DATASHEET

ESIGNS
R N E W D NT
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COMME
PLACEM r at
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Data
Sheet
December 29, 2004
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8
8
1-8
Optimized Multiphase PWM Controller
with 6-Bit DAC for VR10.X Application
The ISL6556A controls microprocessor core voltage
regulation by driving up to 4 synchronous-rectified buck
channels in parallel. Multi-phase buck converter architecture
uses interleaved timing to multiply channel ripple frequency
and reduce input and output ripple currents. Lower ripple
results in fewer components, lower component cost, reduced
power dissipation, and smaller implementation area.
The ISL6556A utilizes rDS(ON) current sensing in each
phase for adaptive voltage positioning (droop), channelcurrent balancing, and overcurrent protection. To ensure
droop accuracy, an external NTC compensation circuit can
be used to completely nullify the effect of temperature
related variation in rDS(ON).
A unity gain, differential amplifier is provided for remote
voltage sensing. Any potential difference between remote
and local grounds can be eliminated using the remote-sense
amplifier. The precision threshold-sensitive enable input is
available to accurately coordinate the start up of the
ISL6556A with Intersil MOSFET driver IC. Dynamic-VID™
technology allows seamless on-the-fly VID changes. The
offset pin allows accurate voltage offset settings that are
independent of VID setting. The ISL6556A uses 5V bias and
has a built-in shunt regulator to allow 12V bias using only a
small external limiting resistor.
Ordering Information
PART NUMBER*
TEMP. (°C)
PACKAGE
PKG. DWG. #
ISL6556ACB
0 to 70
28 Ld SOIC
M28.3
ISL6556ACBZ
(Note)
0 to 70
28 Ld SOIC
(Pb-free)
M28.3
ISL6556ACR
0 to 70
32 Ld 5x5B QFN L32.5x5B
ISL6556ACRZ
(Note)
0 to 70
32 Ld 5x5B QFN L32.5x5B
(Pb-free)
ISL6556A
FN9096.3
Features
• Precision Multi-Phase Core Voltage Regulation
- Differential Remote Voltage Sensing
- 0.5% System Accuracy Over Temperature and Life
- Adjustable Reference-Voltage Offset
• Precision rDS(ON) Current Sensing
- Accurate Load-Line Programming
- Accurate Channel-Current Balancing
- Low-Cost, Lossless Current Sensing
• Internal Shunt Regulator for 5V or 12V Biasing
• Microprocessor Voltage Identification Input
- Dynamic VID™ Technology
- 6-Bit VID Input
- 0.8375V to 1.600V in 12.5mV Steps
• Threshold Enable Function for Precision Sequencing
• Overcurrent Protection
• Overvoltage Protection
- No Additional External Components Needed
- OVP Pin to drive optional Crowbar Device
• 2, 3, or 4 Phase Operation up to 1.5MHz per Phase
• QFN Package Option
- QFN Compliant to JEDEC PUB95 MO-220 QFN - Quad
Flat No Leads - Product Outline
- QFN Near Chip Scale Package Footprint; Improves
PCB Efficiency, Thinner in Profile
• Pb-free Available (RoHS Compliant)
NOTE: Intersil Pb-free products employ special Pb-free material
sets; molding compounds/die attach materials and 100% matte tin
plate termination finish, which are 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-020C.
*Add “-T” suffix for tape and reel.
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 321-724-7143 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright © Intersil Americas Inc. 2002-2004. All Rights Reserved. Dynamic VID™ is a trademark of Intersil Americas Inc.
All other trademarks mentioned are the property of their respective owners.
ISL6556A
Pinouts
VID3
PGOOD
OVP
FS
GND
ENLL
EN
VCC
28 LEAD SOIC
TOP VIEW
VID4
32 LEAD QFN
TOP VIEW
OVP
32
31
30
29
28
27
26
25
PGOOD
1
24
PWM4
VID2
2
23
ISEN4
VID1
3
22
ISEN2
VID0
4
21
PWM2
VID12.5
5
20
PWM1
OFS
6
19
ISEN1
DAC
7
18
GND
17
8
2
16
PWM3
15
GND
14
RGND
13
VSEN
12
VDIFF
11
COMP
10
FB
9
OFSOUT
REF
ISEN3
1
28
FS
2
27
EN
VID4
3
26
VCC
VID3
4
25
PWM4
VID2
5
24
ISEN4
VID1
6
23
ISEN2
VID0
7
22
PWM2
VID12.5
8
21
PWM1
OFS
9
20
ISEN1
DAC 10
19
ISEN3
REF 11
18
PWM3
FB 12
17
GND
COMP 13
16
RGND
VDIFF 14
15
VSEN
ISL6556A
ISL6565ACR Block Diagram
OVP
VDIFF PGOOD
RGND
S
x1
OVP
LATCH
VSEN
ENLL
VCC
R
1.24V
POWER-ON
RESET (POR)
EN
Q
THREE-STATE
SOFT START
AND
FAULT LOGIC
OVP
CLOCK AND
SAWTOOTH
GENERATOR

+200mV
OFS
FS
PWM1
PWM

Â
OFFSET
PWM2
PWM
OFSOUT

REF
PWM3
PWM
DAC

VID4
VID3
PWM4
PWM
VID2
DYNAMIC
VID
VID1
D/A
+
E/A
-
VID0
CHANNEL
CURRENT
BALANCE
VID12.5
CHANNEL
DETECT
COMP
ISEN1
I_TRIP
FB
OC

I_TOT
ISEN2
CURRENT
SENSE
ISEN3
ISEN4
GND
3
CHANNEL
SAMPLE
&
HOLD
ISL6556A
Typical Application of ISL6556ACB
+12V
VIN
VCC
PVCC
BOOT
UGATE
PHASE
HIP6601B
+5V
LGATE
PWM
FB
COMP VCC
GND
+12V
DAC
VDIFF
NTC
THERMISTOR
VIN
VSEN
REF
RGND
VCC
PGOOD
PVCC
OVP
PHASE
ISEN1
ISL6556ACB
VID4
VID3
PWM1
PWM2
VID2
ISEN2
BOOT
UGATE
HIP6601B
LGATE
PWM
GND
VID1
PWM3
VID0
PWM4
OFS
FS
RT
VIN
ISEN3
VID12.5
GND
+12V
VCC
ISEN4
EN
PVCC
BOOT
UGATE
PHASE
HIP6601B
LGATE
PWM
VID_PGOOD
(BUFFERED)
GND
+12V
VIN
VCC
PVCC
BOOT
UGATE
PHASE
HIP6601B
LGATE
PWM
4
GND
µP
LOAD
ISL6556A
Typical Application of ISL6556ACR
+12V
VIN
VCC
PVCC
BOOT
UGATE
PHASE
HIP6601B
+5V
LGATE
PWM
FB
COMP VCC
GND
+12V
EN
VDIFF
NTC
THERMISTOR
VIN
DAC
VSEN
RGND
OVP
VCC
REF
PGOOD
PVCC
PHASE
OFSOUT
HIP6601B
ISL6556ACR
VID4
LGATE
ISEN1
VID3
VID2
PWM1
PWM2
VID1
ISEN2
PWM3
VID0
BOOT
UGATE
PWM
GND
+12V
VIN
ISEN3
VID12.5
PWM4
OFS
FS
VCC
ISEN4
EN
GND ENLL
PVCC
RT
BOOT
UGATE
PHASE
HIP6601B
LGATE
PWM
VID_PGOOD
GND
+12V
VIN
VCC
PVCC
BOOT
UGATE
PHASE
HIP6601B
LGATE
PWM
5
GND
µP
LOAD
ISL6556A
ISL6565ACB Block Diagram
OVP
VDIFF PGOOD
RGND
S
x1
VCC
OVP
LATCH
VSEN
R
1.24V
POWER-ON
RESET (POR)
EN
Q
THREE-STATE
SOFT START
AND
FAULT LOGIC
OVP
CLOCK AND
SAWTOOTH
GENERATOR

