IRF IR3831MTRPBF

PD60363
IR3831MPbF
SupIRBuck
TM
HIGHLY EFFICIENT INTEGRATED
SYNCHRONOUS BUCK REGULATOR FOR DDR APPLICATIONS
Features
Description
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The IR3831 SupIRBuckTM is an easy-to-use, fully
integrated and highly efficient DC/DC regulator.
The MOSFETS co-packaged with the on-chip
PWM controller make IR3831 a space-efficient
solution, providing accurate power delivery for
DDR memory applications.
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Wide Input Voltage Range 1.0V to 16V
Wide Output Voltage Range 0.6V to 0.9*Vin
Continuous 8A Load Capability
Integrated Bootstrap-diode
High Bandwidth E/A for excellent transient
performance
Programmable Switching Frequency up to 1.5 MHz
Programmable Over Current Protection (Hiccup)
PGood output
Programmable Soft-Start
Enable Input with Voltage Monitoring Capability
Enhanced Pre-Bias Start-up
Vp input for DDR Tracking applications
-40oC to 125oC operating junction temperature
Thermal Protection
5mm x 6mm Power QFN Package, 0.9 mm height
Lead-free, halogen-free and RoHS compliant
IR3831 is configured to generate termination
voltage (VTT) for DDR memory applications.
IR3831 offers programmability of start up time,
switching frequency and current limit while
operating in wide input and output voltage range.
The switching frequency is programmable from
250kHz to 1.5MHz for an optimum solution.
It also features important protection functions,
such as Pre-Bias startup, hiccup current limit and
thermal shutdown to give required system level
security in the event of fault conditions.
Applications
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Server Applications
Storage Applications
Embedded Telecom Systems
Distributed Point of Load Power Architectures
Netcom Applications
1.0V <Vin<16V
4.5V <Vcc<5.5V
Enable
VDDQ
Vin
Boot
Vo
Vcc
SW
PGood
PGood
OCSet
Vp
Fb
Rt
SS/ SD
Gnd
PGnd
Comp
Fig. 1. Typical application diagram
11/05/10
1
IR3831MPbF
ABSOLUTE MAXIMUM RATINGS
(Voltages referenced to GND unless otherwise specified)
•
Vin ……………………………………………………. -0.3V to 25V
•
Vcc ……………….….…………….……..……….…… -0.3V to 8V (Note2)
•
Boot
……………………………………..……….…. -0.3V to 33V
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SW
…………………………………………..……… -0.3V to 25V(DC), -4V to 25V(AC, 100ns)
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Boot to SW
•
OCSet
•
Input / output Pins
•
PGND to GND ……………...………………………….. -0.3V to +0.3V
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Storage Temperature Range ................................... -55°C To 150°C
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Junction Temperature Range ................................... -40°C To 150°C (Note2)
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ESD Classification …………………………… ……… JEDEC Class 1C
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Moisture sensitivity level………………...………………JEDEC Level [email protected] °C
……..…………………………….…..….. -0.3V to Vcc+0.3V (Note1)
………………………………………….……. -0.3V to 30V, 30mA
……………………………….. ... -0.3V to Vcc+0.3V (Note1)
Note1: Must not exceed 8V
Note2: Vcc must not exceed 7.5V for Junction Temperature between -10oC and -40oC
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.
PACKAGE INFORMATION
SW
5mm x 6mm POWER QFN
VIN
11
12
10
PGnd
θJA = 35 o C / W
θJ -PCB = 2 o C / W
Boot
13
Enable
14
1
ORDERING INFORMATION
11/05/10
Vp
15
Gnd
2
3
4
5
FB COMP Gnd Rt
6
9
VCC
8
PGood
7
SS OCSet
PACKAGE
DESIGNATOR
PACKAGE
DESCRIPTION
PIN COUNT
PARTS PER
REEL
M
IR3831MTRPbF
15
4000
M
IR3831MTR1PbF
15
750
2
IR3831MPbF
Block Diagram
Fig. 2. Simplified block diagram of the IR3831
11/05/10
3
IR3831MPbF
Pin Description
Pin
Name
Description
Track pin. Use External resistors from VDDQ rail. The Vp voltage can
be set to 0.9V for DDR2 application and 0.75 or 0.6V for DDR3
application.
