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1-888-IN
ISL6557A
July 2004
FN9068.3
Multiphase PWM Controller for
CORE Voltage Regulation
Features
The ISL6557A provides core-voltage regulation by driving up
to four interleaved synchronous-rectified buck-converter
channels in parallel. Intersil multi-phase controllers together
with Intersil MOSFET drivers form the basis for the most
reliable power-suppply solutions available to power the
latest industry-leading microprocessors. Multi-phase buck
converter architecture uses interleaved timing to multiply
ripple frequency and reduce input and output ripple currents.
Lower ripple results in lower total component cost, reduced
dissipation, and smaller implementation area. Preconfigured for 4-phase operation, the ISL6557A offers the
flexibility of selectable 2- or 3-phase operation. Simply
connect the unused PWM pins to VCC. The channel
switching frequency is adjustable in the range of 50kHz to
igMHz giving the designer the ultimate flexibility in managing
the balance between high-speed response and good
thermal management.
• Active Channel Current Balancing
New features on the ISL6557A include Dynamic-VID™
technology allowing seamless on-the-fly VID changes with
no need for any additional external components. When the
ISL6557A receives a new VID code, it incrementally steps
the output voltage to the new level. Dynamic VID changes
are fast and reliable with no output voltage overshoot or
undershoot. The RGND and VSEN pins provide inputs for
differential remote voltage sensing to improve regulation and
protection accuracy. A threshold-sensitive enable pin (EN)
can be used with an external resistor divider to optionally set
the power-on voltage level. This allows optional start-up
coordination with Intersil MOSFET drivers or any other
devices powered from a separate supply.
Like other Intersil multiphase controllers, the ISL6557A uses
cost and space-saving rDS(ON) sensing for channel current
balance, dynamic voltage positioning, and overcurrent
protection. Channel current balancing is automatic and
accurate with the integrated current-balance control system.
Overcurrent protection can be tailored to any application with
no need for additional parts. The IOUT pin carries a signal
proportional to load current and can be optionally connected
to FB for accurate load-line regulation.
An integrated DAC decodes the 5-bit logic signal present at
VID4-VID0 and provides an accurate reference for precision
voltage regulation. The high-bandwidth error amplifier,
differential remote-sensing amplifier, and accurate voltage
reference all work together to provide better than 0.8% total
system accuracy, and to enable the fastest transient
response available.
1
• Multi-Phase Power Conversion
• Precision rDS(ON) Current Sensing
- Low Cost
- Lossless
• Precision CORE Voltage Regulation
- Differential Remote Voltage Sensing
- 0.8% System Accuracy
• Microprocessor Voltage Identification Input
- Dynamic VID technology
- 5-Bit VID Input
- 0.800V to 1.550V in 25mV Steps
• Programmable Power-On Bias Level
• Programmable Droop Voltage
• Fast Transient Recovery Time
• Precision Enable Threshold
• Overcurrent Protection
• 2-, 3-, or 4-Phase Operation
• High Ripple Frequency. Channel Frequency Times
Number Channels (100kHz to 6MHz)
• Pb-free available
Ordering Information
PART NUMBER
TEMP. (oC)
PACKAGE
PKG. DWG. #
ISL6557ACB
0 to 70
24-Ld SOIC
M24.3
ISL6557ACBZ
(See Note)
0 to 70
24-Ld SOIC
(Pb-free)
M24.3
*Add “-T” suffix to part number for tape and reel packaging.
NOTE: Intersil Pb-free products employ special Pb-free material sets; molding
compounds/die attach materials and 100% matte tin plate termination finish, which
is 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-020B.
Pinout
ISL6557A 24 PIN (SOIC)
TOP VIEW
VID4 1
24 VCC
VID3 2
23 EN
VID2 3
22 FS
VID1 4
21 PGOOD
VID0 5
20 PWM4
COMP 6
19 ISEN4
FB 7
18 ISEN1
IOUT 8
17 PWM1
VDIFF 9
16 PWM2
VSEN 10
15 ISEN2
RGND 11
14 ISEN3
GND 12
13 PWM3
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 trademark of Intersil Americas Inc.
Copyright © Intersil Americas Inc. 2003, 2004. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners. Dynamic VID™ is a trademark of Intersil Americas Inc.