+200mV
OFS
FS
PWM1
PWM

OFFSET
PWM2
PWM

REF
PWM3
PWM
DAC

VID4
VID3
PWM4
PWM
VID2
DYNAMIC
VID
VID1
D/A
E/A
VID0
CHANNEL
CURRENT
BALANCE
VID12.5
CHANNEL
DETECT
COMP
ISEN1
I_TRIP
FB
OC

I_TOT
CHANNEL
ISEN2
CURRENT
SENSE
ISEN3
ISEN4
GND
6
SAMPLE
&
HOLD
ISL6556A
Absolute Maximum Ratings
Thermal Information
Supply Voltage, VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .+7V
Input, Output, or I/O Voltage (except OVP) . . GND -0.3V to VCC +0.3V
OVP Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .+15V
ESD (Human body model) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .>4kV
ESD (Machine model) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .>300V
ESD (Charged device model) . . . . . . . . . . . . . . . . . . . . . . . . . .>2kV
Thermal Resistance
JA (°C/W)
JC (°C/W)
SOIC Package (Note 1) . . . . . . . . . . . .
60
N/A
QFN Package (Notes 2, 3). . . . . . . . . .
32
3.5
Maximum Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . 150°C
Maximum Storage Temperature Range . . . . . . . . . . . -65°C to 150°C
Maximum Lead Temperature (Soldering 10s) . . . . . . . . . . . . . 300°C
(SOIC - Lead Tips Only)
Operating Conditions
Supply Voltage, VCC (5V bias mode, Note 4) . . . . . . . . . . +5V ±5%
Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to 125°C
CAUTION: Stress above those listed in “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress only rating and operation of the
device at these or any other conditions above those indicated in the operational section of this specification is not implied.
NOTES:
1. JA is measured with the component mounted on a high effective thermal conductivity test board in free air. See Tech Brief TB379 for details.
2. JA 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.
3. For JC, the “case temp” location is the center of the exposed metal pad on the package underside.
Electrical Specifications
Operating Conditions: VCC = 5V or ICC < 25mA (Note 4), TJ = 0°C to 105°C.
Unless Otherwise Specified.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
VCC SUPPLY CURRENT
Nominal Supply
VCC = 5VDC; EN = 5VDC; RT = 100k
ISEN1 = ISEN2 = ISEN3 = ISEN4 = -70A
-
14
18
mA
Shutdown Supply
VCC = 5VDC; EN = 0VDC; RT = 100k
-
10
14
mA
SHUNT REGULATOR
VCC Voltage
VCC tied to 12VDC through 300 resistor, RT = 100k
5.6
5.9
6.2
V
VCC Sink Current
VCC tied to 12VDC through 300 resistor, RT = 100k
-
-
25
mA
VCC Rising
4.2
4.31
4.50
V
VCC Falling
3.7
3.82
4.00
V
EN Rising
1.22
1.24
1.26
V
Hysteresis
-
100
-
mV
Fault Reset
1.10
1.14
1.18
V
ENLL Input Logic Low Level
-
-
0.4
V
ENLL input Logic High Level
0.8
-
-
V
-
-
1
A
POWER-ON RESET AND ENABLE
POR Threshold
ENABLE Threshold
ENLL Leakage Current
ENLL = 5V
REFERENCE VOLTAGE AND DAC
System Accuracy (VID = 1.2V - 1.6V)
(Note 5)
-0.5
-
0.5
%VID
System Accuracy (VID = 0.8375V - 1.1875V)
(Note 5)
-0.8
-
0.8
%VID
-65
-50
-35
A
VID Input Low Level
-
-
0.4
V
VID Input High Level
0.8
-
-
V
VID Pull Up
DAC Source/Sink Current
VID = 010100
-200
-
200
A
OFSOUT Source/Sink Current
(ISL6556ACR Only)
-50
-
50
A
-50
-
50
A
REF Source/Sink Current
7
ISL6556A
Electrical Specifications
Operating Conditions: VCC = 5V or ICC < 25mA (Note 4), TJ = 0°C to 105°C.
Unless Otherwise Specified. (Continued)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Offset resistor connected to ground
485
500
515
mV
VCC=5.00V, offset resistor connected to VCC
2.91
3.00
3.09
V
RT = 100k
-10
-
10
%
0.08
-
1.5
MHz
Sawtooth Amplitude
-
1.5
-
V
Max Duty Cycle
-
66.7
-
%
PIN-ADJUSTABLE OFFSET
Voltage at OFS pin
OSCILLATOR
Accuracy
Adjustment Range
ERROR AMPLIFIER
Open-Loop Gain
RL = 10k to ground
-
80
-
dB
Open-Loop Bandwidth
CL = 100pF, RL = 10k to ground
-
18
-
MHz
Slew Rate
CL = 100pF
4.5
6.0
7.5
V/s
Maximum Output Voltage
4.0
4.3
-
V
Output High Voltage @ 2mA
3.7
-
-
V
Output Low Voltage @ 2mA
-
-
1.35
V
-
20
-
MHz
REMOTE-SENSE AMPLIFIER
Bandwidth
Output High Current
VSEN - RGND = 2.5V
-500
-
500
A
Output High Current
VSEN - RGND = 0.6
-500
-
500
A
PWM OUTPUT
PWM Output Voltage LOW Threshold
Iload = ±500A
-
-
0.3
V
PWM Output Voltage HIGH Threshold
Iload = ±500A
4.3
-
-
V
ISEN1 = ISEN2 = ISEN3 = ISEN4 = 80A, 0°C to 105°C
74
81
91
A
98
110
122
A
-
-
0.4
V
SENSE CURRENT
Sensed Current Tolerance
Overcurrent Trip Level
POWER GOOD AND PROTECTION MONITORS
PGOOD Low Voltage
IPGOOD = 4mA
Undervoltage Offset From VID
VSEN Falling
72
74
76
%VID
Overvoltage Threshold
Voltage above VID, After Soft Start (Note 6)
180
200
220
mV
-
1.63
-
V
1.7
1.8
1.87
V
VCC  POR Threshold, VSEN Falling
-
0.6
-
V
VCC < POR Threshold
-
1.5
-
V
IOVP = -100mA, VCC = 5V
-
1.9
-
V
1.4
-
-
V
Before Enable
VCC < POR Threshold
Overvoltage Reset Voltage
OVP Drive Voltage
Minimum VCC for OVP
NOTES:
4. When using the internal shunt regulator, VCC is clamped to 6.02V (max). Current must be limited to 25mA or less.
5. These parts are designed and adjusted for accuracy with all errors in the voltage loop included.
6. During soft start, VDAC rises from 0 to VID. The overvoltage trip level is the higher of 1.7V and VDAC + 0.2V.
8
ISL6556A
Functional Pin Description
FB and COMP
Supplies all the power necessary to operate the chip. The
controller starts to operate when the voltage on this pin
exceeds the rising POR threshold and shuts down when the
voltage on this pin drops below the falling POR threshold.
Connect this pin directly to a +5V supply or through a series
300 resistor to a +12V supply.
Inverting input and output of the error amplifier respectively.
FB is connected to VDIFF through a resistor. A negative
current, proportional to output current is present on the FB
pin. A properly sized resistor between VDIFF and FB sets
the load line (droop). The droop scale factor is set by the
ratio of the ISEN resistors and the lower MOSFET rDS(ON).
COMP is tied back to FB through an external R-C network to
compensate the regulator.
GND
DAC and REF
Bias and reference ground for the IC.
The DAC output pin is the output of the precision internal
DAC reference. The REF input pin is the positive input of
the error amplifier. In typical applications, a 1k, 1%
resistor is used between DAC and REF to generate a
precise offset voltage. This voltage is proportional to the
offset current determined by the offset resistor from OFS to
ground or VCC. A capacitor is used between REF and
ground to smooth the voltage transition during
Dynamic VID™ operations.
VCC
EN
This pin is a threshold-sensitive enable input for the
controller. Connecting the 12V supply to EN through an
appropriate resistor divider provides a means to synchronize
power-up of the controller and the MOSFET driver ICs.
When EN is driven above 1.24V, the ISL6556A is active
depending on status of ENLL, the internal POR, and pending
fault states. Driving EN below 1.14V will clear all fault states
and prime the ISL6556 to soft start when re-enabled.
ENLL
This pin is implemented in QFN ISL6556A only. It’s a logiclevel enable input for the controller. When asserted to a logic
high, the ISL6556A is active depending on status of EN, the
internal POR, VID inputs and pending fault states.
Deasserting ENLL will clear all fault states and prime the
ISL6556A to soft start when re-enabled.
FS
A resistor, placed from FS to ground will set the switching
frequency. There is an inverse relationship between the
value of the resistor and the switching frequency. See
Figure 13 and Equation 26.
VID4, VID3, VID2, VID1, VID0, and VID12.5
These are the inputs to the internal DAC that provides the
reference voltage for output regulation. Connect these pins
either to open-drain outputs with or without external pull-up
resistors or to active-pull-up outputs. VID4-VID12.5 have
50A internal pull-up current sources that diminish to zero as
the voltage rises above the logic-high level. These inputs
can be pulled up as high as VCC plus 0.3V.
VDIFF, VSEN, and RGND
VSEN and RGND form the precision differential remotesense amplifier. This amplifier converts the differential
voltage of the remote output to a single-ended voltage
referenced to local ground. VDIFF is the amplifier’s output
and the input to the regulation and protection circuitry.
Connect VSEN and RGND to the sense pins of the remote
load.
9
PWM1, PWM2, PWM3, PWM4
Pulse-width modulation outputs. Connect these pins to the
PWM input pins of the Intersil driver IC. The number of
active channels is determined by the state of PWM3 and
PWM4. Tie PWM3 to VCC to configure for 2-phase
operation. Tie PWM4 to VCC to configure for 3-phase
operation.
ISEN1, ISEN2, ISEN3, ISEN4
Current sense inputs. The ISEN1, ISEN2, ISEN3, and
