = 100pF, Load = ±400µA
-
8
-
V/µs
Maximum Output Voltage
VCC = 5V; Load = 1mA
3.90
4.20
4.60
V
Minimum Output Voltage
Load = -1mA
0.6
0.85
1.0
V
(VSEN = 1.5V)
49
55
60
µA
Bandwidth (Note 3)
-
20
-
MHz
Slew Rate (Note 3)
-
8
-
V/µs
93
100
107
µA
REMOTE SENSE DIFFERENTIAL AMPLIFIER
Input Bias Current (VSEN)
OVERCURRENT PROTECTION
OCSET Trip Current
OCSET Accuracy
OC comparator offset (OCSET and ISUM Difference)
-5
0
5
mV
ICOMP Offset
ISEN amplifier offset
-5
0
5
mV
Undervoltage Threshold
VSEN falling
80
82
84
%VID
Undervoltage Hysteresis
VSEN rising - VSEN falling
PROTECTION
Overvoltage Threshold while IC Disabled
2
3
4
%VID
1.62
1.67
1.72
V
DAC +
125mV
DAC +
150mV
DAC +
175mV
V
75
100
125
mV
Overvoltage Threshold
VSEN rising
Overvoltage Hysteresis
VSEN rising - VSEN falling
Open Sense-Line Protection Threshold
IREF Rising and Falling
VDIFF
+ 0.9V
VDIFF +
1V
VDIFF
+ 1.1V
V
OVP Output High Drive Voltage
IOVP = 50mA, VCC = 5V
2.2
3.8
4.4
V
PGOOD Output Low
VCC = 5V; 1kΩ resistor to VCC
0
0.2
0.4
V
UGATE Rise Time (Note 3)
tRUGATE; VPVCC = 12V, 3nF Load, 10% to 90%
-
26
-
ns
LGATE Rise Time (Note 3)
tRLGATE; VPVCC = 12V, 3nF Load, 10% to 90%
-
18
-
ns
UGATE Fall Time (Note 3)
tFUGATE; VPVCC = 12V, 3nF Load, 90% to 10%
-
18
-
ns
LGATE Fall Time (Note 3)
tFLGATE; VPVCC = 12V, 3nF Load, 90% to 10%
-
12
-
ns
UGATE Turn-On Non-overlap (Note 3)
tPDHUGATE; VPVCC = 12V, 3nF Load, Adaptive
-
10
-
ns
LGATE Turn-On Non-overlap (Note 3)
tPDHLGATE; VPVCC = 12V, 3nF Load, Adaptive
-
10
-
ns
1.25
2.0
3.0
Ω
SWITCHING TIME
GATE DRIVE RESISTANCE (Note 4)
Upper Drive Source Resistance
VPVCC = 12V, 150mA Source Current
Upper Drive Sink Resistance
VPVCC = 12V, 150mA Sink Current
0.9
1.6
3.0
Ω
Lower Drive Source Resistance
VPVCC = 12V, 150mA Source Current
0.85
1.4
2.2
Ω
Lower Drive Sink Resistance
VPVCC = 12V, 150mA Sink Current
0.60
0.94
1.35
Ω
Thermal Shutdown Setpoint (Note 4)
-
160
-
°C
Thermal Recovery Setpoint (Note 4)
-
100
-
°C
OVER-TEMPERATURE SHUTDOWN
NOTES:
3. Limits should be considered typical and are not production tested.
4. Limits established by characterization and are not production tested.
6
FN9208.4
September 30, 2008
ISL6308
Timing Diagram
tPDHUGATE
tRUGATE
tFUGATE
UGATE
LGATE
tFLGATE
tRLGATE
tPDHLGATE
Simplified Power System Diagram
+12VIN
+5VIN
Q1
CHANNEL1
Q2
2
REF0,REF1
DAC
ENLL
Q3
OVP
PGOOD
CHANNEL2
VOUT
Q4
ISL6308
Q5
CHANNEL3
Q6
Functional Pin Description
VCC (Pin 6)
Bias supply for the IC’s small-signal circuitry. Connect this
pin to a +5V supply and locally decouple using a quality
1.0µF ceramic capacitor.
PVCC1, PVCC2, PVCC3 (Pins 33, 24, 18)
Power supply pins for the corresponding channel MOSFET
drive. These pins can be connected to any voltage from +5V
to +12V, depending on the desired MOSFET gate drive level.
Note that tying PVCC2 OR PVCC3 to GND has the same
effect as tying 2PH or 3PH to GND for disabling the
corresponding phase
GND (Pin 41)
Bias and reference ground for the IC.
7
ENLL (Pin 37)
This pin is a threshold sensitive (approximately 0.66V) enable
input for the controller. Held low, this pin disables controller
operation. Pulled high, the pin enables the controller for
operation.
FS (Pin 36)
A resistor, placed from FS to ground, will set the switching
frequency. Refer to Equation 43 and Figure 23 for proper
resistor calculation.
3PH and 2PH (Pins 1, 2)
These pins decide how many phases the controller will
operate. Tying both pins to VCC allows for 3-phase operation.
Tying the 3PH pin to GND causes the controller to operate in
2-phase mode, while connecting both 3PH and 2PH GND will
allow for single phase operation.
FN9208.4
September 30, 2008
ISL6308
REF0 and REF1 (Pins 40, 39)
DAC (Pin 3)
These pins make up the 2-bit input that selects the fixed
DAC reference voltage. These pins respond to TTL logic
thresholds. The ISL6308 decodes these inputs to establish
one of four fixed reference voltages; see Table 1 for
correspondence between REF0 and REF1 inputs and
reference voltage settings.
The DAC pin is the direct output of the internal DAC. This pin
is connected to the REF pin using a 1kΩ to 5kΩ resistor.
This pin can be left open if an external reference is used.
These pins are internally pulled high, to approximately 1.2V,
by 40µA (typically) internal current sources; the internal
pull-up current decreases to 0 as the REF0 and REF1
voltages approach the internal pull-up voltage. Both REF0
and REF1 pins are compatible with external pull-up voltages
not exceeding the IC’s bias voltage (VCC).
RGND and VSEN (Pins 10, 11)
RGND and VSEN are inputs to the precision differential
remote-sense amplifier and should be connected to the
sense pins of the remote load.
REF (Pin 4)
The REF input pin is the positive input of the error amplifier.
This pin can be connected to the DAC pin using a resistor
(1kΩ to 5kΩ) when the internal DAC voltage is used as the
reference voltage. When an external voltage reference is
used, it must be connected directly to the REF pin, while the
DAC pin is left unconnected. The output voltage will be
regulated to the voltage at the REF pin unless this voltage
is greater than the voltage at the DAC pin. If an external
reference is used at this pin, its magnitude cannot exceed
1.75V.
A capacitor is used between the REF pin and ground to
smooth the DAC voltage during soft-start.
ICOMP, ISUM, and IREF (Pins 13, 15, 16)
OFST (Pin 5)
ISUM, IREF, and ICOMP are the DCR current sense
amplifier’s negative input, positive input, and output
respectively. For accurate DCR current sensing, connect a
resistor from each channel’s phase node to ISUM and
connect IREF to the summing point of the output inductors.
A parallel R-C feedback circuit connected between ISUM
and ICOMP will then create a voltage from IREF to ICOMP
proportional to the voltage drop across the inductor DCR.
This voltage is referred to as the droop voltage and is added
to the differential remote-sense amplifier’s output.
The OFST pin provides a means to program a DC current for
generating an offset voltage across the resistor between FB
and VDIFF. The offset current is generated via an external
resistor and precision internal voltage references. The
polarity of the offset is selected by connecting the resistor to
GND or VCC. For no offset, the OFST pin should be left
unconnected.
OCSET (Pin 12)
An optional 0.001µF to 0.01µF ceramic capacitor can be
placed from the IREF pin to the ISUM pin to help reduce
common mode noise that might be introduced by the layout.
This is the overcurrent set pin. Placing a resistor from OCSET
to ICOMP, allows a 100µA current to flow out of this pin,
producing a voltage reference. Internal circuitry compares the
voltage at OCSET to the voltage at ISUM, and if ISUM ever
exceeds OCSET, the overcurrent protection activates.
DROOP (Pin 14)
ISEN1, ISEN2 and ISEN3 (Pins 32, 25, 19)
This pin enables or disables droop. Tie this pin to the ICOMP
pin to enable droop. To disable droop, tie this pin to the IREF
pin.
These pins are used for balancing the channel currents by
sensing the current through each channel’s lower MOSFET
when it is conducting. Connect a resistor between the
ISEN1, ISEN2, and ISEN3 pins and their respective phase
node. This resistor sets a current proportional to the current
in the lower MOSFET during its conduction interval.
VDIFF (Pin 9)
VDIFF is the output of the differential remote-sense
amplifier. The voltage on this pin is equal to the difference
between VSEN and RGND added to the difference between
IREF and ICOMP. VDIFF therefore represents the VOUT
voltage plus the droop voltage.
COMP and FB (Pins 7, 8)
The internal error amplifier’s inverting input and output
respectively. FB is connected to VDIFF through an external R
or R-C network depending on the desired type of
compensation (Type II or III). COMP is tied back to FB
through an external R-C network to compensate the regulator.
8
UGATE1, UGATE2, and UGATE3 (Pins 31, 27, 20)
Connect these pins to the upper MOSFETs’ gates. These
pins are used to control the upper MOSFETs and are
monitored for shoot-through prevention purposes. Maximum
individual channel duty cycle is limited to 66%.
BOOT1, BOOT2, and BOOT3 (Pins 30, 26, 21)
These pins provide the bias voltage for the upper MOSFETs’
drives. Connect these pins to appropriately-chosen external
bootstrap capacitors. Internal bootstrap diodes connected to
the PVCC pins provide the necessary bootstrap charge.
FN9208.4
September 30, 2008
ISL6308
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 VIN and ground to
prevent damage to a load device.
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 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.
IL1 + IL2 + IL3, 7A/DIV
IL3, 7A/DIV
PWM3, 5V/DIV
IL2, 7A/DIV
PWM2, 5V/DIV
IL1, 7A/DIV
PWM1, 5V/DIV
FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS
FOR 3-PHASE CONVERTER
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
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.