ISL6557A
Block Diagram
VDIFF
RGND
VCC
PGOOD
x1
-
POWER-ON
+
RESET (POR)
+
1.23V
EN
OV
S LATCH
UV
+
+
VSEN
+
OVP
-
CLOCK AND
SAWTOOTH
GENERATOR

FS
+
-
PWM1
PWM
-
2.1V
x 0.9
+
SOFTSTART
AND FAULT
LOGIC

+
PWM2
PWM
-
-
COMP
+
2.5V

+
PWM3
PWM
-
VID0
-
VID1
VID2
DYNAMIC
VID
D/A
+
VID3
+
VID4
-

-
E/A
+
-
CURRENT
FB
CORRECTION
OC
IOUT
+
I_TOT
IOC
PHASE
NUMBER
CHANNEL
DETECTOR
ISEN1
+

PWM4
PWM
ISEN2
+
+
ISEN3
+
ISEN4
GND
2
ISL6557A
Typical Application - 4-Phase Buck Converter
+12V
VIN
VCC
BOOT
UGATE
PVCC
+5V
PHASE
HIP6601A
DRIVER
PWM
LGATE
GND
IOUT
FB
COMP
VDIFF
VCC
VSEN
EN
+12V
VIN
VCC
BOOT
UGATE
RGND
PVCC
PGOOD
PHASE
ISEN1
VID4
ISL6557A
VID3
VID2
HIP6601A
DRIVER
PWM
PWM1
LGATE
PWM2
GND
ISEN2
VID1
PWM3
VID0
+12V
VIN
ISEN3
FS
P
LOAD
PWM4
RT
VCC
BOOT
UGATE
ISEN4
PVCC
GND
PHASE
HIP6601A
DRIVER
PWM
LGATE
GND
+12V
VIN
VCC
BOOT
UGATE
PVCC
PWM
PHASE
HIP6601A
DRIVER
LGATE
GND
3
ISL6557A
Absolute Maximum Ratings
Thermal Information
Supply Voltage, VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7V
Input, Output, or I/O Voltage . . . . . . . . . . GND -0.3V to VCC + 0.3V
ESD Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5kV
Thermal Resistance (Typical, Note 1)
Recommended Operating Conditions
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +5V 5%
Ambient Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . 0oC to 70oC
JA (oC/W)
SOIC Package . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
Maximum Junction Temperature . . . . . . . . . . . . . . . . . . . . . . .150oC
Maximum Storage Temperature Range . . . . . . . . . . -65oC to 150oC
Maximum Lead Temperature (Soldering 10s) . . . . . . . . . . . . .300oC
(SOIC - Lead Tips Only)
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.
NOTE:
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.)
Operating Conditions: VCC = 5V, TA = 0oC to 70oC, Unless Otherwise Specified.
Electrical Specifications
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
10.5
15
mA
INPUT SUPPLY POWER
Input Supply Current
RT = 100k, EN = 5V
RT = 100k, EN = 0V
Power-On Reset Threshold
Enable Threshold
5
9.2
VCC Rising
4.25
4.38
4.5
V
VCC Falling
3.75
3.86
4.0
V
EN Rising
1.206
1.230
1.254
V
EN Falling
1.106
1.15
1.194
V
100
mV
50
nA
0.8
%VID
-10
A
0.8
V
Enable Hysteresis
60
Enable Current
EN = 3V
mA
SYSTEM ACCURACY
System Accuracy
(Note 2)
VID Pull Up
-0.8
-40
-20
VID Input Low Level
VID Input High Level (Note 3)
2.0
V
OSCILLATOR
Accuracy
-20
Frequency
RT = 110k (±1%)
Adjustment Range
20
250
80
Sawtooth Amplitude
kHz
1500
1.33
Duty-Cycle Range
0
%
kHz
V
75
%
ERROR AMPLIFIER
Open-Loop Gain
RL = 10k to ground
72
dB
Open-Loop Bandwidth
CL = 100pF, RL = 10k to ground
18
MHz
Slew Rate
CL = 100pF, RL = 10k to ground
5
V/s
Maximum Output Voltage
RL = 10k to ground
4.1
V
Input Impedance
80
k
Slew Rate
6
V/s
Bandwidth
10
MHz
3.6
REMOTE-SENSE AMPLIFIER
4
ISL6557A
Operating Conditions: VCC = 5V, TA = 0oC to 70oC, Unless Otherwise Specified. (Continued)
Electrical Specifications
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
-90
-75
-60
A
2.04
2.09
2.13
V
ISEN
Overcurrent Trip Level
PROTECTION and MONITOR
Overvoltage Threshold
VSEN Rising
Undervoltage Threshold
PGOOD Low Voltage
VSEN Falling
VID
V
VSEN Rising
92
%VID
VSEN Falling
90
%VID
IPGOOD = 4mA
0.18
0.4
mV
NOTES:
2. These parts are designed and adjusted for accuracy within the system tolerance given in the Electrical Specifications. The system tolerance
accounts for offsets in the differential and error amplifiers; reference-voltage inaccuracies; temperature drift; and the full DAC adjustment range.
3. VID input levels above 2.9V may produce an reference-voltage offset inaccuracy.
Functional Pin Descriptions
input to the external regulation circuitry and the internal
protection circuitry. Connect VSEN and RGND to the sense
pins of the remote load.
VID4 1
24 VCC
VID3 2
23 EN
VID2 3
22 FS
VID1 4
21 PGOOD
VID0 5
20 PWM4
Return for VCC and signal ground for the IC.
COMP 6
19 ISEN4
PWM3, PWM2, PWM1, PWM4 (Pins 13, 16, 17, 20)
FB 7
18 ISEN1
IOUT 8
17 PWM1
Pulse-width modulation outputs. These logic outputs tell the
driver IC(s) when to turn the MOSFETs on and off.
VDIFF 9
16 PWM2
VSEN 10
15 ISEN2
RGND 11
14 ISEN3
GND 12
13 PWM3
VID4, VID3, VID2, VID1, VID0 (Pins 1, 2, 3, 4, 5)
These are the inputs to the internal DAC that provides the
reference voltage for output regulation. Connect these pins
to either open-drain or active-pull-up type outputs. Pulling
these pins above 2.9V can cause a reference offset
inaccuracy.
FB (Pin 7) and COMP (Pin 6)
The internal error amplifier’s inverting input and output
respectively. These pins are connected to an external R-C
network to compensate the regulator.