ISEN4 pins are held to a virtual ground such that a resistor
connected between them and the drain terminal of the
associated lower MOSFET will carry a current proportional
to the current flowing through the related channel. The
current is determined by the negative voltage developed
across the lower MOSFET’s rDS(ON) which is the channel
current scaled by the inverse of the rDS(ON). The current is
used as a reference for channel balancing, protection, and
load-line regulation (via the FB pin). Inactive channels
should have their respective current sense inputs left open.
PGOOD
PGOOD is used as an indication of the end of soft-start per
the microprocessor specification. It is an open-drain logic
output that is low impedance until the soft start is
completed. It will be pulled low again once the undervoltage
point is reached.
OFS
The OFS pin provides a means to program a dc current for
generating an offset voltage at the REF input. 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 OFS pin should be left unterminated.
ISL6556A
OFSOUT (ISL6556ACR only)
OFSOUT is the output of the offset-current generating
circuit. It must be connected either to REF (recommended)
or FB to generate a dc offset.
OVP
Overvoltage protection pin. This pin pulls to VCC and is
latched when an overvoltage condition is detected. Connect
this pin to the gate of an SCR or MOSFET tied from VIN or
VOUT to ground to prevent damage to the load. This pin may
be pulled as high as 15V to ground with an external resistor.
However, it is only capable of pulling low when VCC is above
2V.
Operation
Multi-Phase Power Conversion
Microprocessor load current profiles have changed to the
point that the advantages of multi-phase power conversion
are impossible to ignore. The technical challenges
associated with producing a single-phase converter which is
both cost-effective and thermally viable have forced a
change to the cost-saving approach of multi-phase. The
ISL6556A controller helps in simplifying the implementation
by integrating vital functions and requiring minimal output
components. The block diagrams on pages 2 and 3 provide
top level views of multi-phase power conversion using the
ISL65556ACB and ISL6556ACR controllers.
IL1 + IL2 + IL3, 7A/DIV
IL3, 7A/DIV
PWM3, 5V/DIV
IL2, 7A/DIV
PWM2, 5V/DIV
IL1, 7A/DIV
ripple frequency of any one phase. In addition, the peak-topeak amplitude of the combined inductor currents is reduced
in proportion to the number of phases (Equations 1 and 2).
Increased ripple frequency and lower ripple amplitude mean
that the designer can use less per-channel inductance and
lower total output capacitance for any performance
specification.
Figure 1 illustrates the multiplicative effect on output ripple
frequency. The three channel currents (IL1, IL2, and IL3)
combine to form the AC ripple current and the DC load
current. The ripple component has three times the ripple
frequency of each individual channel current. Each PWM
pulse is terminated 1/3 of a cycle after the PWM pulse of the
previous phase. The peak-to-peak current for each phase is
about 7A, and the dc components of the inductor currents
combine to feed the load.
To understand the reduction of ripple current amplitude in the
multi-phase circuit, examine the equation representing an
individual channel’s peak-to-peak inductor current.
 V IN -V OUT  V OUT
I PP = -------------------------------------------------L fS V
(EQ. 1)
IN
In Equation 1, VIN and VOUT are the input and output
voltages respectively, L is the single-channel inductor value,
and fS 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. Outputvoltage 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 L, PP = -------------------------------------------------------L fS V
(EQ. 2)
IN
PWM1, 5V/DIV
1s/DIV
FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS
FOR 3-PHASE CONVERTER
Interleaving
The switching of each channel in a multi-phase converter is
timed to be symmetrically out of phase with each of the other
channels. In a 3-phase converter, each channel switches 1/3
cycle after the previous channel and 1/3 cycle before the
following channel. As a result, the three-phase converter has
a combined ripple frequency three times greater than the
10
Another benefit of interleaving is to reduce input ripple
current. Input capacitance is determined in part by the
maximum input ripple current. Multi-phase 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 36A to a 1.5V
load from a 12V input. The RMS input capacitor current is
5.9A. Compare this to a single-phase converter also
stepping down 12V to 1.5V at 36A. The single-phase
ISL6556A
converter has 11.9A RMS input capacitor current. The
single-phase converter must use an input capacitor bank
with twice the RMS current capacity as the equivalent threephase converter.
INPUT-CAPACITOR CURRENT, 10A/DIV
CHANNEL
Channel
33
input current
INPUT
CURRENT
10A/DIV
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 4. When the modified
VCOMP voltage crosses the sawtooth ramp, the PWM output
transitions high. The MOSFET driver detects the change in
state of the PWM signal, turns off the synchronous MOSFET
and turns on the upper MOSFET. The PWM signal remains
high until the pulse termination signal commands the
beginning of the next cycle by triggering the PWM signal low.
Current Sensing
CHANNEL 2
INPUT CURRENT
10A/DIV
CHANNEL 1
INPUT CURRENT
10A/DIV
1s/DIV
FIGURE 2. CHANNEL INPUT CURRENTS AND INPUTCAPACITOR RMS CURRENT FOR 3-PHASE
CONVERTER
Figures 14, 15 and 16 in the section entitled Input Capacitor
Selection 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. Figure 17 shows the single
phase input-capacitor RMS current for comparison.
PWM Operation
The timing of each converter leg is set by the number of
active channels. The default channel setting for the
ISL6556A is four. One switching cycle is defined as the time
between 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
the channel-1 PWM output to go low. The PWM1 transition
signals the 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 terminates 1/4 of a cycle after PWM1. The PWM3
output follows another 1/4 of a cycle after PWM2. PWM4
terminates another 1/4 of a cycle after PWM3.
If PWM3 is connected to VCC, two channel operation is
selected and the PWM2 pulse terminates 1/2 of a cycle later.
Connecting PWM4 to VCC selects three channel operation
and the pulse-termination times are spaced in 1/3 cycle
increments.
Once a PWM signal transitions low, it is held low for a
minimum of 1/3 cycle. This forced off time is required to
11
During the forced off time following a PWM transition low, the
controller senses channel current by sampling the voltage
across the lower MOSFET rDS(ON) (see Figure 3). A groundreferenced operational amplifier, internal to the ISL6556A, 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
after sufficient settling time every switching cycle. The
sampled current, In, is used for channel-current balance, loadline regulation, overcurrent protection, and module current
sharing. From Figure 3, the following equation for In is derived
r DS  ON 
I n = I L ---------------------R ISEN
(EQ. 3)
where IL is the channel current.
If rDS(ON) sensing is not desired, an independent currentsense resistor in series with the lower MOSFET source can
serve as a sense element. The circuitry shown in Figure 3
represents channel n of an N-channel converter. This
circuitry is repeated for each channel in the converter, but
may not be active depending upon the status of the PWM3
and PWM4 pins as described under PWM Operation
section.
VIN
In
r DS  ON 
I
SEN = I L ------------------------R ISEN
SAMPLE
&
HOLD
CHANNEL N
UPPER MOSFET
IL
ISEN(n)
+
RISEN
I
r
L DS  ON 
+
CHANNEL N
LOWER MOSFET
ISL6556A INTERNAL CIRCUIT
EXTERNAL CIRCUIT
FIGURE 3. INTERNAL AND EXTERNAL CURRENT-SENSING
CIRCUITRY
ISL6556A
Channel-Current Balance
The sampled current, In, from each active channel is used to
gauge both overall load current and the relative channel
current carried in each leg of the converter. The individual
sample currents are summed and divided by the number of
active channels. The resulting average current, IAVG,
provides a measure of the total load current demand on the
converter and the appropriate level of channel current. Using
Figures 3 and 4, the average current is defined as
I1 + I2 +  + IN
I AVG = ---------------------------------------N
(EQ. 4)
I OUT r DS  ON 
I AVG = ---------------------------------N
R ISEN
where N is the number of active channels and IOUT is the
total load current.
VCOMP
+
+
-
PWM1
SAWTOOTH SIGNAL
f(j)
I4 *
IER
IAVG
-
N