Inverting input to the error amplifier. This pin is connected directly to the
output of the regulator via resistor divider to set the output voltage and
provide feedback to the error amplifier.
Output of error amplifier. An external resistor and capacitor network is
typically connected from this pin to Fb pin to provide loop
compensation.
1
Vp
2
Fb
3
Comp
4
Gnd
5
Rt
Set the switching frequency. Connect an external resistor from this pin
to Gnd to set the switching frequency.
6
SS/SD
¯¯
Soft start / shutdown. This pin provides user programmable soft-start
function. Connect an external capacitor from this pin to Gnd to set the
start up time of the output voltage. The converter can be shutdown by
pulling this pin below 0.3V.
7
OCSet
Current limit set point. A resistor from this pin to SW pin will set the
current limit threshold.
8
PGood
Power Good status pin. Output is open drain. Connect a pull up resistor
from this pin to Vcc. If unused, it can be left open.
9
VCC
10
PGnd
11
SW
Switch node. This pin is connected to the output inductor.
12
VIN
Input voltage connection pin.
13
Boot
14
Enable
15
Gnd
11/05/10
Signal ground for internal reference and control circuitry.
This pin powers the internal IC and drivers. A minimum of 1uF high
frequency capacitor must be connected from this pin to the power
ground (PGnd).
Power Ground. This pin serves as a separated ground for the MOSFET
drivers and should be connected to the system’s power ground plane.
Supply voltage for high side driver. Connect a 0.1uF capacitor from this
pin to SW.
Enable pin to turn on and off the device.
Signal ground for internal reference and control circuitry.
4
IR3831MPbF
Recommended Operating Conditions
Symbol
Vin
Vcc
Boot to SW
Vo
Io
Fs
Tj
Definition
Input Voltage
Supply Voltage
Supply Voltage
Output Voltage
Output Current
Switching Frequency
Junction Temperature
Min
Max
Units
1.0
4.5
4.5
0.6
0
225
-40
16
5.5
5.5
0.90*Vin
8
1650
125
V
A
kHz
o
C
Electrical Specifications
Unless otherwise specified, these specification apply over 4.5V< Vcc<5.5V, Vp=0.6V, Vin=12V,
0oC<Tj< 125oC. Typical values are specified at Ta = 25oC.
Parameter
Symbol
Test Condition
Min
TYP
MAX
Units
Power Loss
Power Loss
Ploss
Vcc=5V, Vin=12V, Vo=0.75V, Io=8A,
Fs=400kHz, L=0.6uH, Note4
1.28
W
MOSFET Rds(on)
Top Switch
Rds(on)_Top
Bottom Switch
o
VBoot -Vsw =5V, ID=10A, Tj=25 C
o
Rds(on)_Bot
Vcc=5V, ID=10A, Tj=25 C
ICC(Standby)
SS=0V, No Switching, Enable low
16.7
23
8.7
12
mΩ
Supply Current
VCC Supply Current (Standby)
Vcc Supply Current (Dyn)
ICC(Dyn)
500
SS=3V, Vcc=5V, Fs=500kHz
Enable high
12
μA
mA
Under Voltage Lockout
VCC-Start-Threshold
VCC_UVLO_Start
Vcc Rising Trip Level
4.06
4.26
4.46
VCC-Stop-Threshold
VCC_UVLO_Stop
Vcc Falling Trip Level
3.76
3.96
4.16
VCC-Hysteresis
Vcc-Hys
Supply ramping up and down
0.25
0.3
0.38
Enable-Start-Threshold
Enable_UVLO_Start
Supply ramping up
1.14
1.2
1.36
Enable-Stop-Threshold
Enable_UVLO_Stop
Supply ramping down
0.9
1.0
1.06
Enable-Hysteresis
Enable_Hys
Supply ramping up and down
0.16
0.2
0.24
Enable leakage current
Ien
Enable=3.3V
11/05/10
18
V
μA
5
IR3831MPbF
Electrical Specifications (continued)
Unless otherwise specified, these specification apply over 4.5V< Vcc<5.5V, Vp=0.6V, Vin=12V,
0oC<Tj< 125oC. Typical values are specified at Ta = 25oC.