( V IN – V OUT ) ⋅ V OUT
I PP = --------------------------------------------------------L ⋅ F SW ⋅ V IN
(EQ. 1)
In Equation 1, VIN and VOUT are the input and output
voltages respectively, L is the single-channel inductor value,
and FSW 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 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.
( V IN – N ⋅ V OUT ) ⋅ V OUT
I C, PP = ------------------------------------------------------------------L ⋅ F SW ⋅ V
(EQ. 2)
IN
FN9208.4
September 30, 2008
ISL6308
Another benefit of interleaving is to reduce input ripple
current. Input capacitance is determined in part by the
maximum input ripple current. Multiphase topologies can
improve overall system cost and size by lowering input ripple
current and allowing the designer to reduce the cost of input
capacitance. The example in Figure 2 illustrates input
currents from a three-phase converter combining to reduce
the total input ripple current.
The converter depicted in Figure 2 delivers 1.5V to a 36A load
from a 12V input. The RMS input capacitor current is 6.1A.
Compare this to a single-phase converter also stepping down
12V to 1.5V at 36A. The single-phase converter has a 13.3A
RMS input capacitor current. The single-phase converter
must use an input capacitor bank with twice the RMS current
capacity as the equivalent three-phase converter.
INPUT-CAPACITOR CURRENT
CHANNEL 3
INPUT CURRENT
terminates one third of a cycle after the PWM1 pulse. The
PWM3 pulse terminates one third of a cycle after PWM2.
If PVCC3 is left open or connected to ground, two channel
operation is selected and the PWM2 pulse terminates one
half of a cycle after the PWM1 pulse terminates. If both
PVCC3 and PVCC2 are left open or connected to ground,
single channel operation is selected. The 2PH and 3PH
inputs can also be used to accomplish this function. Once a
PWM pulse transitions low, it is held low for a minimum of
one third cycle. This forced off time is required to ensure an
accurate current sample. Current sensing is described in the
next section. After the forced off time expires, the PWM
output is enabled. The PWM output state is driven by the
position of the error amplifier output signal, VCOMP, minus
the current correction signal relative to the sawtooth ramp as
illustrated in Figure 3. When the modified VCOMP voltage
crosses the sawtooth ramp, the PWM output transitions
high. The internal MOSFET driver detects the change in
state of the PWM signal and turns off the synchronous
MOSFET and turns on the upper MOSFET. The PWM signal
will remain high until the pulse termination signal marks the
beginning of the next cycle by triggering the PWM signal low.
Channel Current Balance
CHANNEL 2
INPUT CURRENT
CHANNEL 1
INPUT CURRENT
FIGURE 2. CHANNEL INPUT CURRENTS AND INPUTCAPACITOR RMS CURRENT FOR 3-PHASE
CONVERTER
Figures 24, 25 and 26 in the section entitled “Input Capacitor
Selection” on page 25 can be used to determine the input
capacitor RMS current based on load current, duty cycle,
and the number of channels. They are provided as aids in
determining the optimal input capacitor solution.
PWM Operation
The timing of each converter leg is set by the number of
active channels. The default channel setting for the ISL6308
is three. One switching cycle is defined as the time between
the internal PWM1 pulse termination signals. The pulse
termination signal is the internally generated clock signal
that triggers the falling edge of PWM1. The cycle time of the
pulse termination signal is the inverse of the switching
frequency set by the resistor between the FS pin and
ground. Each cycle begins when the clock signal commands
PWM1 to go low. The PWM1 transition signals the internal
channel 1 MOSFET driver to turn off the Channel 1 upper
MOSFET and turn on the Channel 1 synchronous MOSFET.
In the default channel configuration, the PWM2 pulse
10
One important 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 parallel MOSFETs and the
expense of using expensive heat sinks and exotic magnetic
materials.
In order to realize the thermal advantage, it is important that
each channel in a multi-phase converter be controlled to carry
about the same amount of current at any load level. To
achieve this, the currents through each channel must be
sampled every switching cycle. The sampled currents, In,
from each active channel are summed together and divided
by the number of active channels. The resulting cycle average
current, IAVG, provides a measure of the total load current
demand on the converter during each switching cycle.
Channel current balance is achieved by comparing the
sampled current of each channel to the cycle average current,
and making the proper adjustment to each channel pulse
width based on the error. Intersil’s patented current-balance
method is illustrated in Figure 3, with error correction for
Channel 1 represented. In Figure 3, the cycle average
current, IAVG, is compared with the Channel 1 sample, I1, to
create an error signal IER.
The filtered error signal modifies the pulse width
commanded by VCOMP to correct any unbalance and force
IER toward zero. The same method for error signal
correction is applied to each active channel.
FN9208.4
September 30, 2008
ISL6308
+
VCOMP
+
FILTER
PWM1
-
TO GATE
CONTROL
LOGIC
each channel in the converter, but may not be active
depending on the status of the PVCC3 and PVCC2 pins, as
described in “PWM Operation” on page 10.
SAWTOOTH SIGNAL
f(s)
VIN
I3
IER
IAVG
-
Σ
÷N
In
+
r
DS ( ON )
I
SEN = I L x ------------------------R
ISEN
CHANNEL N
UPPER MOSFET
I2
SAMPLE
AND
HOLD
I1
NOTE: CHANNEL 2 AND 3 ARE OPTIONAL.
IL
ISEN(n)
-
RISEN
+
I x r DS ( ON )
L
FIGURE 3. CHANNEL 1 PWM FUNCTION AND CURRENTBALANCE ADJUSTMENT
+
CHANNEL N
LOWER MOSFET
Current Sampling
In order to realize proper current balance, the currents in
each channel must be sampled every switching cycle. This
sampling occurs during the forced off-time, following a PWM
transition low. During this time the current sense amplifier
uses the ISEN inputs to reproduce a signal proportional to
the inductor current, IL. This sensed current, ISEN, is simply
a scaled version of the inductor current. The sample window
opens exactly one sixth of the switching period, tSW, after
the PWM transitions low. The sample window then stays
open the rest of the switching cycle until PWM transitions
high again, as illustrated in Figure 4.
The sampled current, at the end of the tSAMPLE, is
proportional to the inductor current and is held until the next
switching period sample. The sampled current is used only
for channel current balance.
IL
EXTERNAL CIRCUIT
FIGURE 5. ISL6308 INTERNAL AND EXTERNAL CURRENTSENSING CIRCUITRY FOR CURRENT BALANCE
The ISL6308 senses the channel load current by sampling
the voltage across the lower MOSFET rDS(ON), as shown in
Figure 5. A ground-referenced operational amplifier, internal
to the ISL6308, is connected to the PHASE node through a
resistor, RISEN. The voltage across RISEN is equivalent to
the voltage drop across the rDS(ON) of the lower MOSFET
while it is conducting. The resulting current into the ISEN pin
is proportional to the channel current, IL. The ISEN current is
sampled and held as described in “Current Sampling” on
page 11. From Figure 5, Equation 3 for In is derived where IL
is the channel current.
r DS ( ON )
I n = I L ⋅ ---------------------R ISEN
(EQ. 3)
Output Voltage Setting
PWM
SWITCHING PERIOD
ISEN
SAMPLING PERIOD
NEW SAMPLE
CURRENT
OLD SAMPLE
CURRENT
TIME
FIGURE 4. SAMPLE AND HOLD TIMING
The ISL6308 supports MOSFET rDS(ON) current sensing to
sample each channel’s current for channel current balance.
The internal circuitry, shown in Figure 5 represents Channel
N of an N-channel converter. This circuitry is repeated for
11
ISL6308 INTERNAL CIRCUIT
The ISL6308 uses a digital to analog converter (DAC) to
generate a reference voltage based on the logic signals at the
REF1, REF0 pins. The DAC decodes the 2-bit logic signals
into one of the discrete voltages shown in Table 1. Each REF0
and REF1 pins are pulled up to an internal 1.2V voltage by
weak current sources (40µA current, decreasing to 0 as the
voltage at the REF0, REF1 pins varies from 0 to the internal
1.2V pull-up voltage). External pull-up resistors or active-high
output stages can augment the pull-up current sources, up to
a voltage of 5V. The DAC pin must be connected to REF pin
through a 1kΩ to 5kΩ resistor and a filter capacitor (0.022µF)
is connected between REF and GND.
The ISL6308 accommodates the use of external voltage
reference connected to REF pin if a different output voltage
is required. The DAC voltage must be set at least as high as
the external reference. The error amp internal non-inverting
input is the lower of REF or (DAC +300mV).
A third method for setting the output voltage is to use a
resistor divider (RP1, RS1) from the output terminal (VOUT)
FN9208.4
September 30, 2008
ISL6308
to VSEN pin to set the output voltage level as shown in
Figure 6. This method is good for generating voltages up to
2.3V (with the REF voltage set to 1.5V).
circuitry that controls voltage regulation is illustrated in
Figure 6.
(EQ. 5)
V OUT = V REF ± V OFST – V DROOP
For this case, the output voltage can be obtained as follows:
( R S1 + R P1 )
V OUT = V REF ⋅ ---------------------------------- −
+ V OFS – V DROOP
R
EXTERNAL CIRCUIT
R2
C1
COMP
(EQ. 4)
P1
It is recommended to choose resistor values of less than
500Ω for RS1 and RP1 resistors in order to get better output
voltage DC accuracy.
DAC
REF0
VDA (V)
0
0
0.600
0
1
0.900
1
0
1.200
1
1
1.500
In order to regulate the output voltage to a specified level, the
ISL6308 uses the integrating compensation network shown in
Figure 6. This compensation network insures that the steady
state error in the output voltage is limited only to the error in
the reference voltage (output of the DAC or the external
voltage reference) and offset errors in the OFS current
source, remote sense and error amplifiers. Intersil specifies
the guaranteed tolerance of the ISL6308 to include the
combined tolerances of each of these elements, except when
an external reference or voltage divider is used, then the
tolerances of these components has to be taken into account.