+
I3 *
to cool more effectively than the other(s) due to nearby air
flow or heat sinking components. The other channel(s) may
have more difficulty cooling with comparatively less air flow
and heat sinking. The hotter channels may also be located
close to other heat-generating components tending to drive
their temperature even higher. In these cases, the proper
selection of the current sense resistors (RISEN in Figure 3)
introduces channel current unbalance into the system.
Increasing the value of RISEN in the cooler channels and
decreasing it in the hotter channels moves all channels into
thermal balance at the expense of current balance.
Voltage Regulation
The integrating compensation network shown in Figure 5
assures that the steady-state error in the output voltage is
limited only to the error in the reference voltage (output of
the DAC) and offset errors in the OFS current source,
remote-sense and error amplifiers. Intersil specifies the
guaranteed tolerance of the ISL6556A to include the
combined tolerances of each of these elements.
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 Intersil MOSFET drivers and
regulate the converter output to the specified reference
voltage. The internal and external circuitry that controls
voltage regulation is illustrated in Figure 5.
I2
EXTERNAL CIRCUIT
RC CC
COMP
I1
NOTE: *Channels 3 and 4 are optional.
DAC
FIGURE 4. CHANNEL-1 PWM FUNCTION AND CURRENTBALANCE ADJUSTMENT
The average current is subtracted from the individual
channel sample currents. The resulting error current, IER, is
filtered to modify VCOMP. The modified VCOMP signal is
compared to a sawtooth ramp signal to produce a modified
pulse width which corrects for any unbalance and drives the
error current toward zero. Figure 4 illustrates Intersil’s
patented current-balance method as implemented on
channel-1 of a multi-phase converter.
Two considerations designers face are MOSFET selection
and inductor design. Both are significantly improved when
channel currents track at any load level. The need for
complex drive schemes for multiple MOSFETs, exotic
magnetic materials, and expensive heat sinks is avoided,
resulting in a cost-effective and easy-to-implement solution
relative to single-phase conversion. Channel-current
balance insures that the thermal advantage of multi-phase
conversion is realized. Heat dissipation in multiple channels
is spread over a greater area than can easily be
accomplished using the single phase approach.
In some circumstances, it may be necessary to deliberately
design some channel-current unbalance into the system. In
a highly compact design, one or two channels may be able
12
ISL6556A INTERNAL CIRCUIT
RREF
REF
CREF
+
FB
RFB
+
VDROOP
-
VOUT+
VOUT-
-
VCOMP
ERROR AMPLIFIER
IAVG
VDIFF
VSEN
+
RGND
DIFFERENTIAL
REMOTE-SENSE
AMPLIFIER
FIGURE 5. OUTPUT VOLTAGE AND LOAD-LINE
REGULATION WITH OFFSET ADJUSTMENT
The ISL6556A incorporates an internal differential remotesense amplifier in the feedback path. The amplifier removes
the voltage error encountered when measuring the output
voltage relative to the local controller ground reference point
resulting in a more accurate means of sensing output
voltage. Connect the microprocessor sense pins to the non-
ISL6556A
inverting input, VSEN, and inverting input, RGND, of the
remote-sense amplifier. The remote-sense output, VDIFF, is
connected to the inverting input of the error amplifier through
an external resistor.
A digital to analog converter (DAC) generates a reference
voltage based on the state of logic signals at pins VID4
through VID12.5. The DAC decodes the 6-bit logic signal
(VID) into one of the discrete voltages shown in Table 1.
Each VID input offers a 20A pull-up to an internal 2.5V
source for use with open-drain outputs. The pull-up current
diminishes to zero above the logic threshold to protect
voltage-sensitive output devices. External pull-up resistors
can augment the pull-up current sources in case leakage
into the driving device is greater than 20A.
TABLE 1. VOLTAGE IDENTIFICATION (VID) CODES
VID4
VID3
VID2
VID1
VID0
VID12.5
VDAC
0
1
0
1
0
0
0.8375V
0
1
0
0
1
1
0.8500V
0
1
0
0
1
0
0.8625V
0
1
0
0
0
1
0.8750V
0
1
0
0
0
0
0.8875V
0
0
1
1
1
1
0.9000V
0
0
1
1
1
0
0.9125V
0
0
1
1
0
1
0.9250V
0
0
1
1
0
0
0.9375V
0
0
1
0
1
1
0.9500V
0
0
1
0
1
0
0.9625V
0
0
1
0
0
1
0.975V0
0
0
1
0
0
0
0.9875V
0
0
0
1
1
1
1.0000V
0
0
0
1
1
0
1.0125V
0
0
0
1
0
1
1.0250v
0
0
0
1
0
0
1.0375V
0
0
0
0
1
1
1.0500V
0
0
0
0
1
0
1.0625V
0
0
0
0
0
1
1.0750V
0
0
0
0
0
0
1.0875V
1
1
1
1
1
1
OFF
1
1
1
1
1
0
OFF
1
1
1
1
0
1
1.1000V
1
1
1
1
0
0
1.1125V
1
1
1
0
1
1
1.1250V
1
1
1
0
1
0
1.1375V
1
1
1
0
0
1
1.1500V
13
TABLE 1. VOLTAGE IDENTIFICATION (VID) CODES (Continued)
VID4
VID3
VID2
VID1
VID0
VID12.5
VDAC
1
1
1
0
0
0
1.1625V
1
1
0
1
1
1
1.1750V
1
1
0
1
1
0
1.1875V
1
1
0
1
0
1
1.2000V
1
1
0
1
0
0
1.2125V
1
1
0
0
1
1
1.2250V
1
1
0
0
1
0
1.2475V
1
1
0
0
0
1
1.2500V
1
1
0
0
0
0
1.2625V
1
0
1
1
1
1
1.2750V
1
0
1
1
1
0
1.2875V
1
0
1
1
0
1
1.3000V
1
0
1
1
0
0
1.3125V
1
0
1
0
1
1
1.3250V
1
0
1
0
1
0
1.3375V
1
0
1
0
0
1
1.3500V
1
0
1
0
0
0
1.3625V
1
0
0
1
1
1
1.3750V
1
0
0
1
1
0
1.3875V
1
0
0
1
0
1
1.4000V
1
0
0
1
0
0
1.4125V
1
0
0
0
1
1
1.4250V
1
0
0
0
1
0
1.4375V
1
0
0
0
0
1
1.4500V
1
0
0
0
0
0
1.4625V
0
1
1
1
1
1
1.4750V
0
1
1
1
1
0
1.4875V
0
1
1
1
0
1
1.5000V
0
1
1
1
0
0
1.5125V
0
1
1
0
1
1
1.5250V
0
1
1
0
1
0
1.5375V
0
1
1
0
0
1
1.5500V
0
1
1
0
0
0
1.5625V
0
1
0
1
1
1
1.5750V
0
1
0
1
1
0
1.5875V
0
1
0
1
0
1
1.600V
ISL6556A
For Negative Offset (connect ROFS to GND):
Load-Line Regulation
Some microprocessor manufacturers require a preciselycontrolled output resistance. This dependence of output
voltage on load current is often termed “droop” or “load line”
regulation. By adding a well controlled output impedance,
the output voltage can be level in the direction that works to
control the voltage spike coincident with fast load current
demand changes.
The magnitude of the spike is dictated by the ESR and ESL
of the output capacitors selected. By positioning the no-load
voltage level near the upper specification limit, a larger
negative spike can be sustained without crossing the lower
limit. By adding a well controlled output impedance, the
output voltage under load can effectively be level shifted
down so that a larger positive spike can be sustained without
crossing the upper specification limit.
As shown in Figure 5, a current proportional to the average
current in all active channels, IAVG, flows from FB through a
load-line regulation resistor, RFB. The resulting voltage drop
across RFB is proportional to the output current, effectively
creating an output voltage droop with a steady-state value
defined as:
V DROOP = I AVG R FB
(EQ. 5)
In most cases, each channel uses the same RISEN value to
sense current. A more complete expression for VDROOP is
derived by combining Equations 4 and 5.
I OUT r DS  ON 
V DROOP = ---------------------------------- R FB
N
R ISEN
0.5  R REF
R OFS = -----------------------------V OFFSET
(EQ. 8)
FB
DAC
DYNAMIC
VID D/A
RREF
E/A
REF
VCC
or
GND
-
ROFS
+
+
OFS
0.5V
-
ISL6556ACB
2.0V
GND
VCC
FIGURE 6. OUTPUT VOLTAGE OFFSET PROGRAMMING
WITH ISL6556ACB (28-LEAD SOIC)
(EQ. 6)
FB
Output-Voltage Offset Programming
The ISL6556A allows the designer to accurately adjust the
offset voltage. When a resistor, ROFS, is connected between
OFS and VCC, the voltage across it is regulated to 2.0V.
This causes a proportional current (IOFS) to flow into OFS. If
ROFS is connected to ground, the voltage across it is
regulated to 0.5V, and IOFS flows out of OFS. A resistor
between DAC and REF, RREF, is selected so that the
product (IOFS x RREF) is equal to the desired offset voltage.
These functions are shown in Figures 6 and 7.
DYNAMIC
VID D/A
RREF
E/A
REF
OFSOUT
As evident in Figure 7, the OFSOUT pin must be connected
to the REF pin for this current injection to function in
ISL6556ACR. The current flowing through RREF creates an
offset at the REF pin, which is ultimately duplicated at the
output of the regulator.
Once the desired output offset voltage has been determined,
use the following formulae to set ROFS:
For Positive Offset (connect ROFS to VCC):
2  R REF
R OFS = -------------------------V OFFSET
14
VCC
or
GND
2.0V
-
ROFS
+
+
0.5V
VCC
(EQ. 7)
DAC
-
ISL6556ACR
OFS
GND
FIGURE 7. OUTPUT VOLTAGE OFFSET PROGRAMMING
WITH ISL6556ACR (32-LEAD QFN)
ISL6556A
Dynamic VID
Modern microprocessors need to make changes to their
core voltage as part of normal operation. They direct the
core-voltage regulator to do this by making changes to the
VID inputs during regulator operation. The power
management solution is required to monitor the DAC inputs
and respond to on-the-fly VID changes in a controlled
manner. Supervising the safe output voltage transition within