Parameter
Symbol
Test Condition
Min
TYP
MAX
Units
0.625
0.7
0.775
V
225
250
275
Oscillator
Rt Voltage
Frequency
FS
Rt=59K
Rt=28.7K
450
500
550
Rt=9.31K, Note4
1350
1500
1650
kHz
Vramp
Note4
1.8
Vp-p
Ramp Offset
Ramp (os)
Note4
0.6
V
Min Pulse Width
Dmin(ctrl)
Note4
50
Note4
130
Ramp Amplitude
Fixed Off Time
Max Duty Cycle
Dmax
Fs=250kHz
92
Vfb-Vp
Vp=0.6V
-5
200
ns
%
Error Amplifier
Input Offset Voltage
Vos
0
+5
Input Bias Current
IFb(E/A)
-1
+1
Input Bias Current
IVp(E/A)
-1
+1
Sink Current
Isink(E/A)
0.40
0.85
12
Isource(E/A)
8
10
13
Source Current
Slew Rate
Gain-Bandwidth Product
DC Gain
mV
μA
mA
SR
Note4
7
12
20
V/μs
GBWP
Note4
20
30
40
MHz
Gain
Note4
100
110
120
dB
Vcc=4.5V
3.4
3.5
3.7
V
120
220
mV
1
V
μA
Maximum Voltage
Vmax(E/A)
Minimum Voltage
Vmin(E/A)
Note4
Common Mode Voltage
0
Soft Start/SD
Soft Start Current
Soft Start Clamp Voltage
Shutdown Output
Threshold
ISS
Source
Vss(clamp)
14
20
26
2.7
3.0
3.3
SD
0.3
V
Over Current Protection
OCSET Current
OC Comp Offset Voltage
SS off time
IOCSET
VOFFSET
Fs=250kHz
20.8
23.6
26.4
Fs=500kHz
43
48.8
54.6
Fs=1500kHz
136
154
172
Note4
-10
0
+10
SS_Hiccup
4096
μA
mV
Cycles
Bootstrap Diode
Forward Voltage
11/05/10
I(Boot)=30mA
180
260
470
mV
6
IR3831MPbF
Electrical Specifications (continued)
Unless otherwise specified, these specification apply over 4.5V< Vcc<5.5V, Vp=0.6V, Vin=12V,
0oC<Tj< 125oC. Typical values are specified at Ta = 25oC.
Parameter
SYM
Test Condition
Min
TYP
MAX
Units
Thermal Shutdown
Note4
Note4
Thermal Shutdown
Hysteresis
140
20
o
C
Power Good
Power Good upper
Threshold
Delay Comparator
Threshold
Delay Comparator
Hysteresis
PGood Voltage Low
PGood Comparator
Dealy
Leakage Current
VPG+
Fb Rising
PG(Delay)
Relative to charge voltage, SS rising
Delay(hys)
Note4
PG(voltage)
IPGood=-5mA
470
515
560
mV
2
2.1
2.3
V
260
300
340
mV
0.5
V
PG(Delay)
256/Fs
Ileakage
0
s
10
μA
3
40
μA
Switch Node
SW Bias Current
SW=0V, Enable=0V
Isw
SW=0V,Enable=high,SS=3V,Vp=0V,
Note4
Note3: Cold temperature performance is guaranteed via correlation using statistical quality control. Not tested in production.