The ISL6308 incorporates an internal differential remote
sense amplifier in the feedback path. The amplifier removes
the voltage error encountered when measuring the output
voltage relative to the controller ground reference point,
resulting in a more accurate means of sensing output voltage.
Connect the microprocessor sense pins to the non-inverting
input, VSEN, and inverting input, RGND, of the remote-sense
amplifier. The droop voltage, VDROOP, also feeds into the
remote-sense amplifier. The remote-sense output, VDIFF, is
therefore equal to the sum of the output voltage, VOUT, and
the droop voltage. VDIFF is connected to the inverting input of
the error amplifier through an external resistor.
+
CREF
FB
R1
VCOMP
+
VOFS
-
IOFS
ERROR AMPLIFIER
VDIFF
RS1
VSEN
+
VOUT
Voltage Regulation
VID DAC
REF
TABLE 1. ISL6308 DAC VOLTAGE SELECTION TABLE
REF1
ISL6308 INTERNAL CIRCUIT
RP1
+
+
RGND
-
-
DROOP
DIFFERENTIAL
REMOTE-SENSE
AMPLIFIER
VDROOP
+
CSUM
IREF
ICOMP
+
ISENSE
AMP
-
ISUM
FIGURE 6. OUTPUT VOLTAGE AND LOAD-LINE
REGULATION WITH OFFSET ADJUSTMENT
The output of the error amplifier, VCOMP, is compared to the
sawtooth waveform to generate the PWM signals. The PWM
signals control the timing of the Internal MOSFET drivers
and regulate the converter output so that the voltage at FB is
equal to the voltage at REF. This will regulate the output
voltage to be equal to Equation 5. The internal and external
12
FN9208.4
September 30, 2008
ISL6308
Load Line (Droop) Regulation
+
PHASE1
The droop voltage, VDROOP, is created by sensing the
current through the output inductors. This is accomplished
by using a continuous DCR current sensing method.
Inductor windings have a characteristic distributed
resistance or DCR (Direct Current Resistance). For
simplicity, the inductor DCR is considered as a separate
lumped quantity, as shown in Figure 7. The channel current,
IL, flowing through the inductor, passes through the DCR.
Equation 6 shows the S-domain equivalent voltage, VL,
across the inductor.
V L ( s ) = I L ⋅ ( s ⋅ L + DCR )
(EQ. 6)
The inductor DCR is important because the voltage dropped
across it is proportional to the channel current. By using a
simple R-C network and a current sense amplifier, as shown
in Figure 7, the voltage drop across all of the inductors DCRs
can be extracted. The output of the current sense amplifier,
VDROOP, can be shown to be proportional to the channel
currents IL1, IL2, and IL3, shown in Equation 7.
s⋅L
⎛ ------------+ 1⎞
R COMP
⎝ DCR
⎠
VDROOP ( s ) = -------------------------------------------------------------------------- ⋅ ----------------------- ⋅ ( I + I + I ) ⋅ DCR
L1 L2 L3
(s ⋅ R
⋅C
+ 1)
R
COMP
COMP
S
(EQ. 7)
If the R-C network components are selected such that the
R-C time constant matches the inductor L/DCR time
constant, then VDROOP is equal to the sum of the voltage
drops across the individual DCRs, multiplied by a gain. As
Equation 8 shows, VDROOP is therefore proportional to the
total output current, IOUT.
R COMP
V DROOP = --------------------- ⋅ I OUT ⋅ DCR
RS
13
L1
PHASE2
VOUT
COUT
RS
L2
IOUT
DCR
INDUCTOR
I
The Droop is an optional feature in the ISL6308. It can be
enabled by connecting ICOMP pin to DROOP pin, as shown
in Figure 6. To disable it, connect the DROOP pin to IREF
pin.
DCR
INDUCTOR
I
L2
RS
-
ISUM
+
As shown in Figure 6, a voltage, VDROOP, proportional to the
total current in all active channels, IOUT, feeds into the
differential remote-sense amplifier. The resulting voltage at
the output of the remote-sense amplifier is the sum of the
output voltage and the droop voltage. As Equation 4 shows,
feeding this voltage into the compensation network causes
the regulator to adjust the output voltage so that it’s equal to
the reference voltage minus the droop voltage.
VL(s)
L1
-
In some high current applications, a requirement on a
precisely controlled output impedance is imposed. This
dependence of output voltage on load current is often
termed “droop” or “load line” regulation.
ICOMP
CCOMP
RCOMP
DROOP
VDROOP
+
IREF
CSUM
(Optional)
ISL6308
FIGURE 7. DCR SENSING CONFIGURATION
By simply adjusting the value of RS, the load line can be set
to any level, giving the converter the right amount of droop at
all load currents. It may also be necessary to compensate for
any changes in DCR due to temperature. These changes
cause the load line to be skewed, and cause the R-C time
constant to not match the L/DCR time constant. If this
becomes a problem a simple negative temperature
coefficient resistor network can be used in the place of
RCOMP to compensate for the rise in DCR due to
temperature.
Output Voltage Offset Programming
The ISL6308 allows the designer to accurately adjust the
offset voltage by connecting a resistor, ROFS, from the OFS
pin to VCC or GND. When ROFS is connected between OFS
and VCC, the voltage across it is regulated to 1.5V. This
causes a proportional current (IOFS) to flow into the OFS pin
and out of the FB pin. If ROFS is connected to ground, the
voltage across it is regulated to 0.5V, and IOFS flows into the
FB pin and out of the OFS pin. The offset current flowing
through the resistor between VDIFF and FB will generate
the desired offset voltage which is equal to the product
(IOFS x R1). These functions are shown in Figures 8 and 9.
(EQ. 8)
FN9208.4
September 30, 2008
ISL6308
Advanced Adaptive Zero Shoot-Through Deadtime
Control (Patent Pending)
VDIFF
+
VOFS
-
R1
The integrated drivers incorporate a unique adaptive
deadtime control technique to minimize deadtime, resulting
in high efficiency from the reduced freewheeling time of the
lower MOSFET body-diode conduction, and to prevent the
upper and lower MOSFETs from conducting simultaneously.
This is accomplished by ensuring either rising gate turns on
its MOSFET with minimum and sufficient delay after the
other has turned off.
VREF
E/A
FB
IOFS
-
OFS
0.5V
-
ISL6308
ROFS
1.5V
+
+
GND
VCC
GND
FIGURE 8. POSITIVE OFFSET OUTPUT VOLTAGE
PROGRAMMING
VDIFF
VOFS
+
R1
During turn-off of the lower MOSFET, the PHASE voltage is
monitored until it reaches a -0.3V/+0.8V trip point for a
forward/reverse current, at which time the UGATE is
released to rise. An auto-zero comparator is used to correct
the rDS(ON) drop in the phase voltage preventing false
detection of the -0.3V phase level during rDS(ON) conduction
period. In the case of zero current, the UGATE is released
after 35ns delay of the LGATE dropping below 0.5V. During
the phase detection, the disturbance of LGATE falling
transition on the PHASE node is blanked out to prevent
falsely tripping. Once the PHASE is high, the advanced
adaptive shoot-through circuitry monitors the PHASE and
UGATE voltages during a PWM falling edge and the
subsequent UGATE turn-off. If either the UGATE falls to less
than 1.75V above the PHASE or the PHASE falls to less than
+0.8V, the LGATE is released to turn on.
VREF
Internal Bootstrap Device
E/A
All three integrated drivers feature an internal bootstrap
Schottky diode. Simply adding an external capacitor across
the BOOT and PHASE pins completes the bootstrap circuit.
The bootstrap function is also designed to prevent the
bootstrap capacitor from overcharging due to the large
negative swing at the PHASE node. This reduces voltage
stress on the boot to phase pins.
FB
IOFS
VCC
-
ROFS
+
+
OFS
-
ISL6308
1.5V
0.5V
GND
VCC
FIGURE 9. NEGATIVE OFFSET OUTPUT VOLTAGE
PROGRAMMING
Q GATE
C BOOT_CAP ≥ -------------------------------------ΔV BOOT_CAP
(EQ. 11)
Q G1 • PVCC
Q GATE = -------------------------------- • N Q1
V GS1
Once the desired output offset voltage has been determined,
use the following formulas to set ROFS:
For Positive Offset (connect ROFS to GND):
0.5 ⋅ R 1
R OFS = -------------------------V OFFSET
The bootstrap capacitor must have a maximum voltage
rating above PVCC + 5V and its capacitance value can be
chosen from Equation 11:
(EQ. 9)
where QG1 is the amount of gate charge per upper MOSFET
at VGS1 gate-source voltage and NQ1 is the number of
control MOSFETs. The ΔVBOOT_CAP term is defined as the
allowable droop in the rail of the upper gate drive. Figure 10
shows the boot capacitor ripple voltage as a function of boot
capacitor value and total upper MOSFET gate charge.
For Negative Offset (connect ROFS to VCC):
1.5 ⋅ R 1
R OFS = -------------------------V OFFSET
(EQ. 10)
14
FN9208.4
September 30, 2008
ISL6308
1.6
ISL6308 INTERNAL CIRCUIT
EXTERNAL CIRCUIT
1.4
VCC
CBOOT_CAP (µF)
1.2
PVCC1
1.0
+12V
0.8
POR
CIRCUIT
0.6
ENABLE
COMPARATOR
QGATE = 100nC
50nC
0.2
ENLL
+
0.4
1.40kΩ
20nC
0.0
0.0
0.1
10.7kΩ
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.66V
1.0
ΔVBOOT_CAP (V)
FIGURE 10. BOOTSTRAP CAPACITANCE vs BOOT RIPPLE
VOLTAGE
SOFT-START
AND
FAULT LOGIC
Gate Drive Voltage Versatility
The ISL6308 provides the user flexibility in choosing the
gate drive voltage for efficiency optimization. The controller
ties the upper and lower drive rails together. Simply applying
a voltage from 5V up to 12V on PVCC sets both gate drive
rail voltages simultaneously.
Initialization
Prior to initialization, proper conditions must exist on the
ENLL, VCC, PVCC and the REF0 and REF1 pins. When the
conditions are met, the controller begins soft-start. Once the
output voltage is within the proper window of operation, the
controller asserts PGOOD.