the DAC range of the processor without discontinuity or
disruption is a necessary function of the core-voltage
regulator.
The ISL6556A checks the VID inputs six times every
switching cycle. If the VID code is found to have changed,
the controller waits half of a complete cycle before executing
a 12.5mV change. If during the half-cycle wait period, the
difference between DAC level and the new VID code
changes sign, no change is made. If the VID code is more
than 1 bit higher or lower than the DAC (not recommended),
the controller will execute 12.5mV changes six times per
cycle until VID and DAC are equal. It is for this reason that it
is important to carefully control the rate of VID stepping in 1bit increments.
In order to ensure the smooth transition of output voltage
during VID change, a VID step change smoothing network
composed of RREF and CREF is required for an ISL6556A
based voltage regulator. The selection of RREF is based on
the desired offset as detailed above in Output-Voltage Offset
Programming. The selection of CREF is based on the time
duration for 1 bit VID change and the allowable delay time.
Assuming the microprocessor controls the VID change at 1
bit every TVID, the relationship between the time constant of
RREF and CREF network and TVID is given by Equation 9.
C REF R REF = 4 T VID
(EQ. 9)
Typically RREF is selected to be 1k, so with a VID step
change rate of 5s per bit, the value of CREF is 22nF based
on Equation 9.
Initialization
Prior to initialization, proper conditions must exist on the
enable inputs and VCC. When the conditions are met, the
controller begins soft-start. Once the output voltage is within
the proper window of operation, PGOOD asserts logic 1.
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 ISL6556A
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 ISL6556A is guaranteed. Hysteresis between the
15
rising and falling thresholds assure that once enabled,
the ISL6556A will not inadvertently turn off unless the
bias voltage drops substantially (see Electrical
Specifications).
ISL6556A INTERNAL CIRCUIT
EXTERNAL CIRCUIT
VCC
POR
CIRCUIT
+12V
10.7k
ENABLE
COMPARATOR
EN
+
-
1.40k
1.24V
ENLL
(ISL6556ACR only)
SOFT START
AND
FAULT LOGIC
FIGURE 8. POWER SEQUENCING USING THRESHOLDSENSITIVE ENABLE (EN) FUNCTION
2. The ISL6556A features an enable input (EN) for power
sequencing between the controller bias voltage and
another voltage rail. The enable comparator holds the
ISL6556A in shutdown until the voltage at EN rises above
1.24V. The enable comparator has about 100mV of
hysteresis to prevent bounce. It is important that the
driver ICs reach their POR level before the ISL6556A
becomes enabled. The schematic in Figure 8
demonstrates sequencing the ISL6556A with the
HIP660X family of Intersil MOSFET drivers, which require
12V bias.
3. (ISL6556ACR only) The voltage on ENLL must be logic
high to enable the controller. This pin is typically
connected to the VID_PGOOD. The ISL6556ACB has
this signal internally connected high.
4. The VID code must not be 111111 or 111110. These codes
signal the controller that no load is present. The controller
will enter shut-down mode after receiving either of these
codes and will execute soft start upon receiving any other
code. These codes can be used to enable or disable the
controller but it is not recommended. After receiving one
of these codes, the controller executes a 2-cycle delay
before changing the overvoltage trip level to the shutdown level and disabling PWM. Overvoltage shutdown
cannot be reset using one of these codes.
To enable the controller, VCC must be greater than the POR
threshold; the voltage on EN must be greater than 1.24V; for
ISL6556ACR, ENLL must be logic high; and VID cannot be
equal to 111111 or 111110. When each of these conditions is
true, the controller immediately begins the soft-start
sequence.
ISL6556A
Soft-Start
Power Good Signal
During soft start, the DAC voltage ramps linearly from zero to
the programmed VID level. The PWM signals remain in the
high-impedance 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 preexisting charge on the output as the controller attempted to
regulate to zero volts at the beginning of the soft-start cycle.
The soft-start time, tSS, begins with a delay period equal to
64 switching cycles followed by a linear ramp with a rate
determined by the switching period, 1/fSW.
The power good pin (PGOOD) is an open-drain logic output
that transitions high when the converter is operating after
soft start.
64 + 1280  VID
t SS = ----------------------------------------f SW
PGOOD pulls low during shutdown and releases high after
a successful soft start. PGOOD only transitions low when an
undervoltage condition is detected or the controller is
disabled by a reset from EN, ENLL, POR, or one of the
no-CPU VID codes. After an undervoltage event, PGOOD
will return high unless the controller has been disabled.
PGOOD does not automatically transition low upon
detection of an overvoltage condition.
(EQ. 10)
PGOOD
For example, a regulator with 250kHz switching frequency
having VID set to 1.35V has tSS equal to 6.912ms.
During the first 640 cycles of soft start (704 cycles following
enable) the DAC voltage increments the reference in 25mV
steps. The remainder of soft start sees the DAC ramping
with 12.5mV steps.
UV
OC
-
+
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.
SOFT START, FAULT
AND CONTROL LOGIC
+
VDIFF
100A
+
I1
REPEAT FOR
EACH CHANNEL
75%
DAC
REFERENCE
-
OV
OC
-
+
100A
IAVG
OVP
VOUT, 500mV/DIV
VID + 0.2V
FIGURE 10. POWER GOOD AND PROTECTION CIRCUITRY
Undervoltage Detection
2ms/DIV
EN, 5V/DIV
The undervoltage threshold is set at 75% of the VID code.
When the output voltage at VSEN is below the undervoltage
threshold, PGOOD gets pulled low.
Overvoltage Protection
500s/DIV
FIGURE 9. SOFT-START WAVEFORMS WITH AN UN-BIASED
OUTPUT. FSW = 500kHz
Fault Monitoring and Protection
The ISL6556A actively monitors output voltage and current
to detect fault conditions. Fault monitors trigger protective
measures to prevent damage to a microprocessor load. One
common power good indicator is provided for linking to
external system monitors. The schematic in Figure 10
outlines the interaction between the fault monitors and the
power good signal.
16
When VCC is above 1.4V, but otherwise not valid as defined
under Power on Reset in Electrical Specifications, the
overvoltage trip circuit is active using auxiliary circuitry. In
this state, an overvoltage trip occurs if the voltage at VSEN
exceeds 1.8V.
With valid VCC, the overvoltage circuit is sensitive to the
voltage at VDIFF. In this state, the trip level is 1.7V prior to
valid enable conditions being met as described in Enable
and Disable. The only exception to this is when the IC has
been disabled by an overvoltage trip. In that case the
overvoltage trip point is VID plus 200mV. During soft start,
the overvoltage trip level is the higher of 1.7V or VID plus
200mV. Upon successful soft start, the overvoltage trip level
is 200mV above VID. Two actions are taken by the
ISL6556A
ISL6556A to protect the microprocessor load when an
overvoltage condition occurs.
At the inception of an overvoltage event, all PWM outputs
are commanded low until the voltage at VSEN falls below
0.6V with valid VCC or 1.5V otherwise. This causes the
Intersil drivers to turn on the lower MOSFETs and pull the
output voltage below a level that might cause damage to the
load. The PWM outputs remain low until VDIFF falls to the
programmed DAC level when they enter a high-impedance
state. The Intersil drivers respond to the high-impedance
input by turning off both upper and lower MOSFETs. If the
overvoltage condition reoccurs, the ISL6556A will again
command the lower MOSFETs to turn on. The ISL6556A will
continue to protect the load in this fashion as long as the
overvoltage condition recurs.
Simultaneous to the protective action of the PWM outputs, the
OVP pin pulls to VCC delivering up to 100mA to the gate of a
crowbar MOSFET or SCR placed either on the input rail or the
output rail. Turning on the MOSFET or SCR collapses the
power rail and causes a fuse placed further up stream to blow.
The fuse must be sized such that the MOSFET or SCR will
not overheat before the fuse blows. The OVP pin is tolerant to
12V (see Absolute Maximum Ratings), so an external resistor
pull up can be used to augment the driving capability. If using
a pull up resistor in conjunction with the internal overvoltage
protection function, care must be taken to avoid nuisance trips
that could occur when VCC is below 2V. In that case, the
controller is incapable of holding OVP low.
Once an overvoltage condition is detected, normal PWM
operation ceases until the ISL6556A is reset. Cycling the
voltage on EN or ENLL or VCC below the POR-falling
