IRF GRM188R71H103KA01D Dual output, 4a/phase, highly integrated supirbuck Datasheet

Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
FEATURES
IR3891
DESCRIPTION
 Single 5V to 21V application
The IR3891 SupIRBuckTM is an easy-to-use, fully
integrated and highly efficient DC/DC regulator. The
onboard PWM controller and MOSFETs make IR3891
a space-efficient solution, providing accurate power
delivery for low output voltage.
 Wide Input Voltage Range from 1V to 21V with
external Vcc
 Output Voltage Range: 0.5V to 0.86*PVin
 Dual output, 4A/Phase
 Enhanced Line/Load Regulation with FeedForward
 Programmable Switching Frequency up to 1.5MHz
 Internal Digital Soft-Start
 Enable input with Voltage Monitoring Capability
The switching frequency is programmable from
300kHz to 1.5MHz for an optimum solution.
 Thermally compensated current limit and Hiccup
Mode Over Current Protection
 External synchronization with Smooth Clocking
 Precision Reference Voltage (0.5V +/-1%)
 Seq pin for Sequencing Applications
 Integrated MOSFETs, drivers and Bootstrap diode
 Thermal Shut Down
IR3891 is a versatile regulator which offers
programmability of switching frequency and a fixed
current limit while operating in wide input and output
voltage range.
It also features important protection functions,
Over Voltage Protection (OVP), Pre-Bias
hiccup current limit and thermal shutdown
required system level security in the event
conditions.
APPLICATIONS
 Open Feedback Line Protection
 Over Voltage Protection
 Sever Applications
 Interleaved Phases to reduce Input Capacitors
 Netcom Applications
 Monotonic Start-Up
 Set Top Box Applications
o
such as
startup,
to give
of fault
o
 Operating Junction Temp: -40 C<Tj<125 C
 Storage Applications
 Small Size 5mm x 6mm PQFN
 Embedded telecom Systems
 Lead-free, Halogen-free, and RoHS Compliant
 Distributed Point of Load Power Architectures
 Computing Peripheral Voltage regulators
 General DC-DC Converters
ORDERING INFORMATION
Base Part
Number
1
Standard Pack
Package Type
Orderable Part
Form
Quantity
Number
IR3891
PQFN 5mm x 6mm
Tape and Reel
750
IR3891MTR1PBF
IR3891
PQFN 5mm x 6mm
Tape and Reel
4000
IR3891MTRPBF
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
BASIC APPLICATION
93
91
89
Efficiency [%]
87
85
83
81
79
77
75
73
0.5
1
1.5
2
2.5
Load Current[A]
1.2V
3
3.5
4
1.8V
Figure 2: Efficiency [Vin=12V, Fsw=600kHz]
Figure 1: IR3891 Basic Application Circuit
PIN DIAGRAM
5mm X 6mm POWER QFN
Top View
22
20
21
19
18 PGnd2
PGnd1 23
PGnd1 24
SW1
17 PGnd2
SW2
PGnd1 25
16 PGnd2
PVin1 26
15 PVin2
PVin1 27
14 PVin2
Boot1 28
13 Boot2
PGood1 29
12 PGood2
Comp1 30
11 Comp2
10 FB2
FB1 31
1
2
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2
3
© 2013 International Rectifier
4
5
6
7
8
9
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
FUNCTIONAL BLOCK DIAGRAM
Vin
VCC/LDO_out
LDO
+
UVLO
VCC
+
Gnd
Comp
VREF
Seq*
+
+
+ E/A
-
FAULT
CONTROL
FAULT
PVin
-
+
FB
HDin
Intl_SS
FAULT
UVEN
Rff
Vin
SOFT
START
POR
EN
Boot
THERMAL TSD
SHUTDOWN
OV/OFLP
POR
OC
VLDO_REF -
fb
UVLO
UVEN
VREF
POR
UVLO
POR
VREF
FB
UV/
OV/OLFP
GATE
DRIVE
LOGIC
FAULT
LDin
SSOK
CONTROL
LOGIC
HDrv
SW
VCC
LDrv
PGnd
OC
OVER CURRENT
PROTECTION
UV/
OV/OLFP
Rt/Sync
Vsns
PGood
*The Seq pin is only available for channel 2
Figure 3: IR3891 Simplified Block Diagram (one phase)
3
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
PIN DESCRIPTIONS
PIN #
PIN NAME
1, 9
Vsns 1/2
2, 8
EN 1/2
3
Vin
4
VCC/LDO_out
5
GND
6
Seq
7
Rt/Sync
10, 31
FB 2/1
11, 30
Comp 2/1
12, 29
PGood 2/1
13, 28
Boot 2/1
14, 15, 26,
27
PVin 2/1
Input voltage for power stage.
16, 17, 18,
23, 24, 25
PGnd 2/1
Power Ground. These pins serve as a separated ground for the
MOSFET drivers and should be connected to the system’s power ground
plane.
19, 20, 21,
22
SW 2/1
4
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PIN DESCRIPTION
Sense pins for over-voltage protection and PGood. A resistor divider
with the same ratio as the respective feedback resistor divider should be
connected between each Vsns pin and its respective Vout.
Enable pins for turning on and off the regulator.
Input voltage for Internal LDO. A 1.0µF capacitor should be connected
between this pin and PGnd. If external supply is connected to VCC pin,
this pin should be shorted to VCC pin.
Input Bias Voltage, output of the internal LDO. Place a minimum 2.2µF
cap from this pin to PGnd.
Signal ground for internal reference and control circuitry.
Input to error amplifier for sequencing purposes. Can be left floating for
non-sequencing applications. It is only connected to the Error-Amplifier
of channel 2.
Multi-function pin to set switching frequency. Use an external resistor
from this pin to Gnd to set the free-running switching frequency. Or use
an external clock signal to connect to this pin through a diode, the
device’s switching frequency is synchronized with the external clock.
Inverting inputs to the error amplifiers. These pins are connected directly
to the outputs of the regulator via resistor dividers to set the output
voltages and provide feedback to the error amplifiers.
Output of the error amplifiers. External resistor and capacitor networks
are typically connected from these pins to its respective Fb pin to provide
loop compensation.
Power Good status pins are open drain outputs. The pins are typically
connected to VCC via pull up resistors.
Supply voltages for high side drivers, 100nF capacitors should be
connected between these pins and their respective SW pin.
Switch nodes. These pins are connected to the output inductors.
© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
ABSOLUTE MAXIMUM RATINGS
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only and functional operation of the device at these or any other conditions beyond those indicated in the
operational sections of the specifications are not implied.
PVin
Vin
VCC
SW
BOOT
BOOT to SW
EN, PGood
Other Input/Output pins
PGnd to GND
Junction Temperature Range
Storage Temperature Range
Machine Model
ESD
-0.3V to 25V
-0.3V to 25V
-0.3V to 8V (Note 1)
-0.3V to 25V (DC), -4V to 25V (AC, 100ns)
-0.3V to 33V
-0.3V to VCC + 0.3V (Note 2)
-0.3V to VCC + 0.3V (Note 2)
-0.3V to 3.9V
-0.3V to + 0.3V
-40°C to 150°C
-55°C to 150°C
Class A
Human Body Model
Class 1C
Charged Device Model
Class III
Moisture Sensitivity level
JEDEC Level 2 @ 260°C
RoHS Compliant
Yes
Note:
1. VCC must not exceed 7.5V for Junction Temperature between -10°C and -40°C.
2. Must not exceed 8V.
THERMAL INFORMATION
Thermal Resistance, Junction to Case Top (θJC_TOP)
36 °C/W
Thermal Resistance, Junction to PCB (θJB)
3.6 °C/W
Thermal Resistance, Junction to Ambient (θJA) (Note 3)
24.7 °C/W
Note:
3. Thermal resistance (θJA) is measured with components mounted on a high effective thermal conductivity
test board in free air.
5
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
ELECTRICAL SPECIFICATIONS
RECOMMENDED OPERATING CONDITIONS
SYMBOL
DEFINITION
MIN
1.0
5.0
4.5
4.5
0.5
0
300
-40
PVin
Input Bus Voltage *
Vin
Supply Voltage
VCC
Supply Voltage **
Boot to SW
Supply Voltage
VO
Output Voltage
IO
Output Current
Fs
Switching Frequency
TJ
Junction Temperature
SW1/2 node must not exceed 25V
When VCC is connected to an externally regulated supply, also connect Vin.
*
**
UNIT
MAX
21
21
7.5
7.5
0.86 * PVin
4
1500
125
V
A / Phase
kHz
°C
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, these specifications apply over, 6.8V < Vin=PVin < 21V in 0°C < TJ < 125°C.
Typical values are specified at Ta = 25°C.
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNIT
Power Stage
Power Losses
PLOSS
Top Switch
Rds(on)_Top
Bottom Switch
Rds(on)_Bot
Bootstrap Diode
Forward Voltage
SW Leakage Current
Dead Band Time
Vin = 12V, Vout1 = 1.8V,
Vout2 = 1.2V, IO =
4A/phase, Fs = 600kHz,
L1 =2.2uH, L2=1.5uH,
Note 4
VBoot - Vsw= 5.3V, IO =
4A, Tj = 25°C
Vcc = 5.3V, IO = 4A, Tj =
25°C
1.38
Tdb
SW = 0V, Enable = 0V
SW = 0V, Enable = high,
VSeq = 0V
Note 4
27.5
36.4
19.5
24.2
300
450
mV
1
µA
2
µA
30
ns
mΩ
I(Boot) = 10mA
ISW
W
10
20
Supply Current
VIN Supply Current
(standby)
Iin(Standby)
VIN Supply Current
(dynamic)
Iin(Dyn)
EN = Low, No Switching
100
EN = High, Fs = 600kHz,
µA
175
mA
12.0
17
5.3
5.6
V
0.75
V
VCC LDO Output
Output Voltage
VCC Dropout
6
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Vcc
Vcc_drop
© 2013 International Rectifier
Vin(min) = 6.8V, Io = 060mA, Cload = 2.2uF
Icc = 60mA, Cload =
2.2uF
5
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
CONDITIONS
MIN
IR3891
PARAMETER
SYMBOL
TYP
MAX
UNIT
Short Circuit Current
Ishort
120
mA
Vrt
1.0
V
Oscillator
Rt Voltage
Frequency Range
Fs
Ramp Amplitude
Vramp
Min Pulse Width
Tmin(ctrl)
Max Duty Cycle
Dmax
Fixed Off Time
Toff
Rt = 80.6K
270
300
330
Rt = 39.2K
540
600
660
Rt = 15K
1350
1500
1650
Vin = 6.8V, Vin slew rate
max = 1V/μs, Note 4
1.02
Vin = 12V, Vin slew rate
max = 1V/μs, Note 4
1.80
Vin = 21V, Vin slew rate
max = 1V/μs, Note 4
3.15
Vcc=Vin = 5V, For
external Vcc operation,
Note 4
0.75
Note 4
Fs = 300kHz,
Vin=Pvin=12V
Vp-p
60
ns
86
Note 4
%
200
Sync Frequency Range
Fsync
270
Sync Pulse Duration
Tsync
100
High
3
Sync Level Threshold
kHz
250
ns
1650
kHz
200
Low
ns
V
0.6
Error Amplifier
Seq Input Offset
Voltage
Input Bias Current
Seq Input impedance
Sink Current
Source Current
Slew Rate
Gain-Bandwidth
Product
DC Gain
Vos_VSeq
IFb(E/A)
Rin_Seq(E/A)
+3
%
-200
+200
nA
Internal Seq pull-up
resistor
300
kΩ
0.4
0.85
1.2
mA
Isource(E/A)
3
4
7
mA
Note 4
7
12
20
V/µs
Note 4
20
30
40
MHz
Note 4
80
90
110
dB
1.7
2
2.3
V
120
220
mV
0.77
V
SR
GBWP
Gain
Vmax(E/A)
Minimum Voltage
Vmin(E/A)
Vseq Common Mode
Voltage
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-3
Isink(E/A)
Maximum Voltage
7
VSeq – Vfb;
VSeq=250mV
0
© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
PARAMETER
SYMBOL
CONDITIONS
MIN
IR3891
TYP
MAX
UNIT
Reference Voltage
Feedback Voltage
Vfb
VSeq=3.3V
0°C < Tj < 85°C
Accuracy
-40°C < Tj < 125°C,
Note 5
0.5
V
-1
+1
-1.5
+1.5
%
Soft Start
Soft Start Ramp Rate
Ramp (SS_start)
0.14
0.18
0.22
4.8
6.0
7.2
65
0.1
115
20.48
70
0.3
120
75
0.5
125
mV /
µs
Fault Protection
Current Limit
ICC
Hiccup blanking time
OFLP Trip Threshold
OFLP Fault Prop Delay
OVP Trip Threshold
OVP Trip Threshold
Hysteresis
Tblk_Hiccup
OFLP(threshold)
OFLP(delay)
OVP(threshold)
OVP_Hys
OVP Comparator Delay
Thermal Shutdown
Thermal Hysteresis
VCC-Start-Threshold
VCC-Stop-Threshold
Vcc=5.3V, Tj = 25°C
Note 4
Fb Falling
Vsns Rising
Vsns falling from above
120% of Vref, Sync_FET
turns off afterwards
OVP(delay)
VCC_UVLO_Start
VCC_UVLO_Stop
Note 4
Note 4
VCC Rising Trip Level
VCC Falling Trip Level
4.0
3.7
A/
Phase
ms
%Vref
µs
%Vref
25
mV
2
140
20
4.2
3.9
µs
°C
°C
4.4
4.1
V
Input / Output Signals
Enable-Start-Threshold
EN_UVLO_Start
Supply ramping up
1.14
1.2
1.26
V
Enable-Stop-Threshold
EN_UVLO_Stop
Supply ramping down
0.95
1
1.05
Enable leakage current
Ien
Enable=3.3V
3
4.5
µA
Power Good upper
VPG(upper)
Vsns Rising
80
85
90
%Vref
Threshold
Power Good lower
VPG(lower)
Vsns Falling
75
80
85
%Vref
Threshold
Lower Threshold Delay
VPG(lower)_Dly
Vsns Rising
1
1.3
1.6
ms
PGood Voltage Low
PG(voltage)
IPgood= -5mA
0.5
V
Note:
4. Guaranteed by design but not tested in production.
5. Cold temperature performance is guaranteed via correlation using statistical quality control. Not tested in
production.
8
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
TYPICAL EFFICIENCY AND POWER LOSS CURVES
PVin = 12V, Vcc = Internal LDO, Io=0-4A, Fs= 600kHz, Room Temperature, No Air Flow. Note that the losses of
the inductor, input and output capacitors are also considered in the efficiency and power loss curves. The table
below shows the indicator used for each of the output voltages in the efficiency measurement while running a
single channel and disabling the other.
VOUT (V)
1.0
1.2
1.8
3.3
5.0
9