IOUT (Pin 8)
The current out of this pin is proportional to output current
and is used for load-line regulation and load sharing. The
scale factor is set by the ratio of the ISEN resistors
(connected to pins 14, 15, 18, and 19) to the lower MOSFET
rDS(ON).
GND (Pin 12)
ISEN3, ISEN2, ISEN1, ISEN4 (PINS 14, 15, 18, 19)
Current sense inputs. A resistor connected between these
pins and the respective phase nodes has a current
proportional to the current in the lower MOSFET during its
conduction interval. The current is used as a reference for
channel balancing, load sharing, protection, and load-line
regulation.
PGOOD (Pin 21)
PGOOD is an open-drain logic output that changes to a logic
low when the differential output voltage at VDIFF swings
below 90% of the DAC setting or above 2.1V.
FS (Pin 22)
This pin has two functions. A resistor placed from FS to
ground sets the switching frequency. There is an inverse
relationship between the value of the resistor and the
switching frequency. This pin can also be used to disable the
controller. To disable the controller, pull this pin below 1V.
EN (Pin 23)
This is the threshold-sensitive enable input for the controller.
To enable the controller, pull this pin above 1.23V.
VDIFF (Pin 9), VSEN (Pin 10), RGND (Pin 11)
VCC (Pin 24)
VSEN and RGND are the inputs to the differential remotesense amplifier. VDIFF is the output and it serves as the
Bias supply voltage for the controller. Connect this pin to a
5V power supply.
5
ISL6557A
VIN
COMP
-
-
+
PWM
CIRCUIT
+
ISEN1
RISEN1
-
DAC
&
REFERENCE
VIN
PWM
CIRCUIT
+
L1
HIP6601A
FB
+
PWM1
PWM2
AVERAGE
IOUT
L2
HIP6601A
ERROR
AMPLIFIER
PWM
CIRCUIT
VOUT
ISEN2
CO
RISEN2
VIN
+
CURRENT
SENSE
-
CURRENT
SENSE
-
CURRENT
SENSE
PWM3
+
+
VDIFF
-
RGND
L3
HIP6601A
+
x1
P
LOAD
ISEN3
RISEN3
VSEN
FIGURE 1. SIMPLIFIED BLOCK DIAGRAM OF THE ISL6557A IN A 3-PHASE CONVERTER
Operation
Multi-Phase Power Conversion
Multi-phase power conversion provides the most costeffective power solution when load currents are no longer
easily supported by single-phase converters. Although its
greater complexity presents additional technical challenges,
the multi-phase approach offers cost-saving advantages with
improved response time, superior ripple cancellation, and
excellent thermal distribution.
that the designer can use less per-channel inductance and
lower total output capacitance for any performance
specification.
IL1 + IL2 + IL3, 7A/DIV
IL3, 7A/DIV
PWM3, 5V/DIV
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 4-phase converter, each channel switches 1/4
cycle after the previous channel and 1/4 cycle before the
following channel. As a result, the four-phase converter has
a combined ripple frequency four times greater than the
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
6
IL2, 7A/DIV
PWM2, 5V/DIV
IL1, 7A/DIV
PWM1, 5V/DIV
1s/DIV
FIGURE 2. PWM AND INDUCTOR-CURRENT WAVEFORMS
FOR 3-PHASE CONVERTER
ISL6557A
Figure 2 (previous page) 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, or 1.33s, after the
PWM pulse of the previous phase. The peak-to-peak current
waveforms 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 L 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.
 V IN – N V OUT  V OUT
I PP = -----------------------------------------------------------L fS V
(EQ. 2)
IN
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.
INPUT-CAPACITOR CURRENT, 10A/DIV
CHANNEL 3
INPUT CURRENT
10A/DIV
CHANNEL 2
INPUT CURRENT
10A/DIV
CHANNEL 1
INPUT CURRENT
10A/DIV
1s/DIV
FIGURE 3. CHANNEL INPUT CURRENTS AND INPUTCAPACITOR RMS CURRENT FOR 3-PHASE
CONVERTER
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
7
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 3 illustrates input
currents from a three-phase converter combining to reduce
the total input ripple current.
The converter depicted in Figure 3 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 down
12V to 1.5V at 36A. The single-phase 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 three-phase converter.
Figures 15, 16 and 17 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 18 shows the single
phase input-capacitor rms current for comparisson.
PWM OPERATION
The number of active channels selected determines the
timing for each channel. By default, the timing mode for the
ISL6557A is 4-phase. The designer can select 2-phase
timing by connecting PWM3 to VCC or 3-phase timing by
connecting PWM4 to VCC.
One switching cycle for the ISL6557A is defined as the time
between PWM1 pulse termination signals (the internal signal
that initiates a falling edge on PWM1). The cycle time is the
inverse of the switching frequency selected by the resistor
connected between the FS pin and ground (see Switching
Frequency). Each cycle begins when a clock signal
commands the channel-1 PWM output to go low. This
signals the channel-1 MOSFET driver to turn off the channel1 upper MOSFET and turn on the channel-1 synchronous
MOSFET. If two-channel operation is selected, the PWM2
pulse terminates 1/2 of a cycle later. If three channels are
selected the PWM2 pulse terminates 1/3 of a cycle after
PWM1, and the PWM3 output will follow after another 1/3 of
a cycle. When four channels are selected, the pulsetermination times are spaced in 1/4 cycle increments.