Enable and Disable
While in shutdown mode, the PWM outputs are held in a
high-impedance state to assure the drivers remain off. The
following input conditions must be met before the ISL6308 is
released from shutdown mode.
1. The bias voltage applied at VCC must reach the internal
power-on reset (POR) rising threshold. Once this
threshold is reached, proper operation of all aspects of
the ISL6308 is guaranteed. Hysteresis between the rising
and falling thresholds assure that once enabled, the
ISL6308 will not inadvertently turn off unless the bias
voltage drops substantially (see “Electrical
Specifications” on page 5).
15
FIGURE 11. POWER SEQUENCING USING THRESHOLDSENSITIVE ENABLE (ENLL) FUNCTION
2. The voltage on ENLL must be above 0.66V. The EN input
allows for power sequencing between the controller bias
voltage and another voltage rail. The enable comparator
holds the ISL6308 in shutdown until the voltage at ENLL
rises above 0.66V. The enable comparator has 100mV of
hysteresis to prevent bounce.
3. The driver bias voltage applied at the PVCC pins must
reach the internal power-on reset (POR) rising threshold.
In order for the ISL6308 to begin operation, PVCC1 is the
only pin that is required to have a voltage applied that
exceeds POR. However, for 2 or 3-phase operation
PVCC2 and PVCC3 must also exceed the POR
threshold. Hysteresis between the rising and falling
thresholds assure that once enabled, the ISL6308 will not
inadvertently turn off unless the PVCC bias voltage drops
substantially (see “Electrical Specifications” on page 5).
When each of these conditions is true, the controller
immediately begins the soft-start sequence.
Soft-Start
During soft-start, the DAC voltage ramps linearly from zero to
the programmed level. The PWM signals remain in the highimpedance state until the controller detects that the ramping
DAC level has reached the output-voltage level. This protects
the system against the large, negative inductor currents that
would otherwise occur when starting with a pre-existing
charge on the output as the controller attempted to regulate to
zero volts at the beginning of the soft-start cycle. The Output
soft-start time, tSS, begins with a delay period equal to 64
switching cycles after the ENLL has exceeded its POR level,
FN9208.4
September 30, 2008
ISL6308
followed by a linear ramp with a rate determined by the
switching period, 1/Fsw.
64 + DAC ⋅ 1280
t SS = -------------------------------------------F SW
* CONNECT DROOP TO IREF
TO DISABLE THE DROOP FEATURE.
(EQ. 12)
ROCSET
DROOP*
For example, a regulator with 450kHz switching frequency
having REF voltage set to 1.2V has tSS equal to 3.55ms.
OUTPUT PRECHARGED
ABOVE DAC LEVEL
OCSET
IREF
+
ISEN
ISUM
VDROOP
100µA
+
OC
+
VDIFF
-
The ISL6308 also has the ability to start-up into a pre-charged
output as shown in Figure 12, without causing any
unnecessary disturbance. The FB pin is monitored during
soft-start, and should it be higher than the equivalent internal
ramping reference voltage, the output drives hold both
MOSFETs off. Once the internal ramping reference exceeds
the FB pin potential, the output drives are enabled, allowing
the output to ramp from the pre-charged level to the final level
dictated by the reference setting. Should the output be
pre-charged to a level exceeding the reference setting, the
output drives are enabled at the end of the soft-start period,
leading to an abrupt correction in the output voltage down to
the “reference set” level.
- VOCSET +
+
A 100mV offset exists on the remote-sense amplifier at the
beginning of soft-start and ramps to zero during the first 640
cycles of soft-start (704 cycles following enable). This
prevents the large inrush current that would otherwise occur
should the output voltage start out with a slight negative
bias.
ICOMP
-
+1V
DAC + 150mV
SOFT-START, FAULT
AND CONTROL LOGIC
VOVP
OV
VSEN
+
+
PGOOD
x1
-
-
RGND
UV
+
0.82 x DAC ISL6308 INTERNAL CIRCUITRY
FIGURE 13. POWER-GOOD AND PROTECTION CIRCUITRY
OUTPUT PRECHARGED
BELOW DAC LEVEL
GND>
VOUT (0.5V/DIV)
One common power-good indicator is provided for linking to
external system monitors. The schematic in Figure 13
outlines the interaction between the fault monitors and the
power-good signal
Power-Good Signal
GND>
ENLL (5V/DIV)
T1 T2
T3
FIGURE 12. SOFT-START WAVEFORMS FOR ISL6308-BASED
MULTI-PHASE CONVERTER
Fault Monitoring and Protection
The ISL6308 actively monitors output voltage and current to
detect fault conditions. Fault monitors trigger protective
measures to prevent damage to the load.
16
The power-good pin (PGOOD) is an open-drain logic output
that transitions high when the converter is operating after
soft-start. PGOOD pulls low during shutdown and releases
high after a successful soft-start. PGOOD transitions low
when an undervoltage, overvoltage, or overcurrent condition
is detected or when the controller is disabled by a reset from
ENLL or POR. If after an undervoltage or overvoltage event
occurs the output returns to within under and overvoltage
limits, PGOOD will return high.
Undervoltage Detection
The undervoltage threshold is set at 82% of the REF voltage.
When the output voltage (VSEN-RGND) is below the
undervoltage threshold, PGOOD gets pulled low. No other
action is taken by the controller. PGOOD will return high if the
output voltage rises above 85% of the REF voltage.
FN9208.4
September 30, 2008
ISL6308
Overvoltage Protection
The ISL6308 constantly monitors the difference between the
VSEN and RGND voltages to detect if an overvoltage event
occurs. Before, and during soft-start, while the DAC/REF is
ramping up, the overvoltage trip level is VOVP; upon
successful soft-start, the overvoltage trip level changes to
VOVP_rise (typically REF+150mV, see Table Electrical
Specifications on page 5). Should an OVP event be
triggered for either case, the threshold hysteresis is 100mV
(typical) below its trip point, as sensed at the VSEN pins.
Actions are taken by the ISL6308A to protect the load when
an overvoltage condition occurs, until the output voltage falls
back within set limits.
At the inception of an overvoltage event, all LGATE signals
are commanded high, and the PGOOD signal is driven low.
This causes the controller to turn on the lower MOSFETs
and pull the output voltage below a level that might cause
damage to the load. The LGATE outputs remain high until
VDIFF falls to within the overvoltage limits explained above.
The ISL6308 will continue to protect the load in this fashion
as long as the overvoltage condition recurs.
During an OVP event, the OVP pin is pulled high and
PGOOD is pulled low. In addition to the OVP pin behavior,
the normal control loop behavior commands all LGATE
signals high as soon as the output voltage strays higher than
the set point, essentially protecting against OVP events long
before the OVP pin is triggered. Thusly, the OVP output is a
duplicate method of protection, allowing the user to
implement secondary measures of protection for cases
where normal circuit behavior is insufficient, or an additional
level of protection is desired. The ISL6308 continues to
protect the output in the fashion described for as long as the
overvoltage condition is present and bias is applied to the
circuit.
For designs where the output voltage is scaled with a
resistor divider (as shown in Figure 6), the highest DAC
reference should normally be selected, in order to give the
most useful fixed OVP trip point during soft-start, as well as
after soft-start, where the OVP trip is referenced to the DAC.
Once an overvoltage condition ends the ISL6308 continues
normal operation and PGOOD returns high.
Pre-POR Overvoltage Protection
Prior to PVCC and VCC exceeding their POR levels, the
ISL6308 is designed to protect the load from any overvoltage
events that may occur. This is accomplished by means of an
internal 10kΩ resistor tied from PHASE to LGATE, which
turns on the lower MOSFET to control the output voltage
until the overvoltage event ceases or the input power supply
17
cuts off. For complete protection, the low side MOSFET
should have a gate threshold well below the maximum
voltage rating of the load/microprocessor.
In the event that during normal operation the PVCC or VCC
voltage falls back below the POR threshold, the pre-POR
overvoltage protection circuitry reactivates to protect from
any more pre-POR overvoltage events.
Open Sense Line Protection
In the case that either of the remote sense lines, VSEN or
GND, become open, the ISL6308 is designed to detect this
and shut down the controller. This event is detected by
monitoring the voltage on the IREF pin, which is a local
version of VOUT sensed at the outputs of the inductors.
If VSEN or RGND become opened, VDIFF falls, causing the
duty cycle to increase and the output voltage on IREF to
increase. If the voltage on IREF exceeds “VDIFF+1V”, the
controller will shut down. Once the voltage on IREF falls
below “VDIFF+1V”, the ISL6308 will restart at the beginning
of soft-start.
Overcurrent Protection
The ISL6308 detects overcurrent events by comparing the
droop voltage, VDROOP, to the OCSET voltage, VOCSET, as
shown in Figure 13. The droop voltage, set by the external
current sensing circuitry, is proportional to the output current
as shown in Equation 8. A constant 100µA flows through
ROCSET, creating the OCSET voltage. When the droop
voltage exceeds the OCSET voltage, the overcurrent
protection circuitry activates. Since the droop voltage is
proportional to the output current, the overcurrent trip level,
IMAX, can be set by selecting the proper value for ROCSET,
as shown in Equation 13.
I MAX ⋅ R COMP ⋅ DCR
R OCSET = ---------------------------------------------------------100μA ⋅ R S
(EQ. 13)
Once the output current exceeds the overcurrent trip level,
VDROOP will exceed VOCSET, and a comparator will trigger
the converter to begin overcurrent protection procedures. At
the beginning of overcurrent shutdown, the controller turns
off both upper and lower MOSFETs. The system remains in
this state for a period of 4096 switching cycles. If the
controller is still enabled at the end of this wait period, it will
attempt a soft-start (as shown in Figure 14). If the fault
remains, the trip-retry cycles will continue indefinitely until
either the controller is disabled or the fault is cleared. Note
that the energy delivered during trip-retry cycling is much
less than during full-load operation, so there is no thermal
hazard.