threshold will reset the controller. Cycling the VID codes will
not reset the controller.
Overcurrent Protection
ISL6556A has two levels of overcurrent protection. Each
phase is protected from a sustained overcurrent condition on
a delayed basis, while the combined phase currents are
protected on an instantaneous basis.
In instantaneous protection mode, the ISL6556A takes
advantage of the proportionality between the load current
and the average current, IAVG to detect an overcurrent
condition. See the Channel-Current Balance section for
more detail on how the average current is measured. The
average current is continually compared with a constant
100A reference current as shown in Figure 10. Once the
average current exceeds the reference current, a
comparator triggers the converter to shutdown.
In individual overcurrent protection mode, the ISL6556A
continuously compares the current of each channel with the
same 100A reference current. If any channel current
exceeds the reference current continuously for eight
consecutive cycles, the comparator triggers the converter to
shutdown.
17
At the beginning of overcurrent shutdown, the controller
places all PWM signals in a high-impedance state
commanding the Intersil MOSFET driver ICs to turn off both
upper and lower MOSFETs. The system remains in this state
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. If the fault remains, trip-retry cycles continue
indefinitely as shown in Figure 11 until either 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 during this
kind of operation.
OUTPUT CURRENT, 50A/DIV
0A
OUTPUT VOLTAGE,
500mV/DIV
0V
2ms/DIV
FIGURE 11. OVERCURRENT BEHAVIOR IN HICCUP MODE.
FSW = 500kHz
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 below. In
addition to this guide, Intersil provides complete reference
designs that include schematics, bills of materials, and example
board layouts for all common microprocessor 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; and
the total board space available for power-supply circuitry.
Generally speaking, the most economical solutions are
those in which each phase handles between 15 and 20A. 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 may be
pushed as high above 30A per phase, but these designs
ISL6556A
require heat sinks and forced air to cool the MOSFETs,
inductors and heat-dissipating surfaces.
the required time for this commutation is t1 and the
approximated associated power loss is PUP,1.
MOSFETs
I M I PP  t 1 
P UP,1  V IN  -----  ----  f
 N- + -------2   2 S
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 heat dissipated in the lower MOSFET is
simple, since virtually all of the heat loss in the lower
MOSFET is due to current conducted through the channel
resistance (rDS(ON)). In Equation 11, IM is the maximum
continuous output current; IPP is the peak-to-peak inductor
current (see Equation 1); d is the duty cycle (VOUT/VIN); and
L is the per-channel inductance.
I L, 2PP  1-d 
 I M 2
P LOW 1 = r DS  ON   -----  1 – d  + ----------------------------12
 N
(EQ. 11)
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
lower-MOSFET body diode. This term is dependent on the
diode forward voltage at IM, VD(ON); the switching
frequency, fS; and the length of dead times, td1 and td2, at
the beginning and the end of the lower-MOSFET conduction
interval respectively.
I
I M I PP
 M
P LOW 2 = V D  ON  f S  ----- t d1 +  ------  N- + -------N
2
I PP
-------- t
2  d2
(EQ. 12)
Thus 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 upperMOSFET 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 reverserecovery 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 13,
18
(EQ. 13)
At turn on, the upper MOSFET begins to conduct and this
transition occurs over a time t2. In Equation 14, the
approximate power loss is PUP,2.
 I M I PP  t 2 
P UP, 2  V IN  ----- – ---------  ----  f S
2  2
N
(EQ. 14)
A third component involves the lower MOSFET’s reverserecovery charge, Qrr. Since the inductor current has fully
commutated to the upper MOSFET before the lowerMOSFET’s body diode can draw all of Qrr, it is conducted
through the upper MOSFET across VIN. The power
dissipated as a result is PUP,3 and is approximately
P UP,3 = V IN Q rr f S
(EQ. 15)
Finally, the resistive part of the upper MOSFETs is given in
Equation 16 as PUP,4.
The total power dissipated by the upper MOSFET at full load
can now be approximated as the summation of the results
from Equations 13, 14, 15 and 16. 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.
2
 I M
I PP2
P UP,4  r DS  ON   ----- d + ---------12
 N
(EQ. 16)
Current Sensing
The ISEN pins are denoted ISEN1, ISEN2, ISEN3 and
ISEN4. The resistors connected between these pins and the
respective phase nodes determine the gains in the load-line
regulation loop and the channel-current balance loop as well
as setting the overcurrent trip point. 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 17 (see also Figure 3).
r DS  ON 
R ISEN = ---------------------70 10 – 6
I FL
-------N
(EQ. 17)
In certain circumstances, it may be necessary to adjust the
value of one or more ISEN resistor. 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 the section entitled Channel-Current Balance).
Choose RISEN,2 in proportion to the desired decrease in
ISL6556A
T
R ISEN ,2 = R ISEN ----------2
T 1
yields a solution that is always stable with very close to ideal
transient performance.
C2 (OPTIONAL)
(EQ. 18)
In Equation 18, make sure that T2 is the desired
temperature rise above the ambient temperature, and T1 is
the measured temperature rise above the ambient
temperature. While a single adjustment according to Equation
18 is usually sufficient, it may occasionally be necessary to
adjust RISEN two or more times to achieve optimal thermal
balance between all channels.
RC
CC
FB
+
RFB
VDROOP
VDIFF
The load-line regulation resistor is labeled RFB in Figure 5.
Its value depends on the desired full-load droop voltage
(VDROOP in Figure 5). If Equation 17 is used to select each
ISEN resistor, the load-line regulation resistor is as shown
in Equation 19.
(EQ. 19)
If one or more of the ISEN resistors is adjusted for thermal
balance, as in Equation 18, the load-line regulation resistor
should be selected according to Equation 20 where IFL is the
full-load operating current and RISEN(n) is the ISEN resistor
connected to the nth ISEN pin.
V DROOP
R FB = -------------------------------I FL r DS  ON 
 RISEN  n 
IOUT
-
Load-Line Regulation Resistor
V DROOP
R FB = ------------------------6
70 10
COMP
ISL6556A
temperature rise in order to cause proportionally less current
to flow in the hotter phase.
(EQ. 20)
n
FIGURE 12. COMPENSATION CONFIGURATION FOR
LOAD-LINE REGULATED ISL6556A CIRCUIT
The feedback resistor, RFB, has already been chosen as
outlined in Load-Line Regulation Resistor. Select a target
bandwidth for the compensated system, f0. The target
bandwidth must be large enough to assure adequate
transient performance, but smaller than 1/3 of the perchannel switching frequency. The values of the
compensation components depend on the relationships of f0
to the L-C pole frequency and the ESR zero frequency. For
each of the following three, there is a separate set of
equations for the compensation components.
Case 1:
1
------------------- > f 0
2 LC
2f 0 V pp LC
R C = R FB ----------------------------------0.75V
Compensation
IN
0.75V IN
C C = ----------------------------------2V PP R FB f 0
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 Regulation.
COMPENSATING 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, RC and CC.
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
19
Case 2:
1
1
-------------------  f 0 < ----------------------------2C  ESR 
2 LC
V PP  2  2 f 02 LC
R C = R FB -------------------------------------------0.75 V
(EQ. 21)
IN
0.75V IN
C C = -----------------------------------------------------------2
 2  f 02 V PP R FB LC
Case 3:
1
f 0 > -----------------------------2C  ESR 
2 f 0 V pp L
R C = R FB ----------------------------------------0.75 V IN  ESR 
0.75V IN  ESR  C
C C = -----------------------------------------------2V PP R FB f 0 L
In Equations 21, 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
ISL6556A
resistance of the bulk output-filter capacitance; and VPP is
the peak-to-peak sawtooth signal amplitude as described in
Figure 4 and Electrical Specifications.
Once selected, the compensation values in Equations 21
assure a stable converter with reasonable transient
performance. In most cases, transient performance can be
improved by making adjustments to RC. Slowly increase the
value of RC while observing the transient performance on an
oscilloscope until no further improvement is noted. Normally,
CC will not need adjustment. Keep the value of CC from