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LOUT (uH)
1.5
1.5
2.2
3.3
3.3
P/N
7443340150 (Wurth Elektronik)
7443340150 (Wurth Elektronik)
7443340220 (Wurth Elektronik)
7443340330 (Wurth Elektronik)
7443340330 (Wurth Elektronik)
© 2013 International Rectifier
DCR (mΩ)
4.4
4.4
4.4
6.5
6.5
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
TYPICAL EFFICIENCY AND POWER LOSS CURVES
PVin = 12V, Vin = Vcc = 5V, Io=0-4A, Fs= 600kHz, Room Temperature, No Air Flow. Note that the losses of the
inductor, input and output capacitors are also considered in the efficiency and power loss curves. The table
below shows the indicator used for each of the output voltages in the efficiency measurement while running a
single channel and disabling the other.
VOUT (V)
1.0
1.2
1.8
3.3
5.0
10
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LOUT (uH)
1.5
1.5
2.2
3.3
3.3
P/N
PCMB065T-1R5MS (Cyntec)
PCMB065T-1R5MS (Cyntec)
7443340220 (Wurth Elektronik)
7443340330 (Wurth Elektronik)
7443340330 (Wurth Elektronik)
© 2013 International Rectifier
DCR (mΩ)
6.7
6.7
4.4
6.5
6.5
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
TYPICAL EFFICIENCY AND POWER LOSS CURVES
PVin = 5V, Vcc = 5V, Io=0-4A, Fs = 600kHz, Room Temperature, No Air Flow. Note that the losses of the
inductor, input and output capacitors are also considered in the efficiency and power loss curves. The table
below shows the indicator used for each of the output voltages in the efficiency measurement while running a
single channel and disabling the other.
VOUT (V)
1.0
1.2
1.8
3.3
11
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LOUT (uH)
1.0
1.5
1.5
1.5
© 2013 International Rectifier
P/N
PCMB065T-1R0MS (Cyntec)
PCMB065T-1R5MS (Cyntec)
PCMB065T-1R5MS (Cyntec)
PCMB065T-1R5MS (Cyntec)
DCR (mΩ)
5.6
6.7
6.7
6.7
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
MOSFET RDSON VARIATION OVER TEMPERATURE
12
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
TYPICAL OPERATING CHARACTERISTICS (-40°C TO +125°C)
13
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
14
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© 2013 International Rectifier
IR3891
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
15
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© 2013 International Rectifier
IR3891
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
THEORY OF OPERATION
DESCRIPTION
The IR3891 uses a PWM voltage mode control
scheme with external compensation to provide good
noise immunity and maximum flexibility in selecting
inductor values and capacitor types.
The switching frequency is programmable from
300KHz to 1.5MHz and provides the capability of
optimizing the design in terms of size and
performance.
IR3891 provides precisely regulated output voltage
programmed via two external resistors from 0.5V to
0.86*PVin.
The IR3891 operates with an internal low drop out
regulator (LDO) which is connected to the VCC pin.
This allows operation with a single supply. When
using the internal LDO supply, the Vin pin should be
connected the PVin pin. If an external bias is used, it
should be connected to the VCC pin and the Vin pin
should be shorted to the VCC pin.
voltage
exceeds
its
precise
threshold
(EN_UVLO_START), the respective channel turns on.
The precise threshold allows the user to implement an
Under-Voltage Lockout (UVLO) function. By deriving
the EN pin voltage from the bus voltage (PVin)
through a suitable resistor divider, the user can set a
PVin threshold voltage. The resistor divider scales the
PVin voltage for the EN pin. Only after the bus
voltage reaches or exceeds this level will the voltage
at the Enable pin exceeds its threshold and enable the
respective IR3891 channel. By connecting IR3891 in
this configuration, the user can enable the part by
applying PVin and ensures the IR3891 does not turn
on until the bus voltage reaches the desired level
(Figure 4). Therefore, in addition to being a logic input
pin that enables channels on IR3891, the EN pin also
offers UVLO functionality. UVLO functionality is
particularly desirable for high output voltage
applications, where it is beneficial to disable the
IR3891 until PVin exceeds the desired output voltage
level.
The device utilizes the on-resistance of the low side
MOSFET (sync FET) as a current sense element.
This method enhances the converter’s efficiency and
reduces cost by eliminating the need for an external
current sense resistor.
IR3891 includes two low Rds(on) MOSFETs using
IR’s HEXFET technology.
These are specifically
designed for high efficiency applications.
UNDER-VOLTAGE LOCKOUT AND POR
The under-voltage lockout circuits monitor the voltage
on the VCC pin and the EN1/2 pins. They ensure that
the MOSFET driver outputs remain in the off state
whenever either of these signals drops below the set
thresholds. Normal operation resumes once VCC and
EN rise above their thresholds.
Figure 4: Normal Startup: IR3891 Channel starts when
PVin reaches 10.2V by connecting EN to PVin using a
resistor divider.
The POR (Power On Ready) signal is high when all
these signals reach the valid logic level (see system
block diagram).
ENABLE
The EN pin offers another level of flexibility for startup.
Each channel of the IR3891 is controlled by a
separate EN pin. When the voltage at an EN pin
16
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
PVin=Vin
Vcc
EN1/2
> 1.2V
Intl_SS 1/2
Vo 1/2
Figure 7: Pre-bias Start Up
Figure 5: Recommended startup for Normal operation
HDRv
PVin=Vin
12.5%
Vcc
EN2
...
...
LDRv
> 1.2V
16
...
...
25%
...
...
87.5%
...
...
16
...
End of
PB
...
Intl_SS 2
EN1
> 1.2V
SOFT-START
Intl_SS 1
Vo1
Vo2
Figure 6: Recommended startup for sequencing
operation (ratiometric or simultaneous)
Figure 5 shows the recommended start-up sequence
for the normal (non-sequencing) operation of IR3891,
when EN pins are used as a logic input. Figure 6
shows the recommended startup sequence for
sequenced operation of IR3891.
PRE-BIAS STARTUP
IR3891 begins each start up by pre-charging the
output to prevent oscillation and disturbances to the
output voltage. The buck converter starts in an
asynchronous fashion and keeps the synchronous
MOSFET (Sync FET) off until the first gate signal for
control MOSFET (Ctrl FET) is generated. Figure 7
shows a typical pre-bias sequence. The sync FET
always starts with a narrow pulse width (12.5% of the
switching period). The pulse width increase after 16
pulses by 12.5% until the output reaches steady state
value. There are 16 pulses for each step. Figure 8
shows the series of 16 x 8 startup pulses.
17
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Figure 8: Pre-bias startup pulses
© 2013 International Rectifier
IR3891 has an internal digital soft-start to control the
output voltage rise and to limit the current surge
during start-up. To ensure the correct start-up, the
soft-start sequence initiates when the EN and VCC
rise above their UVLO thresholds and generates
Power On Ready (POR) signal. The internal soft-start
rises with the typical rate of 0.2mV/µS from 0V to
1.5V. Figure 9 shows the waveforms during soft-start.
The normal Vout start-up time is fixed, and is equal to:
Tstart 
0.65V  0.15V   2.5mS
0.2mV / S
(1)
During the soft-start the over-current protection (OCP)
and the over-voltage protection (OVP) is enabled to
protect the device from short circuit or over voltage
events.
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
the external clock solely and no resistor is required. If
the external clock is applied after the converter turns
on, or the converter switching frequency needs to
toggle between the external clock frequency and the
internal free-running frequency, an external resistor
from Rt/Sync pin to GND is required to set the free
running frequency.
Figure 9: Theoretical operation waveforms during softstart (non-sequencing)
OPERATING FREQUENCY
The switching frequency can be programmed between
300KHz-1.5MHz by connecting an external resistor
from Rt/Sync pin to GND. Table 1 tabulates the
oscillator frequency versus Rt.
When an external clock is applied to Rt/Sync pin after
the converter runs in steady state with its free-running
frequency, a transition from the free-running frequency
to the external clock frequency will happen. The
switching frequency gradually synchronizes to the
external clock frequency regardless of which one is
faster. On the contrary, when the external clock signal
is removed from Rt/Sync pin, the switching frequency
gradually returns to the free-running frequency. In
order to minimize the impact from these transitions to
output voltage, a diode is recommended to add
between the external clock and Rt/Sync pin. Figure 10
shows the timing diagram of these transitions.
Table 1: Switching Frequency (Fs) vs. External
Resistor (Rt)
Rt (KΩ)
80.6
60.4
48.7
39.2
34
29.4
26.1
23.2
21
19.1
17.6
16.2
15
Freq
(KHz)
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
1500
EXTERNAL SYNCHRONIZATION
IR3891 incorporates an internal phase lock loop (PLL)
circuit which enables synchronization of the internal
oscillator to an external clock.
This function is
important to avoid sub-harmonic oscillations due to
beat frequency for embedded systems when multiple
point-of-load (POL) regulators are used. A multiplefunction pin, Rt/Sync, is used to connect the external
clock. If the external clock is present before the
converter turns on, Rt/Sync pin can be connected to
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© 2013 International Rectifier
Synchronize to the
external clock
Free Running
Frequency
Return to freerunning freq
...
SW
Gradually change
Gradually change
...
Fs1
SYNC
Fs1
Fs2
Figure 10: Timing diagram for synchronization to an
external clock (Fs1>Fs2 or Fs1<Fs2)
An internal compensation circuit is used to change the
PWM ramp slope according to the clock frequency
applied on Rt/Sync pin. Thus, the effective amplitude
of the PWM ramp (Vramp), which is used in
compensation loop calculation, has minor impact from
the variation of the external synchronization signal.
Vin variation also affects the ramp amplitude, which is
discussed separately in Feed-Forward section.
SHUTDOWN
IR3891 shutdown occurs when VCC drops below its
threshold or a fault occurs. When VCC falls below
VCC_UVLO_STOP, the part detects an UVLO event
and the part turns off. Over-Voltage Protection, OverCurrent Protection and Thermal Shutdown also cause
the IR3891 shutdown. Faults are discussed in more
detail below.
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
Each channel of the IR3891 can be shutdown
separately by pulling the channel EN pin below its low
threshold. Each EN pin controls only one channel to
allow the user to operate each independently.
OVER CURRENT PROTECTION (CURRENT LIMIT
AND HICCUP MODE)
The over-current protection is performed by sensing
current through the RDS(on) of the Sync FET. This
method enhances the converter’s efficiency and
reduces cost by eliminating a current sense resistor.
The current limit is pre-set internally and compensated
to maintain an almost constant limit over temperature.
IR3891 determines over-current events when the
Synchronous FET is on. OCP circuit samples this
current for 40 nsec typically after the rising edge of the
PWM set pulse which has a width of 12.5% of the
switching period. The PWM pulse starts at the falling
edge of the PWM set pulse. This makes valley
current sense more robust as current is sensed close
to the bottom of the inductor downward slope where
transient and switching noise are lower and helps to
prevent false tripping due to noise and transient. An
OC condition is detected if the load current exceeds
the threshold, the converter enters into hiccup mode.
PGood will go low and the internal soft start signal will
be pulled low.
I OCP  I LIMIT
IOCP
ILIMIT
∆i
i