Once a channel’s PWM pulse terminates, it remains low for
a minimum of 1/4 cycle. This forced off time is required to
assure an accurate current sample as described in Current
Sensing. Following the 1/4-cycle forced off time, the
controller enables the PWM output. Once enabled, the PWM
output transitions high when the sawtooth signal crosses the
adjusted error-amplifier output signal, VCOMP as illustrated
in Figures 1 and 5. This is the signal for the MOSFET driver
to turn off the synchronous MOSFET and turn on the upper
MOSFET. The output will remain high until the clock signals
the beginning of the next cycle by commanding the PWM
pulse to terminate.
ISL6557A
CURRENT SENSING
Intersil multi-phase controllers sense current by sampling the
voltage across the lower MOSFET during its conduction
interval. MOSFET rDS(ON) sensing is a no-added-cost
method to sense current for load-line regulation, channelcurrent balance, module current sharing, and overcurrent
protection. If desired, an independent current-sense resistor
in series with the lower-MOSFET source can serve as a
sense element in place of the MOSFET rDS(ON).
VIN
I
In
SAMPLE
&
HOLD
r DS  ON 
SEN = I L ------------------------R
ISEN
CHANNEL N
UPPER MOSFET
IL
ISEN(n)
-
RISEN
+
I L r DS  ON 
+
CHANNEL N
LOWER MOSFET
ISL6557A INTERNAL CIRCUIT
EXTERNAL CIRCUIT
FIGURE 4. INTERNAL AND EXTERNAL CURRENT-SENSING
CIRCUITRY
The ISEN input for each channel uses a ground-referenced
amplifier to reproduce a signal proportional to the channel
current (Figure 4). After sufficient settling time, the sensed
current is sampled, and the sample is used for current
balance, load-line regulation and overcurrent protection. The
ISL6557A samples channel current once each cycle.
Figure 4 shows how the sampled current, In, is created from
the channel current IL. The circuitry in Figure 4 represents
the current measurement and sampling circuitry for channel
n in an N-channel converter. This circuitry is repeated for
each channel in the converter but may not be active in
channels 3 and 4 depending on the particular
implementation (see PWM Operation).
channel currents combines 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.
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
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, a proper
selection of the current sense resistors (RISEN in Figure 4)
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.
OVERCURRENT PROTECTION
The average current, IAVG in Figure 5, is continually
compared with a constant 75A reference current. If the
average current at any time exceeds the reference current,
the comparator triggers the converter to shut down. All PWM
signals are placed in a high-impedance state which signals
the drivers to turn off both upper and lower MOSFETs. The
system remains in this state while the controller counts 2048
phase-clock cycles.
VCOMP
+
+
-
PWM1
SAWTOOTH SIGNAL
f(j)
I4 *
IER
IAVG
-
N
+

I3 *
I2
CHANNEL-CURRENT BALANCE
Another 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 multiple parallel
MOSFETs and the expense of using expensive heat sinks
and exotic magnetic materials.
In order to fully realize the thermal advantage, it is important
that each channel in a multi-phase converter be controlled to
deliver about the same current at any load level. Intersil
multi-phase controllers guarantee current balance by
comparing each channel’s current to the average current
delivered by all channels and making an appropriate
adjustment to each channel’s pulse width based on the error.
Intersil’s patented current-balance method is illustrated in
Figure 5 where the average of the 2, 3, or 4 sampled
8
I1
FIGURE 5. CHANNEL-1 PWM FUNCTION AND CURRENTBALANCE ADJUSTMENT
NOTE: *Channels 3 and 4 are optional.
This is followed by a soft-start attempt (see Soft-Start). If the
soft-start attempt is successful, operation will continue as
normal. Should the soft-start attempt fail, the ISL6557A
repeats the 2048-cycle wait period and follows with another
soft-start attempt. This hiccup mode of operation continues
ISL6557A
indefinitely as shown in Figure 6 as long as the controller is
enabled or until the overcurrent condition resolves.
OUTPUT CURRENT, 20A/DIV
The integrating compensation network shown in Figure 7
assures that the steady-state error in the output voltage is
limited to the error in the reference voltage (output of the
DAC) plus offset errors in the remote-sense and error
amplifiers. Intersil specifies the guaranteed tolerance of the
ISL6557A and all Intersil controllers to include all variations
in the amplifiers and reference so that the output voltage
remains within the specified system tolerance.
TABLE 1. VOLTAGE IDENTIFICATION CODES
0A
VID4
VID3
VID2
VID1
VID0
VDAC
1
1
1
1
1
Off
1
1
1
1
0
0.800
1
1
1
0
1
0.825
1
1
1
0
0
0.850
1
1
0
1
1
0.875
1
1
0
1
0
0.900
VOLTAGE REGULATION
1
1
0
0
1
0.925
The ISL6557A uses a digital to analog converter (DAC) to
generate a reference voltage based on the logic signals at
pins VID4 to VID0. The DAC decodes the a 5-bit logic signal
(VID) into one of the discrete voltages shown in Table 1. Each
VID input offers a 20A pull up to 2.5V for use with open-drain
outputs. External pull-up resistors or active-high output
stages can augment the pull-up current sources, but a slight
accuracy error can occur if they are pulled above 2.9V.
1
1
0
0
0
0.950
1
0
1
1
1
0.975
1
0
1
1
0
1.000
1
0
1
0
1
1.025
1
0
1
0
0
1.050
1
0
0
1
1
1.075
The DAC-selected reference voltage is connected to the noninverting input of the error amplifier, and the output of the differential remote-sense amplifier usually gets connected to
the error amplifier as shown in Figure 7. The remote-sense
amplifier eliminates voltage differences between local and
remote ground to provide a more accurate means of sensing
output voltage.