FN9208.4
September 30, 2008
ISL6308
OUTPUT CURRENT
the lower MOSFET is due to current conducted through the
channel resistance (rDS(ON)). In Equation 14, IM is the
maximum continuous output current, IPP is the peak-to-peak
inductor current (see Equation 1), and d is the duty cycle
(VOUT/VIN).
I L, 2PP ⋅ ( 1 – d )
⎛ I M⎞ 2
·
P LOW, 1 = r DS ( ON ) ⋅ ⎜ -----⎟ ⋅ ( 1 – d ) + ------------------------------------12
⎝ N⎠
0A
OUTPUT VOLTAGE
0V
FIGURE 14. OVERCURRENT BEHAVIOR IN HICCUP MODE
An additional term can be added to the lower-MOSFET loss
equation to account for additional loss accrued during the dead
time when inductor current is flowing through the lowerMOSFET body diode. This term is dependent on the diode
forward voltage at IM, VD(ON), the switching frequency, FSW,
and the length of dead times, td1 and td2, at the beginning and
the end of the lower-MOSFET conduction interval respectively.
I M I PP
I M I PP
P LOW, 2 = V D ( ON ) ⋅ F SW ⋅ ⎛ ----- + ---------⎞ ⋅ t d1 + ⎛ ----- – ---------⎞ ⋅ t d2
⎝N
⎝N
2 ⎠
2 ⎠
(EQ. 15)
General Design Guide
This design guide is intended to provide a high-level
explanation of the steps necessary to create a multi-phase
power converter. It is assumed that the reader is familiar with
many of the basic skills and techniques referenced in the
following. In addition to this guide, Intersil provides complete
reference designs that include schematics, bills of materials,
and example board layouts for many applications.
Power Stages
The first step in designing a multi-phase converter is to
determine the number of phases. This determination
depends heavily on the cost analysis which in turn depends
on system constraints that differ from one design to the next.
Principally, the designer will be concerned with whether
components can be mounted on both sides of the circuit
board, whether through-hole components are permitted, the
total board space available for power-supply circuitry, and
the maximum amount of load current. Generally speaking,
the most economical solutions are those in which each
phase handles between 25A and 30A. All surface-mount
designs will tend toward the lower end of this current range.
If through-hole MOSFETs and inductors can be used, higher
per-phase currents are possible. In cases where board
space is the limiting constraint, current can be pushed as
high as 40A per phase, but these designs require heat sinks
and forced air to cool the MOSFETs, inductors and heat
dissipating surfaces.
MOSFETs
The choice of MOSFETs depends on the current each
MOSFET will be required to conduct, the switching frequency,
the capability of the MOSFETs to dissipate heat, and the
availability and nature of heat sinking and air flow.
LOWER MOSFET POWER CALCULATION
The calculation for the approximate power loss in the lower
MOSFET can be simplified, since virtually all of the loss in
18
(EQ. 14)
The total maximum power dissipated in each lower MOSFET
is approximated by the summation of PLOW,1 and PLOW,2.
UPPER MOSFET POWER CALCULATION
In addition to rDS(ON) losses, a large portion of the upper
MOSFET losses are due to currents conducted across the
input voltage (VIN) during switching. Since a substantially
higher portion of the upper-MOSFET losses are dependent on
switching frequency, the power calculation is more complex.
Upper MOSFET losses can be divided into separate
components involving the upper MOSFET switching times,
the lower-MOSFET body-diode reverse recovery charge, Qrr,
and the upper MOSFET rDS(ON) conduction loss.
When the upper MOSFET turns off, the lower MOSFET does
not conduct any portion of the inductor current until the
voltage at the phase node falls below ground. Once the
lower MOSFET begins conducting, the current in the upper
MOSFET falls to zero as the current in the lower MOSFET
ramps up to assume the full inductor current. In Equation 16,
the required time for this commutation is t1 and the
approximated associated power loss is PUP,1.
I M I PP⎞ ⎛ t 1 ⎞
P UP,1 ≈ V IN ⋅ ⎛ ----- ⋅ ⎜ ---- ⎟ ⋅ F SW
⎝ N- + -------2 ⎠ ⎝ 2⎠
(EQ. 16)
At turn on, the upper MOSFET begins to conduct and this
transition occurs over a time t2. In Equation 17, the
approximate power loss is PUP,2.
⎛ I M I PP⎞
P UP, 2 ≈ V IN ⋅ ⎜ ----- – ---------⎟
2 ⎠
⎝N
⎛t ⎞
⋅ ⎜ ----2 ⎟ ⋅ F SW
⎝ 2⎠
(EQ. 17)
A third component involves the lower MOSFET reverse
recovery charge, Qrr. Since the inductor current has fully
commutated to the upper MOSFET before the lower
MOSFET body diode can recover all of Qrr, it is conducted
FN9208.4
September 30, 2008
ISL6308
through the upper MOSFET across VIN. The power
dissipated as a result is PUP,3.
P UP,3 = V IN ⋅ Q rr ⋅ F SW
(EQ. 18)
Finally, the resistive part of the upper MOSFET is given in
Equation 19 as PUP,4.
2
⎛ I M⎞
I PP2
P UP,4 ≈ r DS ( ON ) ⋅ d ⋅ ⎜ -----⎟ + ---------12
⎝ N⎠
(EQ. 19)
The total power dissipated by the upper MOSFET at full load
can now be approximated as the summation of the results
from Equations 16, 17, 18 and 19. Since the power
equations depend on MOSFET parameters, choosing the
correct MOSFETs can be an iterative process involving
repetitive solutions to the loss equations for different
MOSFETs and different switching frequencies.
In Equations 20 and 21, PQg_Q1 is the total upper gate drive
power loss and PQg_Q2 is the total lower gate drive power
loss; the gate charge (QG1 and QG2) is defined at the
particular gate to source drive voltage PVCC in the
corresponding MOSFET data sheet; IQ is the driver total
quiescent current with no load at both drive outputs; NQ1
and NQ2 are the number of upper and lower MOSFETs per
phase, respectively; NPHASE is the number of active
phases. The IQ*VCC product is the quiescent power of the
controller without capacitive load and is typically 75mW at
300kHz.
PVCC
BOOT
D
CGD
RHI1
RLO1
RG1
Package Power Dissipation
CDS
RGI1
CGS
When choosing MOSFETs it is important to consider the
amount of power being dissipated in the integrated drivers
located in the controller. Since there are a total of three
drivers in the controller package, the total power dissipated
by all three drivers must be less than the maximum
allowable power dissipation for the QFN package.
Calculating the power dissipation in the drivers for a desired
application is critical to ensure safe operation. Exceeding the
maximum allowable power dissipation level will push the IC
beyond the maximum recommended operating junction
temperature of +125°C. The maximum allowable IC power
dissipation for the 6x6 QFN package is approximately 4W at
room temperature. See “Layout Considerations” on page 25
for thermal transfer improvement suggestions.
When designing the ISL6308 into an application, it is
recommended that the following calculation is used to
ensure safe operation at the desired frequency for the
selected MOSFETs. The total gate drive power losses,
PQg_TOT, due to the gate charge of MOSFETs and the
integrated driver’s internal circuitry and their corresponding
average driver current can be estimated with Equations 20
and 21, respectively.
P Qg_TOT = P Qg_Q1 + P Qg_Q2 + I Q ⋅ VCC
G
UGATE
(EQ. 20)
3
P Qg_Q1 = --- ⋅ Q G1 ⋅ PVCC ⋅ F SW ⋅ N Q1 ⋅ N PHASE
2
Q1
S
PHASE
FIGURE 15. TYPICAL UPPER-GATE DRIVE TURN-ON PATH
PVCC
D
CGD
RHI2
RLO2
LGATE
G
RG2
CDS
RGI2
CGS
Q2
S
FIGURE 16. TYPICAL LOWER-GATE DRIVE TURN-ON PATH
The total gate drive power losses are dissipated among the
resistive components along the transition path and in the
bootstrap diode. The portion of the total power dissipated in
the controller itself is the power dissipated in the upper drive
path resistance, PDR_UP, the lower drive path resistance,
PDR_UP, and in the boot strap diode, PBOOT. The rest of the
power will be dissipated by the external gate resistors (RG1
and RG2) and the internal gate resistors (RGI1 and RGI2) of
the MOSFETs. Figures 15 and 16 show the typical upper
and lower gate drives turn-on transition path. The total power
P Qg_Q2 = Q G2 ⋅ PVCC ⋅ F SW ⋅ N Q2 ⋅ N PHASE
(EQ. 21)
3
⎛
⎞
+ Q G2 • N Q2 • N PHASE • F SW + I Q
I DR = --- • Q G1 • N
⎝2
⎠
Q1
19
FN9208.4
September 30, 2008
ISL6308
dissipation in the controller itself, PDR, can be roughly
estimated as:
rise in order to cause proportionally less current to flow in the
hotter phase.
P DR = P DR_UP + P DR_LOW + P BOOT + ( I Q • VCC )
ΔT 2
R ISEN ,2 = R ISEN ⋅ ---------ΔT 1
(EQ. 22)
P Qg_Q1
P BOOT = --------------------3
In Equation 24, make sure that ΔT2 is the desired
temperature rise above the ambient temperature, and ΔT1 is
the measured temperature rise above the ambient
temperature. While a single adjustment according to
Equation 24 is usually sufficient, it may occasionally be
necessary to adjust RISEN two or more times to achieve
optimal thermal balance between all channels.
R LO1
R HI1
⎛
⎞ P Qg_Q1
P DR_UP = ⎜ -------------------------------------- + ----------------------------------------⎟ • --------------------R
+
R
R
+
R
3
⎝ HI1
EXT1
LO1
EXT1⎠
R HI2
R LO2
⎛
⎞ P Qg_Q2
P DR_LOW = ⎜ -------------------------------------- + ----------------------------------------⎟ • --------------------R
+
R
R
+
R
2
⎝ HI2
EXT2
LO2
EXT2⎠
Load Line Regulation Component Selection (DCR
Current Sensing)
R GI2
R EXT2 = R G2 + ------------N Q2
R GI1
R EXT1 = R G1 + ------------N Q1
Current Balancing Component Selection
The ISL6308 senses the channel load current by sampling
the voltage across the lower MOSFET rDS(ON), as shown in
Figure 17. The ISEN pins are denoted ISEN1, ISEN2, and
ISEN3. The resistors connected between these pins and the
respective phase nodes determine the gains in the channel
current balance loop.