2
(2)
= DC current limit hiccup point
= Current Limit Valley Point
= Inductor ripple current
Hiccup mode is when the converter stops and waits
before restarting. The channel waits for Tblk_Hiccup,
2.48 ms typical, before the OC signal resets and
restarts. In normal application, the converter restarts
with a pre-bias sequence and soft-start. Figure 11
shows the timing diagram of the above OC protection.
If another OC event is detected, the part repeats
hiccup mode.
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© 2013 International Rectifier
Figure 11: Timing diagram for pulse-by-pulse current
limit and Hiccup mode
THERMAL SHUTDOWN
IR3891 provides thermal protection. A thermal fault is
detected, when the temperature of the part reaches
the Thermal Shutdown Threshold, 145°C typical. A
thermal fault results in both channels turning off. The
power MOSFETs are disabled during thermal
shutdown. IR3891 automatically restarts when the
temperature of the part drops back below the lower
thermal limit, typically 20°C below the Thermal
Shutdown Threshold.
FEED-FORWARD
Feed-Forward is an important feature which helps with
stability and preserves load transient performance
during PVin changes. In IR3891, Feed-Forward (F.F.)
function is enabled when Vin pin is connected to PVin
pin and Vin>5.0V. The PWM ramp amplitude (Vramp)
is proportionally changed with respect to Vin to
maintain PVin/Vramp ratio.
The ratio is almost
constant throughout the Vin range (as shown in Figure
12). By maintaining a constant PVin/Vramp, the
control loop bandwidth and phase margin are more
constant. F.F. function also helps minimize the effect
of PVin changes on the output voltage.
Feed-Forward is based on the Vin voltage and needs
to be accounted for when calculating IR3891
compensation.
The PVin/Vramp ratio is not
maintained when Vin and PVin are not equal. This is
the case when an external bias voltage for VCC.
When using an external VCC voltage, Vin pin should
be connected to the VCC pin instead of the PVin pin.
Compensation for the configuration should reflect the
separation.
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
Figure 12: Timing diagram for Feed Forward (F.F.)
Function
LOW DROPOUT REGULATOR (LDO)
Figure 14: Internally Biased Single Rail Configuration
IR3891 has an integrated low dropout (LDO) regulator
which can provide gate drive voltage for both drivers.
When using an internally biased configuration, the
LDO draws from the Vin pin and provides a 5.3V
(typ.), as shown in Figure 13. Vin and PVin can be
connected together as shown in the internally biased
single rail configuration, Figure 14.
An external bias configuration can provide gate drive
voltage for the drivers instead of the internal LDO. To
use an external bias, connected to Vin and VCC to the
external bias, as shown in Figure 15. PVin can also
be connected or a different rail can be used.
When using multiple rail configurations, calculate the
compensation Vramp associated with Vin. Vramp is
derived from Vin which can be different from PVin,
refer to Feed-Forward section.
Figure 15: Externally Biased Configuration
OUTPUT VOLTAGE SEQUENCING
IR3891 can accommodate user sequencing options
using Seq, EN1/2, and PGood1/2 pins. In the block
diagram presented on page 3, the error-amplifier (E/A)
has been depicted with three positive inputs. Ideally,
the input with the lowest voltage is used for regulating
the output voltage and the other two inputs are
ignored. In practice the voltages of the other two
inputs should be at least 200mV greater than the
referenced voltage input so that their effects can
completely be ignored.
Figure 13: Internally Biased Configuration
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© 2013 International Rectifier
In normal operating condition, the IR3891 channels
initially follow their internal soft-starts (Intl_SS) and
then references VREF. After Enable goes high,
Intl_SS begins to ramp up from 0V. The FB pin
follows the Intl_SS until it approaches VREF where
the E/A starts to reference the VREF instead of the
Intl_SS (refer to Figure 16). VREF and Seq are not
referenced initially because they are higher than
Intl_SS. VREF is 0.5V, typical. Seq is internally pulled
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
up to approximately 3.3V when left floating in normal
operation and only used by channel 2.
held constant during power-up. Figure 19 shows
examples of the two sequencing modes.
In sequencing mode of operation, Vout2 is initially
regulated with the Seq pin. Vout2 ramps up similar to
the normal operation, but Intl_SS is replaced with Seq.
Seq is kept to ground level until Intl_SS signal reaches
its final value. FB2 follows Seq, until Seq approaches
VREF where the E/A switches reference to the VREF.
Vout2 is then regulated with respect to internal VREF
(refer to Figure 17). The final Seq voltage should
between 0.7V and 3.3V.
IR3891 uses a single configuration to implement both
mode of sequencing operations. Figure 18 shows the
typical circuit configuration for both modes of
sequencing operation. The sequencing mode is
determined by the RA/RB, RE/RF, and RC/RD ratios. If
RE/RF = RC/RD, simultaneous startup is achieved.
Vout2 follows Vout1 until the voltage at the Seq pin
reaches VREF. After the voltage at the Seq pin
exceeds VREF, VREF dictates Vout2. In ratiometric
startup, Vout2 rises at a slower rate than Vout1. The
resistor values are set up in the following way, RA/RB >
RE/RF > RC/RD.
0.65V
OVP
Is Activated
Intl_SS
OVP(Threshold)
OVP(Hys)
VPG(Upper)
LDrv
turned off
VPG(Lower)
FB/Vsns
Table 2 summarizes the required conditions to
achieve simultaneous or ratiometric sequencing
operations.
Table 2: Required Conditions for Simultaneous /
Ratiometric Tracking and Sequencing
Operating Mode
PGood
1.3 mS*
1.3 mS*
* typical filter delay
Figure 16: Timing Diagram for Output Sequence
Figure 17: Timing Diagram for Sequence Startup (Seq
ramping up/down)
Normal
(Non-sequencing,
Non-tracking)
Simultaneous
Sequencing
Ratiometric
Sequencing
Seq
Required
Condition
Floating
―
Ramp up
from 0V
Ramp up
from 0V
RA/RB>RE/RF=RC/RD
RA/RB>RE/RF>RC/RD
Figure 18: Application Circuit for Simultaneous
and Ratiometric Sequencing
IR3891 can perform simultaneous or ratiometric
sequencing operations. Simultaneous sequencing is
when the both outputs rise at the same rate. During
Ratiometric sequencing, the ratio of the two outputs is
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
Vcc
EN2
Intl_SS2
EN1
Vo1 (master)
Vo2 (slave)
(a)
IR3891
Open Feedback Loop protection (OFLP) is devised to
shutdown the channel in case the feedback is broken.
OFLP is activated when the Vsns is above the
VPG(upper) threshold, 0.85*VREF typical,
and
remains active while Vsns is above the VPG(lower)
threshold, 0.80*VREF.
When FB drop below
OFLP(threshold) threshold, 0.70*VREF, OFLP
disables switching and pulls down on PGood. The
part remains disabled until FB rises above
OFLP(threshold) plus OFLP(Hys), 0.75*VREF. This
function does not latch the part off nor does it require
an EN or a VCC toggle to re-enable the part.
Vo1 (master)
Vo2 (slave)
(b)
Figure 19: Typical waveforms for sequencing mode of
operation: (a) simultaneous, (b) ratiometric
OVER-VOLTAGE PROTECTION (OVP)
Over-Voltage protection (OVP) disables the channel
when the output voltage exceeds the over-voltage
threshold. IR3891 achieves OVP by comparing Vsns
pin to the internal over-voltage threshold set at
OVP(threshold), 1.2*VREF typical. Vsns voltage is
determined by an external voltage divider resistor
network connected to the output in typical application.
When Vsns exceeds the over-voltage threshold, an
over-voltage is detected and OV signal asserts after
OVP(delay). The high side drive signal HDrv is turned
off immediately and PGood flags low. The low side
drive signal is kept on until the Vsns voltage drops
below the lower threshold. After that, HDrv is latched
off until a reset is performed by cycling either VCC or
the respective EN.
Figure 21: Timing Diagram for Open Feedback Line
Protection (OFLP)
POWER GOOD OUTPUT
PGood is an open drain pin that monitors the UV,
FAULT and the POR signals. PGood signal asserts
approximately 1.3mS, after Vsns rises above
VGP(Upper) threshold, 0.85*VREF typical, while
FAULT is low and POR is high. It remains asserted
while FAULT is low and POR is high and Vsns stays
above VGP(Lower) threshold, 0.80*VREF typical.
When Vsns falls below VGP(Lower) threshold there is
a typical 2µS delay before PGood goes low. The two
PGood signals are independent of each other and are
set according to their respective channel.
SWITCH NODE PHASE SHIFT
OVP(Hys)
OVP(Threshold)
Vsns
2uS *
PGood
HDrv
LDrv
*typical filter delay
Figure 20: Timing diagram for OVP
OPEN FEEDBACK-LOOP PROTECTION
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© 2013 International Rectifier
The two converters on the IR3891 run interleaving
phases by 180° to reduce input filter requirements.
The two converters are synchronized to the user
programmable oscillator. Channel 1 runs in phase with
the oscillator while channel 2 runs out of phase.
Staggering the switching cycles reduces the time the
converters
draw
current
from
the
supply
simultaneously. The pulses of current drawn from the
input induce voltage ripples across the input capacitor.
The voltage ripple shapes are dependent on the
different loading and output voltages of the two
converters. By switching the converters at different
times, the magnitude of voltage ripples reduces and
input filter requirements become less stringent.
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
MINIMUM ON-TIME CONSIDERATIONS
MAXIMUM DUTY RATIO
The minimum on-time is the shortest amount of time
which the Control FET may be reliably turned on.
Internal delays and gate drive make up a large portion
of the minimum on-time. IR3891 has a minimum ontime of 60nS.
Maximum duty ratio is lower at higher frequencies and
higher Vin voltages. A maximum off-time of 250nS is
specified for IR3891. This provides an upper limit on
the operating duty ratio at any given switching
frequency. The off-time becomes a larger percentage
of the switching period when high switching
frequencies are used. Thus, a lower the maximum
duty ratio can be achieved when frequencies increase.
Any design or application using IR3891 should
operation with a pulse width greater than minimum ontime. This is necessary for the circuit to operate
without jitter and pulse-skipping, which can cause high
inductor current ripple and high output voltage ripple.
ton 
Vout
D