1
0
0
1
0
1.100
1
0
0
0
1
1.125
1
0
0
0
0
1.150
0
1
1
1
1
1.175
0
1
1
1
0
1.200
0
1
1
0
1
1.225
0
1
1
0
0
1.250
OUTPUT VOLTAGE,
500mV/DIV
0V
5ms/DIV
FIGURE 6. OVERCURRENT BEHAVIOR IN HICCUP MODE
.
EXTERNAL CIRCUIT
RC
CC
ISL6557A INTERNAL CIRCUIT
COMP
0
1
0
1
1
1.275
ERROR AMPLIFIER
0
1
0
1
0
1.300
-
0
1
0
0
1
1.325
0
1
0
0
0
1.350
0
0
1
1
1
1.375
0
0
1
1
0
1.400
0
0
1
0
1
1.425
0
0
1
0
0
1.450
0
0
0
1
1
1.475
0
0
0
1
0
1.500
0
0
0
0
1
1.525
0
0
0
0
0
1.550
FB
RFB
+
VDROOP
IAVG
IOUT
+
VCOMP
REFERENCE
VOLTAGE
VDIFF
VOUT
REMOTE
GROUND
VSEN
+
-
RGND
DIFFERENTIAL
REMOTE-SENSE
AMPLIFIER
FIGURE 7. OUTPUT-VOLTAGE AND LOAD-LINE
REGULATION
9
ISL6557A
OVERVOLTAGE PROTECTION
The ISL6557A detects output voltages above 2.1V and
immediately commands all PWM outputs low. This directs
the Intersil drivers turn on the lower MOSFETs and protect
the load by preventing any further increase in output voltage.
Once the output voltage falls to the level set by the VID
code, the PWM outputs enter high-impedance mode. The
Intersil drivers respond by turning off both upper and lower
MOSFETs. If the overvoltage condition reoccurs, the
ISL6557A will again command the lower MOSFETs to turn
on. The ISL6557A will continue to protect the load in this
fashion as long as the overvoltage repeats.
After detecting an overvoltage condition, the ISL6557A
ceases normal PWM operation until it is reset by power cycle
in which VCC is removed below the POR falling threshold
and restored above the POR rising threshold as described in
Enable and Disable and Electrical Specifications.
LOAD-LINE REGULATION
In applications with high transient current slew rates, the
lowest-cost solution for maintaining regulation often requires
some kind of controlled output impedance. Pin 8 of the
ISL6557A carries a current proportional to the average
current of all active channels. The current is equivalent to
IAVG in Figures 5 and 7. Connecting FB and IOUT together
forces IAVG into the summing node of the error amplifier and
produces a voltage drop across the feedback resistor, RFB,
proportional to the output current. In Figure 7, the steadystate value of VDROOP is simply
V DROOP = I AVG R FB
(EQ. 3)
In the case that each channel uses the same value for RISEN to
sense channel current, and this is almost always true, a more
complete expression for VDROOP can be determined from the
expression for IAVG as it is derived from Figures 4 and 5.
I OUT
I AVG = ------------N
r DS  ON 
---------------------R ISEN
I OUT
V DROOP = ------------N
r DS  ON 
---------------------- R FB
R ISEN
(EQ. 4)
ENABLE AND DISABLE
The internal power-on reset circuit (POR) prevents the
ISL6557A from starting before the bias voltage at VCC
reaches the POR-rising threshold as defined in Electrical
Specifications.The POR level is high enough to guarantee
that all parts of the ISL6557A can perform their functions
properly. Built-in hysteresis assures that once enabled, the
ISL6557A will not turn off unless the bias voltage falls to
approximately 0.5V below the POR-rising level. When VCC
10
is below the POR-rising threshold, the PWM outputs are held
in a high-impedance state to assure the drivers remain off.
ISL6557A INTERNAL CIRCUIT
EXTERNAL CIRCUIT
+5V
VCC
10.7k
ENABLE
COMPARATOR
POR
CIRCUIT
OV LATCH
SIGNAL
+
-
+12V
EN
1.40k
1.23V (± 2%)
FIGURE 8. START-UP CONDITION USING THRESHOLDSENSITIVE ENABLE (EN) FUNCTION
After power on, the ISL6557A remains in shut-down mode
until the voltage at the enable input (EN) rises above 1.23V
(±2%). This optional feature prevents the ISL6557A from
operating until the connected voltage rail is available and
above some selectable threshold. For example, the
HIP660X family of MOSFET driver ICs require 12V bias, and
in certain circumstances, it can be important to assure that
the drivers reach their POR level before the ISL6557A
becomes enabled. The schematic in Figure 8 demonstrates
coordination of the ISL6557A with HIP660X family of
MOSFET driver ICs. The enable comparator has about
70mV of hysteresis to prevent bounce. To defeat the
threshold-sensitive enable, connect EN to VCC.
The 11111 VID code is reserved as a signal to the controller
that no load is present. The controller will enter shut-down
mode after receiving this code and will start up upon
receiving any other code.
To enable the controller, VCC must be greater than the POR
threshold; the voltage on EN must be greater than 1.23V and
VID cannot be equal to 11111. Once these conditions are
true, the controller immediately initiates a soft start
sequence.