Select values for these resistors based on the room
temperature rDS(ON) of the lower MOSFETs; the full load
operating current, IFL; and the number of phases, N using
Equation 23.
r DS ( ON )
R ISEN = ----------------------- ⋅
50 ⋅ 10 – 6
I FL
------N
(EQ. 23)
For accurate load line regulation, the ISL6308 senses the total
output current by detecting the voltage across the output
inductor DCR of each channel (as described in “Load Line
(Droop) Regulation” on page 13). As Figure 7 illustrates, an
R-C network is required to accurately sense the inductor DCR
voltage and convert this information into a droop voltage,
which is proportional to the total output current.
Choosing the components for this current sense network is a
two step process. First, RCOMP and CCOMP must be
chosen so that the time constant of this RCOMP-CCOMP
network matches the time constant of the inductor L/DCR.
Then the resistor RS must be chosen to set the current
sense network gain, obtaining the desired full load droop
voltage. Follow the steps below to choose the component
values for this R-C network.
1. Choose an arbitrary value for CCOMP. The recommended
value is 0.01µF.
VIN
CHANNEL N
UPPER MOSFET
IL
ISEN(n)
RISEN
ISL6308
I L x r DS ( ON )
+
CHANNEL N
LOWER MOSFET
FIGURE 17. ISL6308 INTERNAL AND EXTERNAL CURRENTSENSING CIRCUITRY
In certain circumstances, it may be necessary to adjust the
value of one or more ISEN resistors. When the components of
one or more channels are inhibited from effectively dissipating
their heat so that the affected channels run hotter than
desired, choose new, smaller values of RISEN for the affected
phases (see “Channel Current Balance” on page 10). Choose
RISEN,2 in proportion to the desired decrease in temperature
20
(EQ. 24)
2. Plug the inductor L and DCR component values, and the
values for CCOMP chosen in steps 1, into Equation 25 to
calculate the value for RCOMP.
L
R COMP = --------------------------------------DCR ⋅ C COMP
(EQ. 25)
3. Use the new value for RCOMP obtained from Equation 25,
as well as the desired full load current, IFL, full load droop
voltage, VDROOP, and inductor DCR in Equation 26 to
calculate the value for RS.
I FL
R S = ------------------------- ⋅ R COMP ⋅ DCR
V DROOP
(EQ. 26)
Due to errors in the inductance or DCR it may be necessary
to adjust the value of RCOMP to match the time constants
correctly. The effects of time constant mismatch can be seen
in the form of droop overshoot or undershoot during the
initial load transient spike, as shown in Figure 18. Follow the
steps below to ensure the R-C and inductor L/DCR time
constants are matched accurately.
FN9208.4
September 30, 2008
ISL6308
1. Capture a transient event with the oscilloscope set to
about L/DCR/2 (sec/div). For example, with L = 1µH and
DCR = 1mΩ, set the oscilloscope to 500µs/div.
C2 (Optional)
2. Record ΔV1 and ΔV2 as shown in Figure 18.
R2
C1
3. Select a new value, RCOMP,2, for the time constant
resistor based on the original value, RCOMP,1, using
Equation 27.
ΔV 1
R COMP, 2 = R COMP, 1 ⋅ ---------ΔV
COMP
FB
ISL6308
(EQ. 27)
R1
2
VDIFF
4. Replace RCOMP with the new value and check to see that
the error is corrected. Repeat the procedure if necessary.
After choosing a new value for RCOMP, it will most likely be
necessary to adjust the value of RS to obtain the desired full
load droop voltage. Use Equation 26 to obtain the new value
for RS.
ΔV2
ΔV1
VOUT
ITRAN
ΔI
FIGURE 18. TIME CONSTANT MISMATCH BEHAVIOR
FIGURE 19. COMPENSATION CONFIGURATION FOR
LOAD-LINE REGULATED ISL6308 CIRCUIT
Since the system poles and zero are affected by the values
of the components that are meant to compensate them, the
solution to the system equation becomes fairly complicated.
Fortunately, there is a simple approximation that comes very
close to an optimal solution. Treating the system as though it
were a voltage-mode regulator, by compensating the L-C
poles and the ESR zero of the voltage mode approximation,
yields a solution that is always stable with very close to ideal
transient performance.
The feedback resistor, R1, has already been chosen as
outlined in “Load Line Regulation Component Selection (DCR
Current Sensing)” on page 20. Select a target bandwidth for
the compensated system, F0. The target bandwidth must be
large enough to assure adequate transient performance, but
smaller than one third of the per-channel switching frequency.
The values of the compensation components depend on the
relationships of F0 to the L-C double pole frequency and the
ESR zero frequency. For each of the following three, there is a
separate set of equations for the compensation components.
Compensation
The two opposing goals of compensating the voltage
regulator are stability and speed. Depending on whether the
regulator employs the optional load-line regulation as
described in “Load Line (Droop) Regulation” on page 13,
there are two distinct methods for achieving these goals.
Case 1:
2π ⋅ F 0 ⋅ V OSC ⋅ L ⋅ C
R 2 = R 1 ⋅ -----------------------------------------------------------0.66 ⋅ V
IN
0.66 ⋅ V IN
C 1 = -----------------------------------------------2π ⋅ V OSC ⋅ R 1 ⋅ f 0
Compensating the Load-Line Regulated Converter
The load-line regulated converter behaves in a similar
manner to a peak current mode controller because the two
poles at the output filter L-C resonant frequency split with the
introduction of current information into the control loop. The
final location of these poles is determined by the system
function, the gain of the current signal, and the value of the
compensation components, R2 and C1.
21
1
--------------------------- > F 0
2π ⋅ L ⋅ C
Case 2:
1
1
--------------------------- ≤ F 0 < -------------------------------2π ⋅ C ⋅ ESR
2π ⋅ L ⋅ C
2
V OSC ⋅ ( 2π ) 2 ⋅ F 0 ⋅ L ⋅ C
R 2 = R 1 ⋅ ---------------------------------------------------------------0.66 ⋅ V
(EQ. 28)
IN
0.66 ⋅ V IN
C 1 = ------------------------------------------------------------------------------2
( 2π ) 2 ⋅ F 0 ⋅ V OSC ⋅ R 1 ⋅ L ⋅ C
FN9208.4
September 30, 2008
ISL6308
sawtooth wave to provide a pulse-width modulated wave with
an amplitude of VIN at the PHASE node. The PWM wave is
smoothed by the output filter (L and C). The output filter
capacitor bank’s equivalent series resistance is represented by
the series resistor E.
1
F 0 > --------------------------------2π ⋅ C ⋅ ESR
Case 3:
2π ⋅ F 0 ⋅ V OSC ⋅ L
R 2 = R 1 ⋅ ----------------------------------------------0.66 ⋅ V IN ⋅ ESR
0.66 ⋅ V IN ⋅ ESR ⋅ C
C 2 = -------------------------------------------------------------2π ⋅ V OSC ⋅ R 1 ⋅ F 0 ⋅ L
C2
In Equation 28, L is the per-channel filter inductance divided
by the number of active channels; C is the sum total of all
output capacitors; ESR is the equivalent series resistance of
the bulk output filter capacitance; and VPP is the peak-to-peak
sawtooth signal amplitude as described in “Electrical
Specifications” on page 5.
COMP
Compensating the Converter Operating without
Load-Line Regulation
The ISL6308 multi-phase converter operating without load
line regulation behaves in a similar manner to a voltage
mode controller. This section highlights the design
consideration for a voltage-mode controller requiring external
compensation. To address a broad range of applications, a
type-3 feedback network is recommended (see Figure 20).
C2
R2
C1
FB
C3
R3
COMP
R1
ISL6308
VDIFF
FIGURE 20. COMPENSATION CONFIGURATION FOR
NON-LOAD-LINE REGULATED ISL6308 CIRCUIT
Figure 21 highlights the voltage-mode control loop for a
synchronous-rectified buck converter, applicable, with a
small number of adjustments, to the multi-phase ISL6308
circuit. The output voltage (VOUT) is regulated to the reference
voltage, VREF, level. The error amplifier output (COMP pin
voltage) is compared with the oscillator (OSC) modified
22
C3
R3
C1
FB
E/A
+
R1
VREF
Once selected, the compensation values in Equation 28
assure a stable converter with reasonable transient
performance. In most cases, transient performance can be
improved by making adjustments to R2. Slowly increase the
value of R2 while observing the transient performance on an
oscilloscope until no further improvement is noted. Normally,
C1 will not need adjustment. Keep the value of C1 from
Equation 28 unless some performance issue is noted.
The optional capacitor C2, is sometimes needed to bypass
noise away from the PWM comparator (see Figure 19). Keep
a position available for C2, and be prepared to install a high
frequency capacitor of between 22pF and 150pF in case any
leading edge jitter problem is noted.
R2
VDIFF
-
RGND
+
VSEN
VOUT
OSCILLATOR
VIN
VOSC
PWM
CIRCUIT
UGATE
HALF-BRIDGE
DRIVE
L
DCR
PHASE
C
ESR
LGATE
ISL6308
EXTERNAL CIRCUIT
FIGURE 21. VOLTAGE-MODE BUCK CONVERTER
COMPENSATION DESIGN
The modulator transfer function is the small-signal transfer
function of VOUT /VCOMP. This function is dominated by a DC
gain, given by dMAXVIN /VOSC , and shaped by the output
filter, with a double pole break frequency at FLC and a zero at
FCE . For the purpose of this analysis, L and DCR represent
the individual channel inductance and its DCR divided by 3
(equivalent parallel value of the three output inductors), while
C and ESR represents the total output capacitance and its
equivalent series resistance.