Fs PVin  Fs
(3)
In any application that uses IR3891, the following
condition must be satisfied:
t on (min)  t on
(4)
Vout
PVin  Fs
V
 PVin  Fs  out
ton (min)
ton (min) 
(5)
(6)
Feed-Forward from the Vin voltage placed a limitation
on the maximum duty cycle by saturating the
compensation ramp.
By maintaining a constant
Vin/Vramp, the effective Vramp voltage is increased
while the maximum range is remains the same. The
ramp reaches the maximum limit before reaching the
expected level. Reaching the maximum limit ends the
switching cycle prematurely and results in a lower
maximum duty cycle.
Maximum duty cycle is dependent on the Vin and
switching frequency. Figure 22 is a theoretical plot of
the maximum duty cycle vs. the switching frequency
using typical parameter values. It shows how the
maximum duty cycle is influenced by the Vin and the
switching frequency.
The minimum output voltage is limited by the
reference voltage and hence Vout(min) = 0.5V. For
Vout(min) = 0.5V,
 PVin  Fs 
 PVin  Fs 
Vout
ton (min)
(7)
0.5V
 8.33V / S
60nS
Therefore, with an input voltage 16V and minimum
output voltage, the converter should be designed for
switching frequency not to exceed 520kHz.
Conversely, the input voltage (PVin) should not
exceed 5.55V for operation at the maximum
recommended operating frequency (1.5MHz) and
minimum output voltage (0.5V). Increasing the PVin
greater than 5.55V will cause pulse skipping.
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© 2013 International Rectifier
Figure 22: Maximum Duty Cycle vs. Switching
Frequency
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
DESIGN EXAMPLE
The following example is a typical application for
IR3891. The application circuit is shown in
Output Voltage Programming
Output voltage is programmed by reference voltage
and external voltage divider. The FB pin is the
inverting input of the error amplifier, which is internally
referenced to VREF. The divider ratio is set to equal
VREF at the FB pin when the output is at its desired
value. When an external resistor divider is connected
to the output as shown in Figure 24, the output
voltage is defined by using the following equation:
Vin = PVin = 12V (21V Max)
Fs = 600kHz
Channel 1:
Vo = 1.8V
Io = 4A
Ripple Voltage = ± 1% * Vo
∆Vo = ± 5% * Vo (for 50% load transient)
 R 
Vo  Vref  1  5 
 R6 
Channel
2:
Vo = 1.2V
Io = 4A
Ripple Voltage = ± 1% * Vo
∆Vo = ± 5% * Vo (for 50% load transient)
 Vref
R6  R5  
V V
ref
 o
Enabling the IR3891
As explained earlier, the precise threshold of the
Enable lends itself well to implementation of a UVLO
for the Bus Voltage as shown in Figure 23.
Figure 23: Using Enable pin for UVLO implementation
For a typical Enable threshold of VEN = 1.2 V
R2
PVin (min) 
 V EN  1.2
R1  R2
R2  R1
V EN
PVin (min)  V EN
(8)
(9)
Programming the frequency
For Fs = 600 kHz, select Rt = 39.2 KΩ, using Table 1.
www.irf.com