SOFT-START
The soft-start time, tSS, is determined by an 11-bit counter
that increments with every pulse of the phase clock. For
example, a converter switching at 250kHz has a soft-start
time of
2048
T SS = ------------- = 8.2ms
f SW
(EQ. 5)
ISL6557A
During the soft-start interval, the soft-start voltage, VRAMP,
increases linearly from zero to 140% of the programmed
DAC voltage. At the same time a current source, IRAMP, is
decreasing from 160A down to zero. These signals are
connected as shown in Figure 9 (IOUT may or may not be
connected to FB depending on the particular application).
EXTERNAL CIRCUIT
RC
CC
leading to TSS = 4.0ms, tDELAY = 700ns, tRAMP1 = 2.23ms,
and tRAMP2 = 1.17ms.
VOUT, 500mV/DIV
ISL6557A INTERNAL CIRCUIT
COMP
ERROR AMPLIFIER
FB
RFB
EN, 5V/DIV
VCOMP
+
IOUT
REFERENCE
VOLTAGE
IRAMP
VDIFF
VRAMP
tDELAY tRAMP1
IAVG
NOTE: Switching frequency 500kHz and RFB = 2.67k
FIGURE 9. RAMP CURRENT AND VOLTAGE FOR
REGULATING SOFT-START SLOPE
AND DURATION
DYNAMIC VID
The ideal diodes in Figure 9 assure that the controller tries
to regulate its output to the lower of either the reference
voltage or VRAMP. Since IRAMP creates an initial offset
across RFB of RFB times 160A, the first PWM pulses will
not be seen until VRAMP is greater than the RFB IRAMP
offset. This produces a delay after the ISL6557A enables
before the output voltage starts moving. For example, if
VID = 1.5V, RFB = 1k and TSS = 8.3ms, the delay time
can be expressed using Equation 6.
The ISL6557A is capable of executing on-the-fly outputvoltage changes. At the beginning of the phase-1 switching
cycle (defined in the section entitled PWM Operation), the
ISL6557A checks for a change in the VID code. The VID
code is the bit pattern present at pins VID4-VID0 as outlined
in Voltage Regulation. If the new code remains stable for
another full cycle, the ISL6557A begins incrementing the
reference by making 25mV change every two switching
cycles until the it reaches the new VID code.
01110
(EQ. 6)
VREF, 100mV/DIV
1.3V
= 5.27ms
The final portion of the soft-start sequence is the time
remaining after VRAMP reaches VID and before IRAMP gets to
zero. This is also characterized by a slight linear ramp in the
output voltage which, for the current example, exists for a time
(EQ. 8)
= 2.34ms
This behavior is seen in the example in Figure 10 of a converter
switching at 500kHz. For this converter, RFB is set to 2.67k
11
VID, 5V/DIV
1.3V
(EQ. 7)
t RAMP2 = T SS – t RAMP1 – t DELAY
00010
VID CHANGE OCCURS
ANYWHERE HERE
From this point, the soft start ramps linearly until VRAMP
reaches VID. For the system described above, this first
linear ramp will continue for approximately
T SS
t RAMP1 = ---------- – t DELAY
1.4
1ms/DIV
FIGURE 10. SOFT-START WAVEFORMS FOR ISL6557A
BASED MULTI-PHASE BUCK CONVERTER
IDEAL DIODES
T SS
t DELAY = --------------------------------------------------- = 580s
1.4  VID
1 + ---------------------------------------R FB 160  10 – 6
tRAMP2
VOUT, 100mV/DIV
5s/DIV
FIGURE 11. DYNAMIC-VID WAVEFORMS FOR 500KHZ
ISL6557A BASED MULTI-PHASE BUCK
CONVERTER
Since the ISL6557A recognizes VID-code changes only at
the beginnings of switching cycles, up to one full cycle may
pass before a VID change registers. This is followed by a
one-cycle wait before the output voltage begins to change.
Thus, the total time required for a VID change, tDV, is
ISL6557A
dependent on the switching frequency (fS), the size of the
change (VID), and the time before the next switching cycle
begins. The one-cycle uncertainty in Equation 9 is due to the
possibility that the VID code change may occur up to one full
cycle before being recognized. The time required for a
converter running with fS = 500kHz to make a 1.3V to 1.5V
reference-voltage change is between 30s and 32s as
calculated using Equation 9. This example is also illustrated
in Figure 11.

1  2 V ID
1  2 V ID
-----  ----------------- – 1  t DV  -----  -----------------
f S  0.025
f S  0.025 

(EQ. 9)
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 on
either side; and the total board space available for powersupply circuitry. Generally speaking, the most economical
solutions will be for each phase to handle between 15 and
20A. All-surface-mount designs will tend toward the lower
end of this current range and, if through-hole MOSFETs 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 30A per phase, but these designs require
heat sinks and forced air to cool the MOSFETs.
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 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 10, IM is the maximum
continuous output current; IL,PP is the peak-to-peak inductor
current (see Equation 1); d is the duty cycle (VOUT/VIN); and
L is the per-channel inductance.
12
2
I L ,PP  1 – d 
 I M 2
P LOW ,1 = r DS  ON   -----  1 – d  + -----------------------------12
N
 
(EQ. 10)
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 M I PP
 IM I 
PP- t
P LOW , 2 = V D  ON  f S  ----- + --------- t d1 +  ------ – -------2 
2  d2
N
N
(EQ. 11)
Thus the total power dissipated in each lower MOSFET is
approximated by the summation of PL and PD.