Note4: Guaranteed by Design but not tested in production.
11/05/10
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IR3831MPbF
TYPICAL OPERATING CHARACTERISTICS (-40oC - 125oC) Fs=500 kHz
Icc(Stan db y)
Icc(Dyn)
240.0
14.0
13.0
[m A ]
[u A ]
220.0
12.0
200.0
11.0
180.0
10.0
-20
0
20
40
60
80
100
120
-40
-20
0
20
100
120
60
80
100
120
60
80
100
ISS
24.0
23.5
23.0
22.5
22.0
21.5
21.0
-20
0
20
40
60
80
100
- 40
120
- 20
0
20
40
Temp[ oC]
FREQUENCY
IOCSET (500k Hz )
510.0
508.0
506.0
504.0
502.0
500.0
498.0
496.0
494.0
492.0
490.0
[u A]
[kHz ]
80
V fb
T e m p [oC]
-40
-20
0
20
40
60
80
100
120
50.6
50.4
50.2
50.0
49.8
49.6
49.4
49.2
49.0
48.8
48.6
-40
-20
0
20
T e m p[o C]
40
120
Te m p [oC]
V cc(UV LO) Star t
V cc(UV L O) Stop
4.36
4.00
4.34
3.99
4.32
3.98
4.30
4.28
[V]
[V]
60
Te m p[oC]
701.0
700.5
700.0
699.5
699.0
698.5
698.0
697.5
697.0
696.5
696.0
-40
4.26
3.97
3.96
4.24
3.95
4.22
4.20
3.94
-40
-20
0
20
40
60
80
100
120
-40
-20
0
20
Te m p[o C]
40
60
80
100
120
Te m p [oC]
Enab le (UV LO) Star t
En able (UV L O) Stop
1.240
1.030
1.235
1.025
1.230
1.225
1.020
1.220
1.015
[V]
[V]
40
T e m p [o C]
[u A ]
[m V]
-40
1.215
1.010
1.210
1.005
1.205
1.200
1.000
-40
-20
0
20
40
60
Te m p [oC]
11/05/10
80
100
120
-40
-20
0
20
40
60
80
100
120
T e m p[o C]
8
IR3831MPbF
E fficiency (% )
Typical Efficiency and Power Loss Curves
Vin=12V, Vo=0.75V, Io=1A- 8A, Vcc=5V, Fs=400kHz Room Temperature, No Air Flow
90
88
86
84
82
80
78
76
74
72
70
1
2
3
4
5
6
7
8
6
7
8
Iout (A)
0.75V out
1.8
1.6
P ower Loss (W )
1.4
1.2
1
0.8
0.6
0.4
0.2
0
1
2
3
4
5
Iout (A)
0.75V out
11/05/10
9
IR3831MPbF
Typical Efficiency and Power Loss Curves
Vin=5V, Vo=0.75V, Io=1A- 8A, Vcc=5V, Fs=400kHz Room Temperature, No Air Flow
90
89
E fficiency (% )
88
87
86
85
84
83
82
1
2
3
4
Iout (A)
5
6
7
8
6
7
8
0.75V out
1.4
1.2
P ower Loss (W )
1.0
0.8
0.6
0.4
0.2
0.0
1
2
3
4
5
Iout (A)
0.75V out
11/05/10
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IR3831MPbF
Circuit Description
THEORY OF OPERATION
Introduction
The IR3831 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
250kHz to 1.5MHz and provides the capability of
optimizing the design in terms of size and
performance.
IR3831 provides precisely regulated output
voltage programmed via two external resistors
from 0.7V to 0.9*Vin.
If the input to the Enable pin is derived from the
bus voltage by a suitably programmed resistive
divider, it can be ensured that the IR3831 does
not turn on until the bus voltage reaches the
desired level. Only after the bus voltage reaches
or exceeds this level will the voltage at Enable
pin exceed its threshold, thus enabling the
IR3831. Therefore, in addition to being a logic
input pin to enable the IR3831, the Enable
feature, with its precise threshold, also allows the
user to implement an Under-Voltage Lockout for
the bus voltage Vin. This is desirable particularly
for high output voltage applications, where we
might want the IR3831 to be disabled at least
until Vin exceeds the desired output voltage level.