(11)
For the calculated values of R5 and R6, see feedback
compensation section.
Figure 24: Typical application of the IR3891
for programming the output voltage
Bootstrap Capacitor Selection
For PVin (min)=9.2V, R1=49.9K and R2=7.5K ohm is a
good choice.
24
(10)
© 2013 International Rectifier
To drive the Control FET, it is necessary to supply a
gate voltage at least 4V greater than the voltage at the
SW pin, which is connected to the source of the
Control FET. This is achieved by using a bootstrap
configuration, which comprises the internal bootstrap
diode and an external bootstrap capacitor (C1). The
operation of the circuit is as follows: When the sync
FET is turned on, the capacitor node connected to SW
is pulled down to ground. The capacitor charges
towards Vcc through the internal bootstrap diode
(Figure 25), which has a forward voltage drop VD. The
voltage Vc across the bootstrap capacitor C1 is
approximately given as:
Vc  Vcc  VD
(12)
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
When the control FET turns on in the next cycle, the
capacitor node connected to SW rises to the bus
voltage Vin. However, if the value of C1 is
appropriately chosen, the voltage Vc across C1
remains approximately unchanged and the voltage at
the Boot pin becomes:
VBoot  Vin  Vcc  VD
Cvin
+ VD -
(13)
VIN
Boot
Vcc
C1
SW
+
Vc
L
IR3891
PGnd
Figure 25: Bootstrap circuit to generate Vc voltage
Ceramic capacitors are recommended due to their
peak current capabilities. They also feature low ESR
and ESL at higher frequency which enables better
efficiency. For this application, it is advisable to have
4x10uF, 25V ceramic capacitors, C3216X5R1E106K
from TDK.
In addition to these, although not
mandatory, a 1x330uF, 25V SMD capacitor EEVFK1E331P from Panasonic may also be used as a
bulk capacitor and is recommended if the input power
supply is not located close to the converter.
Inductor Selection
Inductors are selected based on output power,
operating frequency and efficiency requirements. A
low inductor value causes large ripple current,
resulting in the smaller size, faster response to a load
transient but may reduce efficiency and cause higher
output noise. Generally, the selection of the inductor
value can be reduced to the desired maximum ripple
current in the inductor (∆i). The optimum point is
usually found between 20% and 50% ripple of the
output current. For the buck converter, the inductor
value for the desired operating ripple current can be
determined using the following relation:
1
i
; t  D 
Fs
t
Vo
L  Vin  Vo  
Vin  i  Fs
Vin  Vo  L 
A bootstrap capacitor of value 0.1uF is suitable for
most applications.
Input Capacitor Selection
The ripple currents generated during the on time of
the control FETs should be provided by the input
capacitor. The RMS value of this ripple for each
channel is expressed by:
I RMS  I o  D  1  D 
D
Vo
Vin
(14)
(15)
Where:
D is the Duty Cycle
IRMS is the RMS value of the input capacitor
current.
Io is the output current.
For channel 1, Io=4A and D = 0.15, the IRMS =
1.43A.
For channel 2, Io=4A and D = 0.1, the IRMS = 1.2A.
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© 2013 International Rectifier
IR3891
Where:
Vin
V0
∆i
Fs
∆t
D
(16)
= Maximum input voltage
= Output Voltage
= Inductor Peak-to-Peak Ripple Current
= Switching Frequency
= On time for Control FET
= Duty Cycle
If ∆i ≈ 20%*Io, then the channel 1 output inductor is
calculated to be 3.2μH. Select L=2.2μH, PCMB065T2R2MS, from Cyntec which provides a compact, low
profile inductor suitable for this application. For
channel 2, the output inductor is calculated to be
2.25μH. Select L=1.5μH, PCMB065T-1R5MS, from
Cyntec.
Output Capacitor Selection
The voltage ripple and transient requirements
determine the output capacitors type and values. The
criterion is normally based on the value of the
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
Effective Series Resistance (ESR). However the
actual capacitance value and the Equivalent Series
Inductance (ESL) are other contributing components.
These components can be described as:
The output LC filter introduces a double pole, 40dB/decade gain slope above its corner resonant
frequency, and a total phase lag of 180o. The resonant
frequency of the LC filter is expressed as follows:
Vo  Vo  ESR   Vo  ESL   Vo (C )
FLC 
V0 ( ESR )  I L  ESR
 V V 
V0( ESL )   in o   ESL
 L 
I L
V0 (C ) 
8  Co  Fs
(18)
2    Lo  Co
Phase
Gain
(17)
Since the output capacitor has a major role in the
overall performance of the converter and determines
the result of transient response, selection of the
capacitor is critical. The IR3891 can perform well with
all types of capacitors.
As a rule, the capacitor must have low enough ESR to
meet output ripple and load transient requirements.
The goal for this design is to meet the voltage ripple
requirement in the smallest possible capacitor size.
Therefore it is advisable to select ceramic capacitors
due to their low ESR and ESL and small size. Four of
TDK
C2012X5R0J226M
(22uF/0805/X5R/6.3V)
capacitors is a good choice for channel 1 and channel
2.
It is also recommended to use a 0.1µF ceramic
capacitor at the output for high frequency filtering.
Feedback Compensation
The IR3891 is a voltage mode controller. The control
loop is a single voltage feedback path including error
amplifier and error comparator. To achieve fast
transient response and accurate output regulation, a
compensation circuit is necessary. The goal of the
compensation network is to have a stable closed-loop
transfer function with a high crossover frequency and
phase margin greater than 45o.
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1
Figure 26 shows gain and phase of the LC filter. Since
we already have 180o phase shift from the output filter
alone, the system runs the risk of being unstable.
0dB
00
-40dB/Decade
Where:
∆V0 = Output Voltage Ripple
∆IL = Inductor Ripple Current
26
IR3891
© 2013 International Rectifier
-900
FLC
Frequency
-1800
FLC
Frequency
Figure 26: Gain and Phase of LC filter
The IR3891 uses a voltage-type error amplifier with
high-gain and high-bandwidth. The output of the
amplifier is available for DC gain control and AC
phase compensation.
The error amplifier can be compensated either in type
II or type III compensation.
Local feedback with Type II compensation is shown in
Figure 27.
This method requires that the output capacitor should
have enough ESR to satisfy stability requirements. If
the output capacitor’s ESR generates a zero at 5kHz
to 50kHz, the zero generates acceptable phase
margin and the Type II compensator can be used.
The ESR zero of the output capacitor is expressed as
follows:
FESR 
1
2    ESR  Co
(19)
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
VO U T
Z IN
FLC = Resonant Frequency of the Output Filter
R5 = Feedback Resistor
C P O LE
R3
C3
R5
Zf
Fb
E /A
R6
C om p
Ve
VR EF
G ain (dB )
IR3891
To cancel one of the LC filter poles, place the zero
before the LC filter resonant frequency pole:
FZ  75%  FLC
FZ  0.75 
H (s) dB
1
2   Lo  Co
(25)
Use equation (22), (23) and (24) to calculate C3.
F
FZ
PO LE
F requency
Figure 27: Type II compensation network
and its asymptotic gain plot
The additional pole is given by:
The transfer function (Ve/Vout) is given by:
Z
Ve
1  sR3C3
 H ( s)   f  
Vout
Z IN
sR5C3
Fp 
(20)
The (s) indicates that the transfer function varies as a
function of frequency. This configuration introduces a
gain and zero, expressed by:
H ( s) 
Fz 
R3
R5
(21)
1
2    R3  C3
(22)
First select the desired zero-crossover frequency (Fo):
Fo  FESR and Fo  (1 / 5 ~ 1 / 10)  Fs
One more capacitor is sometimes added in parallel
with C3 and R3. This introduces one more pole which
is mainly used to suppress the switching noise.
1
C  C POLE
2  3
C3  C POLE
(26)
The pole sets to one half of the switching frequency
which results in the capacitor CPOLE:
CPOLE 
1
1
  R3  FS 
C3