1
F LC = --------------------------2π ⋅ L ⋅ C
1
F CE = --------------------------------2π ⋅ C ⋅ ESR
(EQ. 29)
The compensation network consists of the error amplifier
(internal to the ISL6308) and the external R1-R3, C1-C3
components. The goal of the compensation network is to
provide a closed loop transfer function with high 0dB crossing
frequency (F0; typically 0.1 to 0.3 of FSW) and adequate phase
margin (better than 45°). Phase margin is the difference
FN9208.4
September 30, 2008
ISL6308
between the closed loop phase at F0dB and 180°. The
equations that follow relate the compensation network’s poles,
zeros and gain to the components (R1 , R2 , R3 , C1 , C2 , and
C3) in Figure 20 and 21. Use the following guidelines for
locating the poles and zeros of the compensation network:
1. Select a value for R1 (1kΩ to 5kΩ, typically). Calculate
value for R2 for desired converter bandwidth (F0).
V OSC ⋅ R 1 ⋅ F 0
R 2 = --------------------------------------------d MAX ⋅ V IN ⋅ F LC
(EQ. 30)
If setting the output voltage to be equal to the reference
set voltage as shown in Figure 21, the design procedure
can be followed as presented. However, when setting the
output voltage via a resistor divider placed at the input of
the differential amplifier (as shown in Figure 6), in order
to compensate for the attenuation introduced by the
resistor divider, the obtained R2 value needs be
multiplied by a factor of (RP + RS)/RP. The remainder of
the calculations remain unchanged, as long as the
compensated R2 value is used.
2. Calculate C1 such that FZ1 is placed at a fraction of the FLC,
at 0.1 to 0.75 of FLC (to adjust, change the 0.5 factor to
desired number). The higher the quality factor of the output
filter and/or the higher the ratio FCE/FLC, the lower the FZ1
frequency (to maximize phase boost at FLC).
1
C 1 = ----------------------------------------------2π ⋅ R 2 ⋅ 0.5 ⋅ F LC
(EQ. 31)
d MAX ⋅ V IN
1 + s ( f ) ⋅ ESR ⋅ C
G MOD ( f ) = ------------------------------ ⋅ ----------------------------------------------------------------------------------------------------------2
V OSC
1 + s ( f ) ⋅ ( ESR + DCR ) ⋅ C + s ( f ) ⋅ L ⋅ C
1 + s ( f ) ⋅ R2 ⋅ C1
G FB ( f ) = ---------------------------------------------------- ⋅
s ( f ) ⋅ R1 ⋅ ( C1 + C2 )
(EQ. 34)
1 + s ( f ) ⋅ ( R1 + R3 ) ⋅ C3
⋅ ------------------------------------------------------------------------------------------------------------------------⎛
⎛ C1 ⋅ C2 ⎞ ⎞
( 1 + s ( f ) ⋅ R 3 ⋅ C 3 ) ⋅ ⎜ 1 + s ( f ) ⋅ R 2 ⋅ ⎜ ---------------------⎟ ⎟
⎝
⎝ C 1 + C 2⎠ ⎠
G CL ( f ) = G MOD ( f ) ⋅ G FB ( f )
where, s ( f ) = 2π ⋅ f ⋅ j
COMPENSATION BREAK FREQUENCY EQUATIONS
1
F Z1 = ------------------------------2π ⋅ R 2 ⋅ C 1
1
F Z2 = ------------------------------------------------2π ⋅ ( R 1 + R 3 ) ⋅ C 3
1
F P1 = --------------------------------------------C1 ⋅ C2
2π ⋅ R 2 ⋅ --------------------C1 + C2
1
F P2 = ------------------------------2π ⋅ R 3 ⋅ C 3
3. Calculate C2 such that FP1 is placed at FCE.
C1
C 2 = -------------------------------------------------------2π ⋅ R 2 ⋅ C 1 ⋅ F CE – 1
frequency response of the modulator (GMOD), feedback
compensation (GFB) and closed-loop response (GCL):
(EQ. 35)
(EQ. 36)
(EQ. 37)
(EQ. 38)
(EQ. 32)
4. Calculate R3 such that FZ2 is placed at FLC. Calculate C3
such that FP2 is placed below FSW (typically, 0.5 to 1.0
times FSW). FSW represents the per-channel switching
frequency. Change the numerical factor to reflect desired
placement of this pole. Placement of FP2 lower in
frequency helps reduce the gain of the compensation
network at high frequency, in turn reducing the HF ripple
component at the COMP pin and minimizing resultant
duty cycle jitter.
R1
R 3 = ---------------------F SW
------------ – 1
F LC
(EQ. 33)
1
C 3 = ------------------------------------------------2π ⋅ R 3 ⋅ 0.7 ⋅ F SW
It is recommended a mathematical model is used to plot the
loop response. Check the loop gain against the error
amplifier’s open-loop gain. Verify phase margin results and
adjust as necessary. The following equations describe the
23
Figure 22 shows an asymptotic plot of the DC/DC
converter’s gain vs frequency. The actual Modulator Gain
has a high gain peak dependent on the quality factor (Q) of
the output filter, which is not shown. Using the above
guidelines should yield a compensation gain similar to the
curve plotted. The open loop error amplifier gain bounds the
compensation gain. Check the compensation gain at FP2
against the capabilities of the error amplifier. The closed loop
gain, GCL, is constructed on the log-log graph of Figure 22
by adding the modulator gain, GMOD (in dB), to the feedback
compensation gain, GFB (in dB). This is equivalent to
multiplying the modulator transfer function and the
compensation transfer function and then plotting the
resulting gain.
A stable control loop has a gain crossing with close to a
-20dB/decade slope and a phase margin greater than 45
degrees. Include worst case component variations when
determining phase margin. The mathematical model
presented makes a number of approximations and is
generally not accurate at frequencies approaching or
exceeding half the switching frequency. When designing
compensation networks, select target crossover frequencies
in the range of 10% to 30% of the per-channel switching
frequency, FSW.
FN9208.4
September 30, 2008
ISL6308
FP1
FP2
GAIN
FZ1 FZ2
R2
20 log ⎛ --------⎞
⎝ R1⎠
MODULATOR GAIN
COMPENSATION GAIN
CLOSED LOOP GAIN
OPEN LOOP E/A GAIN
d MAX ⋅ V
IN
20 log --------------------------------V
OSC
0
GFB
LOG
GCL
GMOD
LOG
FLC
FCE
F0
FREQUENCY
FIGURE 22. ASYMPTOTIC BODE PLOT OF CONVERTER GAIN
Output Filter Design
The output inductors and the output capacitor bank together
to form a low-pass filter responsible for smoothing the
pulsating voltage at the phase nodes. The output filter also
must provide the transient energy until the regulator can
respond. Because it has a low bandwidth compared to the
switching frequency, the output filter limits the system
transient response. The output capacitors must supply or
sink load current while the current in the output inductors
increases or decreases to meet the demand.
In high-speed converters, the output capacitor bank is
usually the most costly (and often the largest) part of the
circuit. Output filter design begins with minimizing the cost of
this part of the circuit. The critical load parameters in
choosing the output capacitors are the maximum size of the
load step, ΔI, the load-current slew rate, di/dt, and the
maximum allowable output-voltage deviation under transient
loading, ΔVMAX. Capacitors are characterized according to
their capacitance, ESR, and ESL (equivalent series
inductance).
At the beginning of the load transient, the output capacitors
supply all of the transient current. The output voltage will
initially deviate by an amount approximated by the voltage
drop across the ESL. As the load current increases, the
voltage drop across the ESR increases linearly until the load
current reaches its final value. The capacitors selected must
have sufficiently low ESL and ESR so that the total output
voltage deviation is less than the allowable maximum.
Neglecting the contribution of inductor current and regulator
response, the output voltage initially deviates by an amount
as follows:
di
ΔV ≈ ( ESL ) ⋅ ----- + ( ESR ) ⋅ ΔI
dt
(EQ. 39)
The filter capacitor must have sufficiently low ESL and ESR
so that ΔV < ΔVMAX.
24
Most capacitor solutions rely on a mixture of high frequency
capacitors with relatively low capacitance in combination
with bulk capacitors having high capacitance but limited
high-frequency performance. Minimizing the ESL of the
high-frequency capacitors allows them to support the output
voltage as the current increases. Minimizing the ESR of the
bulk capacitors allows them to supply the increased current
with less output voltage deviation.
The ESR of the bulk capacitors also creates the majority of
the output-voltage ripple. As the bulk capacitors sink and
source the inductor ac ripple current (see “Interleaving” on
page 9 and Equation 2), a voltage develops across the bulk
capacitor ESR equal to IC,PP (ESR). Thus, once the output
capacitors are selected, the maximum allowable ripple
voltage, VPP(MAX), determines the lower limit on the
inductance.
⎛V – N ⋅ V
⎞
OUT⎠ ⋅ V OUT
⎝ IN
L ≥ ( ESR ) ⋅ -------------------------------------------------------------------F SW ⋅ V IN ⋅ V PP( MAX )
(EQ. 40)
Since the capacitors are supplying a decreasing portion of
the load current while the regulator recovers from the
transient, the capacitor voltage becomes slightly depleted.
The output inductors must be capable of assuming the entire
load current before the output voltage decreases more than
ΔVMAX. This places an upper limit on inductance.
Equation 41 gives the upper limit on L for the cases when
the trailing edge of the current transient causes a greater
output-voltage deviation than the leading edge. Equation 42
addresses the leading edge. Normally, the trailing edge
dictates the selection of L because duty cycles are usually
less than 50%. Nevertheless, both inequalities should be
evaluated, and L should be selected based on the lower of
the two results. In each equation, L is the per-channel
inductance, C is the total output capacitance, and N is the
number of active channels.
( 1.25 ) ⋅ N ⋅ C
L ≤ ---------------------------------- ⋅ ΔV MAX – ( ΔI ⋅ ESR ) ⋅ ⎛ V IN – V O⎞
⎝
⎠
( ΔI ) 2
(EQ. 42)
2 ⋅ N ⋅ C ⋅ VO
L ≤ --------------------------------- ⋅ ΔV MAX – ( ΔI ⋅ ESR )
( ΔI ) 2
(EQ. 41)
Switching Frequency
There are a number of variables to consider when choosing
the switching frequency, as there are considerable effects on
the upper MOSFET loss calculation. These effects are
outlined in “MOSFETs” on page 18, and they establish the
upper limit for the switching frequency. The lower limit is
established by the requirement for fast transient response
and small output-voltage ripple as outlined in “Output Filter
Design” on page 24. Choose the lowest switching frequency
that allows the regulator to meet the transient-response
requirements.