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 dependant
on switching frequency, the power calculation is somewhat
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 12,
the required time for this commutation is t1and the
associated power loss is PUP,1.
 I M I L ,PP  t 1 
P UP,1  V IN  ----- + -------------  ----  f S
2  2
N
(EQ. 12)
Similarly, the upper MOSFET begins conducting as soon as
it begins turning on. In Equation 13, this transition occurs
over a time t2, and the approximate the power loss is PUP,2.
 I M I L ,PP  t 2 
P UP, 2  V IN  ----- – -------------  ----  f S
2  2
N
(EQ. 13)
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 recover all of Qrr, it is conducted
through the upper MOSFET across VIN. The power
dissipated as a result is PUP,3 and is simply
P UP,3 = V IN Q rr f S
(EQ. 14)
ISL6557A
Finally, the resistive part of the upper MOSFET’s is given in
Equation 15 as PUP,4.
2
I PP2
 I M
P UP,4 = r DS  ON   ----- d + ---------12
 N
(EQ. 15)
In this case, of course, rDS(ON) is the on resistance of the
upper MOSFET.
The total power dissipated by the upper MOSFET at full load
can now be approximated as the summation of the results
from Equations 12, 13, 14 and 15. Since the power
equations depend on MOSFET parameters, choosing the
correct MOSFETs can be an iterative process that involves
repetitively solving the loss equations for different MOSFETs
and different switching frequencies until converging upon the
best solution.
Current Sensing
Pins 18, 15, 14 and 19 are the ISEN pins denoted ISEN1,
ISEN2, ISEN3 and ISEN4 respectively. The resistors
connected between these pins and the phase nodes
determine the gains in the load-line regulation loop and the
channel-current balance loop. Select the 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 according to Equation 16 (see also
Figure 4).
r DS  ON 
R ISEN = ---------------------50 10 – 6
I FL
-------N
(EQ. 16)
In certain circumstances, it may be necessary to adjust the
value of one or more of the ISEN resistors. This can arise
when the components of one or more channels are inhibited
from dissipating their heat so that the affected channels run
hotter than desired (see the section entitled Channel-Current
Balance). In these cases, chose new, smaller values of RISEN
for the affected phases. Choose RISEN,2 in proportion to the
desired decrease in temperature rise in order to cause
proportionally less current to flow in the hotter phase.
T 2
R ISEN ,2 = R ISEN ---------T 1
ISEN resistor, the load-line regulation resistor is as shown
in Equation 18.
V DROOP
R FB = ------------------------–6
50 10
(EQ. 18)
If one or more of the ISEN resistors was adjusted for thermal
balance as in Equation 17, the load-line regulation resistor
should be selected according to Equation19 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 
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 Regulation, there are two distinct
methods for achieving these goals.
COMPENSATING A 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 effected 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.
C2 (OPTIONAL)
(EQ. 17)
In Equation 17, 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 17 is usually
sufficient, it may occasionally be necessary to adjust RISEN
two or more times to achieve perfect thermal balance
between all channels.
(EQ. 19)
n
RC
CC
COMP
FB
+
RFB
VDROOP
IOUT
ISL6557A
VDIFF
Load-Line Regulation Resistor
The load-line regulation resistor is labeled RFB in Figure 7.
Its value depends on the desired full-load droop voltage
(VDROOP in Figure 7). If Equation 16 is used to select each
13
FIGURE 12. COMPENSATION CONFIGURATION FOR
LOAD-LINE REGULATED ISL6557A CIRCUIT
ISL6557A
resonant frequency and a zero at the ESR frequency. A type
III controller, as shown in Figure 13, provides the necessary
compensation.
C2
RC
CC
COMP
FB
Case 1:
1
-------------------  f 0
2 LC
C1
2f 0 V pp LC
R C = R FB -----------------------------------0.75V IN
R1
+
RFB
VDROOP
-
VDIFF
0.75V IN
C C = -----------------------------------2V PP R FB f 0
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. 20)
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 20, 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
Figure 5 and Electrical Specifications.
Once selected, the compensation values in Equations 20
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 20 unless some performance issue is noted.
The optional capacitor C2, is sometimes needed to bypass
noise away from the PWM comparator (see Figure 5). Keep
a position available for C2, and be prepared to install a highfrequency capacitor of between 22pF and 150pF in case any
jitter problem is noted.
COMPENSATION WITHOUT LOAD-LINE REGULATION
The non load-line regulated converter is accurately modeled
as a voltage-mode regulator with two poles at the L-C
14
IOUT
ISL6557A
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 per-channel 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 three cases
defined below, there is a separate set of equations for the
compensation components.
FIGURE 13. COMPENSATION CIRCUIT FOR ISL6557A BASED
CONVERTER WITHOUT LOAD-LINE
REGULATION
The first step is to choose the desired bandwidth, f0, of the
compensated system. Choose a frequency high enough to
assure adequate transient performance but not higher than
1/3 of the switching frequency. The type-III compensator has
an extra high-frequency pole, fHF. This pole can be used for
added noise rejection or to assure adequate attenuation at
the error-amplifier high-order pole and zero frequencies. A
good general rule is to chose fHF = 10 f0, but it can be higher
if desired. Choosing fHF to be lower than 10 f0 can cause
problems with too much phase shift below the system
bandwidth.