1
  R3  FS
(27)
For an unconditional stability general solution using
any type of output capacitors with a wide range of
ESR values, use local feedback with type III
compensation network. Type III compensation
network is typically used for voltage-mode controller
as shown in Figure 28.
(23)
Use the following equation to calculate R3:
R3 
Vramp  Fo  FESR  R5
2
Vin  FLC
(24)
Where:
Vin = Maximum Input Voltage
Vosc = Amplitude of the oscillator Ramp Voltage
Fo = Crossover Frequency
FESR = Zero Frequency of the Output Capacitor
27
www.irf.com
© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
VOUT
ZIN
R4
R3
C3
R5
Zf
Fb
R6
Ve
Comp
E/ A
VREF
Gain (dB)
FZ1
FZ 2
FP2
FP3
Frequency
Figure 28: Type III Compensation network
and its asymptotic gain plot
Again, the transfer function is given by:
Zf
Ve
 H (s)  
Vout
Z IN
By replacing Zin and Zf, according to Figure 28, the
transfer function can be expressed as:
1  sR3C3 1  sC4 R4  R5 

 C  C3 
1  sR4C4 
sR5 C2  C3 1  sR3  2

C
C
2
3



(28)
The compensation network has three poles and two
zeros and they are expressed as follows:
FP1  0
(29)
1
2  R4  C4
1
1
FP 3 

 C  C3  2  R3  C2

2  R3  2
 C2  C3 
FP 2 
28
(32)
(33)
Cross over frequency is expressed as:
Fo  R3  C 4 
Vin
1

Vramp 2  Lo  C o
(34)
Based on the frequency of the zero generated by the
output capacitor and its ESR, relative to the crossover
frequency, the compensation type can be different.
Table 3 shows the compensation types for relative
locations of the crossover frequency.
|H(s)| dB
H ( s)  
1
2  R3  C3
1
1
FZ 2 

2  C4  R3  R5  2  C4  R5
FZ 1 
C2
C4
IR3891
www.irf.com
(30)
(31)
© 2013 International Rectifier
Table 3: Different types of compensators
Compensator
Type
FESR vs FO
Typical Output
Capacitor
Type II
FLC < FESR < FO <
FS/2
Electrolytic
Type III
FLC < FO < FESR
SP Cap,
Ceramic
The higher the crossover frequency is, the potentially
faster the load transient response will be. However,
the crossover frequency should be low enough to
allow attenuation of switching noise. Typically, the
control loop bandwidth or crossover frequency (Fo) is
selected such that:
Fo  1/5 ~ 1/10 * Fs
The DC gain should be large enough to provide high
DC-regulation accuracy. The phase margin should be
greater than 45o for overall stability.
The specifications for designing channel 1:
Vin = 12V
Vo = 1.8V
Vramp= 1.8V (This is a function of Vin, pls. see
Feed-Forward section)
Vref = 0.5V
Lo = 2.2uH
Co = 4x22uF, ESR≈3mΩ each
It must be noted here that the value of the
capacitance used in the compensator design must be
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
the small signal value. For instance, the small signal
capacitance of the 22uF capacitor used in this design
is 9.5uF at 1.8 V DC bias and 600 kHz frequency. It is
this value that must be used for all computations
related to the compensation. The small signal value
may be obtained from the manufacturer’s datasheets,
design tools or SPICE models. Alternatively, they may
also be inferred from measuring the power stage
transfer function of the converter and measuring the
double pole frequency FLC and using equation (18) to
compute the small signal Co.
These result to:
FLC = 17.4 kHz
FESR = 5.6 MHz
Fs/2 = 300 kHz
Select crossover frequency F0=100 kHz
Since FLC<F0<Fs/2<FESR, Type III is selected to place
the pole and zeros.
Select: C3 = 10 nF
C2 
Calculate R4, R5 and R6:
R4 
FP 2  Fo
1  sin 
 567.1 kHz
1  sin 
Select:
FZ 1  0.5  FZ 2  8.8 kHz and
FP 3  0.5  Fs  300 kHz
Select C4 = 2.2nF.
R5 
2    Fo  Lo  C o  Vramp
C 4  Vin
; R3 = 3.6 kΩ,
Select: R3 = 3.24 kΩ
C3 
29
1
; C3 = 5 nF,
2    FZ 1  R3
www.irf.com
1
; R5 = 3.98 kΩ,
2    C4  FZ 2
Select R5 = 4.02 kΩ
Vref
Vo  Vref
 R5 ; R6 = 1.53 kΩ,
Select R6 = 1.54 kΩ
Setting the Power Good Threshold
In this design IR3891, the PGood outer limits are set
at 85% and 120% of VREF. PGood signal is asserted
1.3ms after Vsns voltage reaches 0.85*0.5V=0.425V.
As long as the Vsns voltage is between the threshold
range, Enable is high, and no fault happens, the
PGood remains high.
The following formula can be used to set the PGood
threshold. Vout (PGood_TH) can be taken as 85% of Vout.
Choose Rsns11=1.54 KΩ.
V

Rsns12   out ( PGood _ TH )  1  Rsns11
 0.85  VREF 
Calculate R3, C3 and C2:
R3 
1
; R4 = 127.6 Ω,
2    C4  FP 2
Select R4 = 130 Ω
R6 
1  sin 
 17.6 kHz
1  sin 
FZ 2  Fo
1
; C2 = 148 pF,
2    FP 3  R3
Select: C2 = 150 pF
Detailed calculation of compensation Type III:
Desired Phase Margin Θ = 70°
IR3891
© 2013 International Rectifier
(35)
Rsns12 = 4.00 kΩ, Select 4.02 kΩ,
OVP comparator also uses Vsns signal for OverVoltage detection. With above values for Rsns22 and
Rsns21, OVP trip point (Vout_OVP) is
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
Vout_ OVP  VREF  1.2 
Rsns11  Rsns12
Rsns11
FP 3  0.5  Fs  300 kHz
(36)
Vout_OVP = 2.17 V
IR3891
Select C4 = 2.2nF.
Calculate R3, C3 and C2:
Selecting Power Good Pull-Up Resistor
The PGood1 and PGood2 are open drain outputs and
require pull up resistors to VCC. The value of the pullup resistors should limit the current flowing into the
each PGood pin to be less than 5mA. A typical value
used is 49.9kΩ.
The specifications for the channel 2 design:
Vin=12V
Vo=1.2V
Vramp=1.8V (This is a function of Vin, pls. see feed
forward section)
Vref=0.5V
Lo=1.5uH
Co=4x22uF, ESR≈3mΩ each
In the calculations, 10uF is used for the 22uF Co
capacitors due to the 1.2V bias and 600 kHz
frequency.
These result to:
FLC = 20.5 kHz
FESR = 5.3 MHz
Fs/2 = 300 kHz
Select crossover frequency F0=100 kHz
Since FLC<F0<Fs/2<FESR, Type III is selected to place
the pole and zeros.
R3 
2    Fo  Lo  C o  Vramp
C 4  Vin
; R3 = 2.57 kΩ,
Select: R3 = 2.87 kΩ
C3 
1
; C3 = 7 nF,
2    FZ 1  R3
Select: C3 = 10 nF
C2 
1
;
2    FP 3  R3
C2 = 206 pF,
Select: C2 = 150 pF
Calculate R4, R5 and R6:
R4 
1
; R4 = 127.6 Ω,
2    C4  FP 2
Select R4 = 130 Ω
R5 
1
; R5 = 3.98 kΩ,
2    C4  FZ 2
Select R5 = 4.02 kΩ
Detailed calculation of compensation Type III:
Desired Phase Margin Θ = 70°
FZ 2  Fo
1  sin 
 17.6 kHz
1  sin 
1  sin 
FP 2  Fo
 567.1 kHz
1  sin 
Select:
FZ 1  0.5  FZ 2  8.8 kHz and
30
www.irf.com
© 2013 International Rectifier
R6 
Vref
Vo  Vref
 R5 ; R6 = 2.84 kΩ,
Select R6 = 2.87 kΩ
Setting the Power Good Threshold
Equation (35) shows how to set values for Rsns12
and Rsns11. Use the same equation to determine
Rsns21 and Rsns22 values, but substitute Rsns22 for
Rsns12 and Rsns21 for Rsns11.
Choose Rsns21=2.87 KΩ.
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator

 Vout ( PGood _ TH )
Rsns 22  
 1  Rsns 21

 0.85  VREF
IR3891
(37)
Rsns22 = 4.02 kΩ
The typical over-voltage threshold is calculated below
for channel 2. With above values for Rsns22 and
Rsns21, OVP trip point (Vout_OVP) is
Vout _ OVP  VREF  1.2 
Rsns 21  Rsns 22 (38)
Rsns 22
Vout_OVP = 1.44 V
31
www.irf.com
© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
APPLICATION DIAGRAM
Vcc
Vin
Rt/Sync
PGND1/2
GND
Seq
PVin1/2
INTERNALLY BIASED SINGLE RAIL
Figure 29: Application circuit for 12V to 1.8V and 1.2V, 4A Point of Load Converter Using the Internal LDO
32
www.irf.com
© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
Suggested Bill of Material for application circuit 12V to 1.8V and 1.2V
Part Reference
Cpvin1
Cpvin2
Cvin
Cvcc
Co1 Co2
Cboot1
Cboot2 Cpvin3
Cc12 Cc22
Cc13 Cc23
Cc11 Cc21
Cpvin2
Cout1 Cout2
Qty
1
4
1
1
Value
330uF
10uF
1.0uF
2.2uF
Description
SMD, electrolytic, 25V, 20%
1206, 25V, X5R, 10%
0603, 25V, X5R, 10%
0603, 16V, X5R, 20%
Manufacturer
Panasonic
TDK
Murata
TDK
Part Number
EEV-FK1E331P
C3216X5R1E106M
GRM188R61E105KA12D
C1608X5R1C225M
6
0.1uF
0603, 25V, X7R, 10%
Murata
GRM188R71E104KA01D
2
2
2
4
8
10nF
150pF
2200pF
10uF
22uF
Murata
Murata
Murata
TDK
TDK
GRM188R71H103KA01D
GRM1885C1H151JA01D
GRM188R71H222KA01D
C3216X5R1E106M
C2012X5R0J226M
L0
1
2.2uH
Cyntec
PCMB065T-2R2MS
L1
1
1.5uH
Cyntec
PCMB065T-1R5MS
Rbd1 Rbd2
Ren12 Ren22
Rpg1 Rpg2
Ren11 Ren21
Rc11 Rc21
Rc12
Rc22
Rfb11 Rsns11
Rfb12 Rsns12
Rfb22 Rsns22
Rfb21 Rsns21
Rt
2
20
0603, 50V, X7R, 10%
0603, 50V, NPO, 5%
0603, 50V, X7R, 10%
1206, 25V, X5R, 20%
0805, 6.3V X5R, 20%
SMD
7.05x6.6x4.8mm,11.2mΩ
SMD 7.05x6.6x4.8mm,
6.0mΩ
Thick Film, 0603, 1/10W, 1%
Panasonic
ERJ-3EKF20R0V
4
49.9K
Thick Film, 0603, 1/10W, 1%
Panasonic
ERJ-3EKF4992V
2
2
1
1
2
7.5K
130
3.24K
2.87K
1.54K
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
ERJ-3EKF7501V
ERJ-3EKF1300V
ERJ-3EKF3241V
ERJ-3EKF2871V
ERJ-3EKF1541V
2
4.02K
Thick Film, 0603, 1/10W, 1%
Panasonic
ERJ-3EKF4021V
2
1
2.87K
39.2K
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
ERJ-3EKF2871V
ERJ-3EKF3922V
U1
1
IR3891
PQFN 5x6mm
Panasonic
Panasonic
International
Rectifier
33
www.irf.com
© 2013 International Rectifier
IR3891MPBF
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
EXTERNALLY BIASED DUAL RAIL
PVin
Vin
Cpvin2
4 x 10 uF
Cvcc
2.2 uF
Cpvin3
2 x 0.1 uF
Cvin
1 uF
PGood1
PG1
EN1
Ren11
7.5 K
Cboot1
0.1 uF
Vin
Rpg2
49.9 K
Vcc
Rpg1
49.9 K
Ren12
49.9 K
PVin1/2
Cpvin1
330 uF
PGood2
PG2
EN2
Boot1
Boot2
SW1
SW2
L1
2.2 uH
Rbd1
20
Rc11
127
Rc12
5.9 K
Rfb12
16.5 K
Rfb11
6.34 K
Cc13
100 pF
Rbd2
20
Cc23
100 pF
FB2
FB1
Vsns1
Rsns11
6.34 K
Vsns2
Rt
39.2 K
Cout2
4 x 22 uF
Cc22
5.6 nF
Cc21
1000 pF
Rc22
3.74 K
Rt/Sync
Cc11
1000 pF
PGND1/2
Rsns12
16.5 K
Vo2
1.5 uH
Comp2
U1
IR3891
Comp1
Cc12
5.6 nF
GND
Cout1
4 x 22 uF
Seq
Co1
0.1 uF
Ren21
7.5 K
Cboot2
0.1 uF
L0
Vo1
Ren22
49.9 K
Co2
0.1 uF
Rsns22
18.2 K
Rc21
140
Rfb22
18.2 K
Rfb21
13 K
Rsns21
13 K
Figure 30: Application circuit for a 12V to 1.8V and 1.2V, 4A Point of Load Converter using external 5V VCC
34
www.irf.com
© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
Suggested Bill of Material for application circuit 12V to 1.8V and 1.2V using external 5V VCC
Part Reference
Cpvin1
Cpvin2
Cvin
Cvcc
Cpvin3 Cboot1
Cboot2 Co1
Co2
Cc11 Cc21
Cc12 Cc22
Cc13 Cc23
Cout1 Cout2
Qty
1
4
1
1
Value
330uF
10uF
1.0uF
2.2uF
Description
SMD, electrolytic, 25V, 20%
1206, 25V, X5R, 10%
0603, 25V, X5R, 10%
0603, 16V, X5R, 20%
Manufacturer
Panasonic
TDK
Murata
TDK
Part Number
EEV-FK1E331P
C3216X5R1E106M
GRM188R61E105KA12D
C1608X5R1C225M
6
0.1uF
0603, 25V, X7R, 10%
Murata
GRM188R71E104KA01D
2
2
2
8
1000pF
5.6nF
100pF
22uF
Murata
Murata
Murata
TDK
GRM188R71H102KA01D
GRM188R71H562KA01D
GRM1885C1H101JA01D
C2012X5R0J226M
L0
1
2.2uH
Cyntec
PCMB065T-2R2MS
L1
1
1.5uH
Cyntec
PCMB065T-1R5MS
Rbd1 Rbd2
Rc11
Rc12
Rc21
Rc22
Ren11 Ren21
Ren12 Ren22
Rpg1 Rpg2
Rfb11 Rsns11
Rfb12 Rsns12
Rfb21 Rsns21
Rfb22 Rsns22
Rt
2
1
1
1
1
2
20
127
5.9K
140
3.74K
7.5K
0603, 50V, X7R, 10%
0603, 50V, X7R, 10%
0603, 50V, NPO, 5%
0805, 6.3V X5R, 20%
SMD 7.05x6.6x4.8mm,
11.2mΩ
SMD 7.05x6.6x4.8mm,
6.0mΩ
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
ERJ-3EKF20R0V
ERJ-3EKF1270V
ERJ-3EKF5901V
ERJ-3EKF1400V
ERJ-3EKF3741V
ERJ-3EKF7501V
4
49.9K
Thick Film, 0603, 1/10W, 1%
Panasonic
ERJ-3EKF4992V
2
2
2
2
1
6.34K
16.5K
13K
18.2K
39.2K
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
ERJ-3EKF6341V
ERJ-3EKF1652V
ERJ-3EKF1302V
ERJ-3EKF1822V
ERJ-3EKF3922V
U1
1
IR3891
PQFN 5x6mm
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
International
Rectifier
35
www.irf.com
© 2013 International Rectifier
IR3891MPBF
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
Vcc
Vin
Rt/Sync
PGND1/2
GND
Seq
PVin1/2
EXTERNALLY BIASED SINGLE RAIL
Figure 31: Application circuit for a 5V to 1.8V and 1.2V, 4A Point of Load Converter
36
www.irf.com
© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
Suggested bill of material for application circuit 5V to 1.8V and 1.2V
Part Reference
Cpvin1
Cpvin2
Cvin
Cvcc
Cpvin3 Cboot1
Cboot2 Co1
Co2
Cc12 Cc22
Cc13 Cc23
Cc11 Cc21
Cout1 Cout2
Qty
1
8
1
1
Value
330uF
10uF
1.0uF
2.2uF
Description
SMD, electrolytic, 25V, 20%
1206, 25V, X5R, 10%
0603, 25V, X5R, 10%
0603, 16V, X5R, 20%
Manufacturer
Panasonic
TDK
Murata
TDK
Part Number
EEV-FK1E331P
C3216X5R1E106M
GRM188R61E105KA12D
C1608X5R1C225M
6
0.1uF
0603, 25V, X7R, 10%
Murata
GRM188R71E104KA01D
2
2
2
8
5.6nF
100pF
2200pF
22uF
Murata
Murata
Murata
TDK
GRM188R71H562KA01D
GRM1885C1H101JA01D
GRM188R71H222KA01D
C2012X5R0J226M
L0 L1
2
1.5uH
Cyntec
PCMB065T-1R5MS
Rbd1 Rbd2
Rc11 Rc21
Rc12
Rc22
Ren11 Ren21
Ren12 Ren22
Rfb11 Rsns11
Rfb12 Rsns12
Rfb22 Rsns22
Rfb21 Rsns21
Rpg1 Rpg2
Rt
2
2
1
1
2
2
2
20
52.3
6.81K
4.02K
21K
41.2K
2.61K
0603, 50V, X7R, 10%
0603, 50V, NPO, 5%
0603, 50V, X7R, 10%
0805, 6.3V X5R, 20%
SMD 7.05x6.6x4.8mm,
6.0mΩ
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
ERJ-3EKF20R0V
ERJ-3EKF52R3V
ERJ-3EKF6811V
ERJ-3EKF4021V
ERJ-3EKF2102V
ERJ-3EKF4122V
ERJ-3EKF2611V
4
6.81K
Thick Film, 0603, 1/10W, 1%
Panasonic
ERJ-3EKF6811V
2
2
1
4.87K
49.9K
39.2K
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
Thick Film, 0603, 1/10W, 1%
ERJ-3EKF4871V
ERJ-3EKF4992V
ERJ-3EKF3922V
U1
1
IR3891
PQFN 5x6mm
Panasonic
Panasonic
Panasonic
International
Rectifier
37
www.irf.com
© 2013 International Rectifier
IR3891MPBF
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
TYPICAL OPERATING WAVEFORMS
Vin=PVin=12V, Vo1=1.8V, Iout1=0-4A, Vo2=1.2V, Iout1=0-4A, Fs=600kHz, Room Temperature, No air flow
Figure 32: Startup with full load
CH1:Vout1, Ch2:Vout2, Ch3:Vin, CH4:Vcc
Figure 33: PGood signals at Startup with full load
CH1:Vout1, Ch2:Vout2, Ch3:PGood1, CH4:PGood2
Figure 34: Channel 1 Startup with Pre-Bias, 1.52V
CH1:Enable1, Ch2:Vout1, Ch4:PGood1
Figure 35: Channel 2 Startup with Pre-Bias, 1.05V
CH1:Enable2, Ch2:Vout2, Ch4:PGood2
Figure 36: Inductor Switch Nodes at full load
CH1:SW1, Ch2:SW2
Figure 37: Output Voltage Ripples at full load
CH1:Vout1, Ch2:Vout2
38
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
TYPICAL OPERATING WAVEFORMS
Vin=PVin=12V, Vo1=1.8V, Iout1=0-4A, Vo2=1.2V, Iout1=0-4A, Fs=600kHz, Room Temperature, No air flow
Figure 38: Vout1 Transient Response, 0A to 1.6A step at 2.5A/µSec
CH1:Vout1, CH2=Vout2, CH4:Iout1
39
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
TYPICAL OPERATING WAVEFORMS
Vin=PVin=12V, Vo1=1.8V, Iout1=0-4A, Vo2=1.2V, Iout1=0-4A, Fs=600kHz, Room Temperature, No air flow
Figure 39: Vout2 Transient Response, 0A to 1.6A step at 2.5A/µSec
CH1:Vout1, CH2=Vout2, CH4:Iout2
40
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
TYPICAL OPERATING WAVEFORMS
Vin=PVin=12V, Vo1=1.8V, Iout1=0-4A, Vo2=1.2V, Iout1=0-4A, Fs=600kHz, Room Temperature, No air flow
Figure 40: CH1 Bode Plot with 4A load, CH2 disabled.
Fo = 84.9 kHz, Phase Margin = 51.9 Degrees
Figure 41: CH2 Bode Plot with 4A load, CH1 disabled.
Fo = 113.1 kHz, Phase Margin = 48.2 Degrees
41
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
LAYOUT RECOMMENDATIONS
The layout is very important when designing high
frequency switching converters. Layout will affect
noise pickup and can cause a good design to perform
with less than expected results.
Make the connections for the power components on
the top layer with wide, copper filled areas or
polygons. In general, it is desirable to make proper
use of power planes and polygons for power
distribution and heat dissipation.
The inductor, input capacitors, output capacitors and
the IR3891 should be as close to each other as
possible. This helps to reduce the EMI radiated by the
power traces due to the high switching currents
through them. Place the input capacitor directly at the
PVin pin of IR3891.
The feedback part of the system should be kept away
from the inductor and other noise sources.
The critical bypass components such as capacitors for
PVin and VCC should be close to their respective
- Ground path length
between VIN- and VOUT1should be minimized with
maximum copper
pins. It is important to place the feedback components
including feedback resistors and compensation
components close to Fb and Comp pins.
In a multilayer PCB use one layer as a power ground
plane and have a control circuit ground (analog
ground), to which all signals are referenced. The goal
is to localize the high current path to a separate loop
that does not interfere with the more sensitive analog
control function. These two grounds must be
connected together on the PC board layout at a single
point. It is recommended to place all the
compensation parts over the analog ground plane on
top layer.
The Power QFN is a thermally enhanced package.
Based on thermal performance it is recommended to
use at least a 4-layers PCB. To effectively remove
heat from the device the exposed pad should be
connected to the ground plane using vias. Figure
42a-d illustrates the implementation of the layout
guidelines outlined above, on the IRDC3891 4-layer
demo board.
Vout1
- Compensation parts
should be placed
as close as possible
to the Comp pins
- Bypass caps should
be placed as close as
possible to their
PVin
- Single point connection
between AGND &
PGND, should be placed
near the part and kept
away from noise sources
AGND
- SW node copper is
kept only at the top
layer to minimize the
switching noise
PGND
- Ground path length
between VIN- and
VOUT2- should be
minimized with
maximum copper
Vout2
Figure 42a: IRDC3891 Demo board Layout Considerations – Top Layer
42
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
PGND
Figure 42b: IRDC3891 Demo board Layout Considerations – Bottom Layer
PGND
AGND
Feedback and Vsns trace
routing should be kept
away from noise sources
Figure 42c: IRDC3891 Demo board Layout Considerations – Mid Layer 1
Vin
PGND
Figure 42d: IRDC3891 Demo board Layout Considerations – Mid Layer 2
43
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
PCB METAL AND COMPONENT PLACEMENT
Evaluations have shown that the best overall
performance is achieved using the substrate/PCB
layout as shown in following figures. PQFN devices
should be placed to an accuracy of 0.050mm on
both X and Y axes. Self-centering behavior is highly
dependent on solders and processes, and
experiments should be run to confirm the limits of
self-centering on specific processes. For further
information, please refer to “SupIRBuck™ Multi-Chip
Module (MCM) Power Quad Flat No-Lead (PQFN)
Board Mounting Application Note.” (AN1132)
Figure 43: PCB Metal Pad Spacing (all Dimensions in mm)
44
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
STENCIL DESIGN