FN9208.4
September 30, 2008
ISL6308
Switching frequency is determined by the selection of the
frequency-setting resistor, RFS. Figure 23 and Equation 43
are provided to assist in selecting the correct value for RFS
R FS = 10
[10.61 – 1.035 ⋅ log ( F
SW
)]
(EQ. 43)
should also be at least 1.25 times greater than the maximum
input voltage. Figures 25 and 26 provide the same input
RMS current information for two-phase and single-phase
designs respectively. Use the same approach for selecting
the bulk capacitor type and number.
INPUT-CAPACITOR CURRENT (IRMS/IO)
0.3
RFS VALUE (kΩ)
200
100
50
20
0.2
0.1
IL,PP = 0
IL,PP = 0.5 IO
IL,PP = 0.75 IO
0
10
1M
500k
200k
SWITCHING FREQUENCY (Hz)
100k
0
0.2
2M
0.4
0.6
0.8
1.0
DUTY CYCLE (VIN/VO)
FIGURE 25. NORMALIZED INPUT-CAPACITOR RMS
CURRENT FOR 2-PHASE CONVERTER
FIGURE 23. RFS vs SWITCHING FREQUENCY
Input Capacitor Selection
INPUT-CAPACITOR CURRENT (IRMS/IO)
0.3
IL,PP = 0
IL,PP = 0.5 IO
IL,PP = 0.25 IO
IL,PP = 0.75 IO
0.2
0.6
INPUT-CAPACITOR CURRENT (IRMS/IO)
The input capacitors are responsible for sourcing the AC
component of the input current flowing into the upper
MOSFETs. Their RMS current capacity must be sufficient to
handle the AC component of the current drawn by the upper
MOSFETs which is related to duty cycle and the number of
active phases.
0.4
0.2
IL,PP = 0
IL,PP = 0.5 IO
IL,PP = 0.75 IO
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (VIN/VO)
0.1
0
FIGURE 26. NORMALIZED INPUT-CAPACITOR RMS
CURRENT FOR SINGLE-PHASE CONVERTER
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (VIN/VO)
FIGURE 24. NORMALIZED INPUT-CAPACITOR RMS
CURRENT FOR 3-PHASE CONVERTER
For a three-phase design, use Figure 24 to determine the
input-capacitor RMS current requirement set by the duty
cycle, maximum sustained output current (IO), and the ratio
of the peak-to-peak inductor current (IL,PP) to IO. Select a
bulk capacitor with a ripple current rating which will minimize
the total number of input capacitors required to support the
RMS current calculated. The voltage rating of the capacitors
25
Low ESL, high-frequency ceramic capacitors are needed in
addition to the input bulk capacitors to suppress leading and
falling edge voltage spikes. The spikes result from the high
current slew rate produced by the upper MOSFET turn on
and off. Place them as close as possible to each upper
MOSFET drain to minimize board parasitics and maximize
suppression.
Layout Considerations
MOSFETs switch very fast and efficiently. The speed with
which the current transitions from one device to another
causes voltage spikes across the interconnecting
impedances and parasitic circuit elements. These voltage
spikes can degrade efficiency, radiate noise into the circuit
FN9208.4
September 30, 2008
ISL6308
and lead to device overvoltage stress. Careful component
layout and printed circuit design minimizes the voltage
spikes in the converter. Consider, as an example, the turnoff
transition of the upper PWM MOSFET. Prior to turnoff, the
upper MOSFET was carrying channel current. During the
turnoff, current stops flowing in the upper MOSFET and is
picked up by the lower MOSFET. Any inductance in the
switched current path generates a large voltage spike during
the switching interval. Careful component selection, tight
layout of the critical components, and short, wide circuit
traces minimize the magnitude of voltage spikes.
There are two sets of critical components in a DC/DC
converter using a ISL6308 controller. The power
components are the most critical because they switch large
amounts of energy. Next are small signal components that
connect to sensitive nodes or supply critical bypassing
current and signal coupling.
It is important to have a symmetrical layout, preferably with
the controller equidistantly located from the three power
trains it controls. Equally important are the gate drive lines
(UGATE, LGATE, PHASE): since they drive the power train
MOSFETs using short, high current pulses, it is important to
size them as large and as short as possible to reduce their
overall impedance and inductance. Extra care should be
given to the LGATE traces in particular since keeping the
impedance and inductance of these traces helps to
significantly reduce the possibility of shoot-through.
Equidistant placement of the controller to the three power
trains also helps to keep these traces equally short (equal
impedances, resulting in similar driving of both sets of
MOSFETs).
The power components should be placed first. Locate the
input capacitors close to the power switches. Minimize the
length of the connections between the input capacitors, CIN,
and the power switches. Locate the output inductors and
output capacitors between the MOSFETs and the load.
Locate the high-frequency decoupling capacitors (ceramic)
as close as practicable to the decoupling target, making use
of the shortest connection paths to any internal planes, such
as vias to GND immediately next, or even onto the capacitor
solder pad.
The critical small components include the bypass capacitors
for VCC and PVCC. Locate the bypass capacitors, CBP,
close to the device. It is especially important to locate the
components associated with the feedback circuit close to
their respective controller pins, since they belong to a high
impedance circuit loop, sensitive to EMI pick-up. It is also
important to place current sense components close to their
respective pins on the ISL6308, including the RISEN
resistors, RS, RCOMP, CCOMP. For proper current sharing
route three separate symmetrical as possible traces from the
corresponding phase node for each RISEN.
A multi-layer printed circuit board is recommended. Figure 27
shows the connections of the critical components for the
converter. Note that capacitors CxxIN and CxxOUT could
each represent numerous physical capacitors. Dedicate one
solid layer, usually the one underneath the component side
of the board, for a ground plane and make all critical
component ground connections with vias to this layer.
Dedicate another solid layer as a power plane and break this
plane into smaller islands of common voltage levels. Keep
the metal runs from the PHASE terminal to inductor LOUT
short. The power plane should support the input power and
output power nodes. Use copper filled polygons on the top
and bottom circuit layers for the phase nodes. Use the
remaining printed circuit layers for small signal wiring. The
wiring traces from the IC to the MOSFETs’ gates and
sources should be sized to carry at least one ampere of
current (0.02” to 0.05”).
All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems.
Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality
Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without
notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and
reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result
from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.
For information regarding Intersil Corporation and its products, see www.intersil.com
26
FN9208.4
September 30, 2008
ISL6308
LOCATE CLOSE TO IC
(MINIMIZE CONNECTION PATH)
R1
C2
C1
KEY
HEAVY TRACE ON CIRCUIT PLANE LAYER
ISLAND ON POWER PLANE LAYER
+12V
R2
ISLAND ON CIRCUIT PLANE LAYER
CHF01
VDIFF
FB
COMP
VIA CONNECTION TO GROUND PLANE
PVCC1
CBIN1
CHF1
BOOT1
+5V
LOCATE NEAR SWITCHING TRANSISTORS;
(MINIMIZE CONNECTION PATH)
CBOOT1
VSEN
RGND
UGATE1
3PH
2PH
PHASE1
LOUT1
ISEN1
VCC
RISEN1
CHF0
LGATE1
ROFST
OFST
+12V
CHF02
FS
PVCC2
CHF2
RFS
CBIN2
BOOT2
CBOOT2
DAC
ISL6308
RREF
UGATE2
PHASE2
REF
CREF
LOUT2
ISEN2
CBOUT
(CHFOUT)
RISEN2
LGATE2
LOAD
REF1
REF0
+12V
OVP
PGOOD
CHF03
CBIN3
PVCC3
LOCATE NEAR LOAD;
(MINIMIZE CONNECTION PATH)
CHF3
+12V
BOOT3
GND
CBOOT3
UGATE3
PHASE3
ENLL
LOUT3
ISEN3
IREF
RISEN3
DROOP
LGATE3
OCSET ICOMP
ISUM
RS
RCOMP
RS
RS
ROCSET
CCOMP
CSUM
FIGURE 27. PRINTED CIRCUIT BOARD POWER PLANES AND ISLANDS
27
FN9208.4
September 30, 2008
ISL6308
Package Outline Drawing
L40.6x6
40 LEAD QUAD FLAT NO-LEAD PLASTIC PACKAGE
Rev 3, 10/06
4X 4.5
6.00
36X 0.50
A
B
6
PIN 1
INDEX AREA
6
PIN #1 INDEX AREA
40
31
30
1
6.00
4 . 10 ± 0 . 15
21
10
0.15
(4X)
11
20
0.10 M C A B
TOP VIEW
40X 0 . 4 ± 0 . 1
4 0 . 23 +0 . 07 / -0 . 05
BOTTOM VIEW
SEE DETAIL "X"
0.10 C
0 . 90 ± 0 . 1
(
C
BASE PLANE
( 5 . 8 TYP )
SEATING PLANE
0.08 C
SIDE VIEW
4 . 10 )
( 36X 0 . 5 )
C
0 . 2 REF
5
( 40X 0 . 23 )
0 . 00 MIN.
0 . 05 MAX.
( 40X 0 . 6 )
DETAIL "X"
TYPICAL RECOMMENDED LAND PATTERN
NOTES:
1. Dimensions are in millimeters.
Dimensions in ( ) for Reference Only.
2. Dimensioning and tolerancing conform to AMSE Y14.5m-1994.
3. Unless otherwise specified, tolerance : Decimal ± 0.05
4. Dimension b applies to the metallized terminal and is measured
between 0.15mm and 0.30mm from the terminal tip.
5. Tiebar shown (if present) is a non-functional feature.
6. The configuration of the pin #1 identifier is optional, but must be
located within the zone indicated. The pin #1 indentifier may be
either a mold or mark feature.
28
FN9208.4
September 30, 2008
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