In the solutions to the compensation equations, there is a
single degree of freedom. For the solutions presented in
Equations 21, RFB is selected arbitrarily. The remaining
compensation components are then selected according to
Equations 21.
C  ESR 
R 1 = R FB ----------------------------------------LC – C  ESR 
LC – C  ESR 
C 1 = ----------------------------------------R FB
0.75V IN
C 2 = ------------------------------------------------------------------ 2  2 f 0 f HF LCR FB V PP
2
R FB V  2 f 0 f HF LC
PP  
R C = --------------------------------------------------------------0.75V IN  2f HF LC – 1


0.75V IN  2f HF LC – 1
C C = ------------------------------------------------------------------ 2 2 V R f f
LC
  PP FB 0 HF
(EQ. 21)
ISL6557A
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
resistance of the bulk output-filter capacitance; and VPP is
the peak-to-peak sawtooth signal amplitude as described in
Figure 5 and “Electrical Specifications”.
Output Filter Design
The output inductors and the output capacitor bank together
form a low-pass filter responsible for smoothing the pulsating
voltage at the phase nodes. The output filter also must
provide the transient energy during the interval of time after
the beginning of the transient until the regulator can fully
respond. Because it has a low bandwidth compared to the
switching frequency, the output filter necessarily limits the
system transient response leaving the output capacitor bank
to 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
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
15
source the inductor ac ripple current (see Interleaving and
Equation 2), a voltage develops across the bulk-capacitor
ESR equal to IPP (ESR). Thus, once the output capacitors
are selected, the maximum allowable ripple voltage,
VPP(MAX), determines the a lower limit on the inductance.
V – N V

OUT V OUT
 IN
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 limits on inductance.
2NCVO
L  --------------------- V MAX – I  ESR 
 I  2
(EQ. 24)
 1.25  NC
L  -------------------------- V MAX – I  ESR   V IN – V O


 I  2
(EQ. 25)
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
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.
Switching Frequency
There are a number of variables to consider when choosing
the switching frequency. There are considerable effects on
the upper-MOSFET loss calculation and, to a lesser extent,
the lower-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 outputvoltage ripple as outlined in Output Filter Design. Choose the
lowest switching frequency that allows the regulator to meet
the transient-response requirements.
Switching frequency is determined by the selection of the
frequency-setting resistor,RT (see the figure Typical
ISL6557A
RT (k)
1000
100
0.3
INPUT-CAPACITOR CURRENT ( IRMS / IO )
Application on page 3). Figure 14 and Equation 26 are
provided to assist in the selecting the correct value for RT.
0.2
0.1
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 )
10
100
1000
SWITCHING FREQUENCY (KHZ)
10000
FIGURE 14. RT VS SWITCHING FREQUENCY
RT = 10
11.09 – 1.13 log  f S  
(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.
Figures 15, 16 and 17 can be used to determine the inputcapacitor rms current as of duty cycle, maximum sustained
output current (IO), and the ratio of the combined peak-topeak inductor current (IL,PP as defined in Equation 1) to the
maximum sustained load current, IO. Figure 18 is provided
as a reference to demonstrate the dramatic reductions in
input-capacitor rms current upon the implementation of the
multiphase topology.
16
FIGURE 15. NORMALIZED INPUT-CAPACITOR RMS
CURRENT VS DUTY CYCLE FOR 2-PHASE
CONVERTER
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
DUTY CYCLE ( VIN / VO )
FIGURE 16. NORMALIZED INPUT-CAPACITOR RMS
CURRENT VS DUTY CYCLE FOR 3-PHASE
CONVERTER
1.0
ISL6557A
IL,PP = 0
IL,PP = 0.5 IO
IL,PP = 0.25 IO
IL,PP = 0.75 IO
0.6
INPUT-CAPACITOR CURRENT ( IRMS / IO )
INPUT-CAPACITOR CURRENT ( IRMS / IO )
0.3
0.2
0.1
0
0
0.2
0.4
0.6
0.8
DUTY CYCLE ( VIN / VO )
FIGURE 17. NORMALIZED INPUT-CAPACITOR RMS
CURRENT VS DUTY CYCLE FOR 4-PHASE
CONVERTER
17
1.0
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 )
FIGURE 18. NORMALIZED INPUT-CAPACITOR RMS
CURRENT VS DUTY CYCLE FOR SINGLE-PHASE
CONVERTER
ISL6557A
Small Outline Plastic Packages (SOIC)
M24.3 (JEDEC MS-013-AD ISSUE C)
N
INDEX
AREA
24 LEAD WIDE BODY SMALL OUTLINE PLASTIC PACKAGE
0.25(0.010) M
H
B M
INCHES
E
-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
SYMBOL
MIN
MAX
MIN
MAX
NOTES
A
0.0926
0.1043
2.35
2.65
-
A1
0.0040
0.0118
0.10
0.30
-
B
0.013
0.020
0.33
0.51
9
C
0.0091
0.0125
0.23
0.32
-
D
0.5985
0.6141
15.20
15.60
3
E
0.2914
0.2992
7.40
7.60
4
e
µ
B S
0.05 BSC
1.27 BSC
-
H
0.394
0.419
10.00
10.65
-
h
0.010
0.029
0.25
0.75
5
L
0.016
0.050
0.40
1.27
6
N

NOTES:
MILLIMETERS
24
0o
24
8o
0o
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.
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18