IR recommends that the larger Power or Land
Area pads are Solder Mask Defined (SMD).
This allows the underlying Copper traces to be
as large as possible, which helps in terms of
current carrying capability and device cooling
capability.
When using SMD pads, the underlying copper
traces should be at least 0.05mm larger (on
each edge) than the Solder Mask window, in
order to accommodate any layer to layer
misalignment. (i.e. 0.1mm in X & Y).
that these are Non Solder Mask Defined or
Copper Defined.

When using NSMD pads, the Solder Resist
Window should be larger than the Copper Pad
by at least 0.025mm on each edge, (i.e.
0.05mm in X & Y), in order to accommodate any
layer to layer misalignment.

Ensure that the solder resist in-between the
smaller signal lead areas are at least 0.15mm
wide, due to the high x/y aspect ratio of the
solder mask strip.
However, for the smaller Signal type leads
around the edge of the device, IR recommends
Figure 44: Stencil Pad Spacing (all dimensions in mm)
45
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© 2013 International Rectifier
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
46
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© 2013 International Rectifier
IR3891
May 08, 2013
Dual output, 4A/Phase, Highly Integrated SupIRBuckTM
Single-Input Voltage, Synchronous Buck Regulator
IR3891
ENVIRONMENTAL QUALIFICATIONS
Industrial
Qualification Level
Moisture Sensitivity Level
5mm x 6mm PQFN
Machine Model
(JESD22-A115A)
ESD
Human Body Model
(JESD22-A114F)
Charged Device Model
(JESD22-C101D)
MSL2
Class A
<200V
Class 1C
1000V to <2000V
Class III
500V to ≤1000V
Yes
RoHS Compliant
Data and specifications subject to change without notice.
Qualification Standards can be found on IR’s Web site.
IR WORLD HEADQUARTERS: 233 Kansas St., El Segundo, California 90245, USA Tel: (310) 252-7105
TAC Fax: (310) 252-7903
Visit us at www.irf.com for sales contact information.
www.irf.com
47
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© 2013 International Rectifier
May 08, 2013
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