Equations 21 unless some performance issue is noted.
The optional capacitor C2, is sometimes needed to bypass
noise away from the PWM comparator (see Figure 12). Keep
a position available for C2, and be prepared to install a highfrequency capacitor of between 22pF and 150pF in case any
leading-edge jitter problem is noted.
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 necessarily limits the
system transient response. The output capacitor 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, I; the load-current slew rate, di/dt; and the
maximum allowable output-voltage deviation under transient
loading, VMAX. 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 outputvoltage deviation is less than the allowable maximum.
Neglecting the contribution of inductor current and regulator
response, the output voltage initially deviates by an amount
di
V   ESL  ----- +  ESR  I
dt
(EQ. 22)
The filter capacitor must have sufficiently low ESL and ESR
so that V < VMAX.
Most capacitor solutions rely on a mixture of high-frequency
capacitors with relatively low capacitance in combination
20
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 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 IN - N V OUT  V OUT
L   ESR  --------------------------------------------------------f S V IN V PP MAX 
(EQ. 23)
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
VMAX. This places an upper limit on inductance.
Equation 24 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 25
addresses the leading edge. Normally, the trailing edge
dictates the selection of L because duty cycles are usually
2NCVO
L  --------------------  V MAX - I  ESR  
 I  2
(EQ. 24)
 1.25  NC
L  --------------------------  V MAX - I  ESR    V - V 
IN
O
 I  2
(EQ. 25)
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.
Input Supply Voltage Selection
The VCC input of the ISL6556A can be connected either
directly to a +5V supply or through a current limiting resistor to
a +12V supply. An integrated 5.8V shunt regulator maintains
the voltage on the VCC pin when a +12V supply is used. A
300 resistor is suggested for limiting the current into the
VCC pin to a worst-case maximum of approximately 25mA.
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, 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-
ISL6556A
voltage ripple as outlined in Output Filter Design. Choose the
lowest switching frequency that allows the regulator to meet
the transient-response requirements.
close as possible to each upper MOSFET drain to minimize
board parasitic impedances and maximize suppression.
0.3
INPUT-CAPACITOR CURRENT (IRMS / IO)
RT (k)
1000
100
0.2
0.1
IL,PP = 0
IL,PP = 0.5 IO
IL,PP = 0.75 IO
0
10
100
1000
SWITCHING FREQUENCY (kHz)
Switching frequency is determined by the selection of the
frequency-setting resistor, RT (see the figures labeled
Typical Application on pages 4 and 5). Figure 13 and
Equation 26 are provided to assist in selecting the correct
value for RT.
·
 10.6258-  1.03167  log  f S  
– 1200
(EQ. 26)
Input Capacitor Selection
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.
For a two phase design, use Figure 14 to determine the
input-capacitor RMS current requirement given the duty
cycle, maximum sustained output current (IO), and the ratio
of the per-phase 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 should also be at least 1.25 times greater
than the maximum input voltage.
Figures 15 and 16 provide the same input RMS current
information for three and four phase designs respectively.
Use the same approach to selecting the bulk capacitor type
and number as described above.
Low capacitance, high-frequency ceramic capacitors are
needed in addition to the bulk capacitors to suppress leading
and falling edge voltage spikes. The result from the high
current slew rates produced by the upper MOSFETs turn on
and off. Select low ESL ceramic capacitors and place one as
21
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (VO/VIN)
FIGURE 14. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 2-PHASE CONVERTER
FIGURE 13. RT vs SWITCHING FREQUENCY
R T = 1.0203  10 
0
10000
0.3
INPUT-CAPACITOR CURRENT (IRMS/IO)
10
IL,PP = 0
IL,PP = 0.5 IO
IL,PP = 0.25 IO
IL,PP = 0.75 IO
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (VO/VIN)
FIGURE 15. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 3-PHASE CONVERTER
ISL6556A
Layout Considerations
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
The following layout strategies are intended to minimize the
impact of board parasitic impedances on converter
performance and to optimize the heat-dissipating capabilities
of the printed-circuit board. These sections highlight some
important practices which should not be overlooked during the
layout process.
0.2
Component Placement
0.1
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (VO/VIN)
FIGURE 16. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 4-PHASE CONVERTER
MULTI-PHASE RMS IMPROVEMENT
Figure 17 is provided as a reference to demonstrate the
dramatic reductions in input-capacitor RMS current upon the
implementation of the multi-phase topology. For example,
compare the input RMS current requirements of a two-phase
converter versus that of a single phase. Assume both
converters have a duty cycle of 0.25, maximum sustained
output current of 40A, and a ratio of IC,PP to IO of 0.5. The
single phase converter would require 17.3 Arms current
capacity while the two-phase converter would only require
10.9 Arms. The advantages become even more pronounced
when output current is increased and additional phases are
added to keep the component cost down relative to the
single phase approach.
INPUT-CAPACITOR CURRENT (IRMS/IO)
0.6
Within the allotted implementation area, orient the switching
components first. The switching components are the most
critical because they carry large amounts of energy and tend
to generate high levels of noise. Switching component
placement should take into account power dissipation. Align
the output inductors and MOSFETs such that space between
the components is minimized while creating the PHASE
plane. Place the Intersil MOSFET driver IC as close as
possible to the MOSFETs they control to reduce the parasitic
impedances due to trace length between critical driver input
and output signals. If possible, duplicate the same
placement of these components for each phase.
Next, place the input and output capacitors. Position one
high-frequency ceramic input capacitor next to each upper
MOSFET drain. Place the bulk input capacitors as close to
the upper MOSFET drains as dictated by the component
size and dimensions. Long distances between input
capacitors and MOSFET drains result in too much trace
inductance and a reduction in capacitor performance. Locate
the output capacitors between the inductors and the load,
while keeping them in close proximity to the microprocessor
socket.
The ISL6556A can be placed off to one side or centered
relative to the individual phase switching components.
Routing of sense lines and PWM signals will guide final
placement. Critical small signal components to place close
to the controller include the ISEN resistors, RT resistor,
feedback resistor, and compensation components.
Bypass capacitors for the ISL6556A and HIP660X driver
bias supplies must be placed next to their respective pins.
Trace parasitic impedances will reduce their effectiveness.
0.4
Plane Allocation and Routing
0.2
Dedicate one solid layer, usually a middle layer, for a ground
plane. Make all critical component ground connections with
vias to this plane. Dedicate one additional layer for power
planes; breaking the plane up into smaller islands of
common voltage. Use the remaining layers for signal wiring.
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 (VO/VIN)
FIGURE 17. NORMALIZED INPUT-CAPACITOR RMS
CURRENT vs DUTY CYCLE FOR SINGLE-PHASE
CONVERTER
22
Route phase planes of copper filled polygons on the top and
bottom once the switching component placement is set. Size
the trace width between the driver gate pins and the
MOSFET gates to carry 1A of current. When routing
components in the switching path, use short wide traces to
reduce the associated parasitic impedances.
ISL6556A
Small Outline Plastic Packages (SOIC)
M28.3 (JEDEC MS-013-AE ISSUE C)
N
INDEX
AREA
28 LEAD WIDE BODY SMALL OUTLINE PLASTIC PACKAGE
H
0.25(0.010) M
B M
INCHES
E
SYMBOL
-B-
1
2
3
L
SEATING PLANE
-A-
h x 45o
A
D
-C-
e
A1
B
0.25(0.010) M
C
0.10(0.004)
C A M
B S
MILLIMETERS
MIN
MAX
NOTES
A
0.0926
0.1043
2.35
2.65
-
0.0040
0.0118
0.10
0.30
-
B
0.013
0.0200
0.33
0.51
9
C
0.0091
0.0125
0.23
0.32
-
D
0.6969
0.7125
17.70
18.10
3
E
0.2914
0.2992
7.40
7.60
4
0.05 BSC
10.00
h
0.01
0.029
0.25
0.75
5
L
0.016
0.050
0.40
1.27
6
8o
0o
28
0o
10.65
-
0.394

0.419
1.27 BSC
H
N
NOTES:
MAX
A1
e

MIN
28
1. Symbols are defined in the “MO Series Symbol List” in Section 2.2
of Publication Number 95.
-
7
8o
Rev. 0 12/93
2. Dimensioning and tolerancing per ANSI Y14.5M-1982.
3. Dimension “D” does not include mold flash, protrusions or gate
burrs. Mold flash, protrusion and gate burrs shall not exceed
0.15mm (0.006 inch) per side.
4. Dimension “E” does not include interlead flash or protrusions. Interlead flash and protrusions shall not exceed 0.25mm (0.010
inch) per side.
5. The chamfer on the body is optional. If it is not present, a visual
index feature must be located within the crosshatched area.
6. “L” is the length of terminal for soldering to a substrate.
7. “N” is the number of terminal positions.
8. Terminal numbers are shown for reference only.
9. The lead width “B”, as measured 0.36mm (0.014 inch) or greater
above the seating plane, shall not exceed a maximum value of
0.61mm (0.024 inch)
10. Controlling dimension: MILLIMETER. Converted inch dimensions are not necessarily exact.
For additional products, see www.intersil.com/en/products.html
Intersil products are manufactured, assembled and tested utilizing ISO9001 quality systems as noted
in the quality certifications found at www.intersil.com/en/support/qualandreliability.html
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
23
ISL6556A
Quad Flat No-Lead Plastic Package (QFN)
Micro Lead Frame Plastic Package (MLFP)
L32.5x5B
32 LEAD QUAD FLAT NO-LEAD PLASTIC PACKAGE
(COMPLIANT TO JEDEC MO-220VHHD-2 ISSUE C
MILLIMETERS
SYMBOL
MIN
NOMINAL
MAX
NOTES
A
0.80
0.90
1.00
-
A1
-
-
0.05
-
A2
-
-
1.00
9
A3
b
0.20 REF
0.18
D
0.30
5,8
5.00 BSC
D1
D2
0.23
9
-
4.75 BSC
3.15
3.30
9
3.45
7,8
E
5.00 BSC
-
E1
4.75 BSC
9
E2
3.15
e
3.30
3.45
7,8
0.50 BSC
-
k
0.25
-
-
-
L
0.30
0.40
0.50
8
L1
-
-
0.15
10
N
32
2
Nd
8
3
Ne
8
3
P
-
-
0.60
9

-
-
12
9
Rev. 1 10/02
NOTES:
1. Dimensioning and tolerancing conform to ASME Y14.5-1994.
2. N is the number of terminals.
3. Nd and Ne refer to the number of terminals on each D and E.
4. All dimensions are in millimeters. Angles are in degrees.
5. Dimension b applies to the metallized terminal and is measured
between 0.15mm and 0.30mm from the terminal tip.
6. The configuration of the pin #1 identifier is optional, but must be
located within the zone indicated. The pin #1 identifier may be
either a mold or mark feature.
7. Dimensions D2 and E2 are for the exposed pads which provide
improved electrical and thermal performance.
8. Nominal dimensions are provided to assist with PCB Land Pattern
Design efforts, see Intersil Technical Brief TB389.
9. Features and dimensions A2, A3, D1, E1, P &  are present when
Anvil singulation method is used and not present for saw
singulation.
10. Depending on the method of lead termination at the edge of the
package, a maximum 0.15mm pull back (L1) maybe present. L
minus L1 to be equal to or greater than 0.3mm.
24
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