The IR3831 operates with an external bias
supply from 4.5V to 5.5V, allowing an extended
operating input voltage range from 1.0V to 16V.
The device utilizes the on-resistance of the low
side MOSFET as current sense element, this
method enhances the converter’s efficiency and
reduces cost by eliminating the need for external
current sense resistor.
IR3831 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 circuit monitors the
input supply Vcc and the Enable input. It assures
that the MOSFET driver outputs remain in the off
state whenever either of these two signals drop
below the set thresholds. Normal operation
resumes once Vcc and Enable rise above their
thresholds.
Fig. 3a. Normal Start up, Device turns on
when the Bus voltage reaches 10.2V
Figure 3b. shows the recommended start-up
sequence for the non-tracking operation of
IR3831, when Enable is used as a logic input.
The POR (Power On Ready) signal is generated
when all these signals reach the valid logic level
(see system block diagram). When the POR is
asserted the soft start sequence starts (see soft
start section).
Enable
The Enable features another level of flexibility for
start up. The Enable has precise threshold which
is internally monitored by Under-Voltage Lockout
(UVLO) circuit. Therefore, the IR3831 will turn on
only when the voltage at the Enable pin exceeds
this threshold, typically, 1.2V.
11/05/10
Fig. 3b. Recommended startup sequence,
Non-Tracking operation
11
IR3831MPbF
Figure 3c. shows the recommended startup
sequence for tracking operation of IR3831 with
Enable used as logic input.
Fig. 5. Pre-Bias startup pulses
Soft-Start
Fig. 3c. Recommended startup sequence,
Sequenced operation
Pre-Bias Startup
IR3831 is able to start up into pre-charged
output,
which
prevents
oscillation
and
disturbances of the output voltage.
The output starts in asynchronous fashion and
keeps the synchronous MOSFET off until the first
gate signal for control MOSFET is generated.
Figure 4 shows a typical Pre-Bias condition at
start up.
The IR3831 has a programmable soft-start to
control the output voltage rise and limit the
current surge at the start-up. To ensure correct
start-up, the soft-start sequence initiates when
the Enable and Vcc rise above their UVLO
thresholds and generate the Power On Ready
(POR) signal. The internal current source
(typically 20uA) charges the external capacitor
Css linearly from 0V to 3V. Figure 6 shows the
waveforms during the soft start.
The start up time can be estimated by:
Tstart =
V p * CSS
20μA
- - - - - - - - - - - - - - - - - - - - (1)
During the soft start the OCP is enabled to
protect the device for any short circuit and over
current condition.
The synchronous MOSFET always starts with a
narrow pulse width and gradually increases its
duty cycle with a step of 25%, 50%, 75% and
100% until it reaches the steady state value. The
number of these startup pulses for the
synchronous MOSFET is internally programmed.
Figure 5 shows a series of 32, 16, 8 startup
pulses.
Fig. 6. Theoretical operation waveforms
during soft-start
Fig. 4. Pre-Bias startup
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12
IR3831MPbF
Operating Frequency
The switching frequency can be programmed
between 250kHz – 1500kHz by connecting an
external resistor from Rt pin to Gnd. Table 1
tabulates the oscillator frequency versus Rt.
Table 1. Switching Frequency and IOCSet vs.
External Resistor (Rt)
Rt (kΩ)
47.5
35.7
28.7
23.7
20.5
17.8
15.8
14.3
12.7
11.5
10.7
9.76
9.31
Fs (kHz)
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
1500
Iocset (μA)
29.4
39.2
48.7
59.07
68.2
78.6
88.6
97.9
110.2
121.7
130.8
143.4
150.3
Shutdown
The IR3831 can be shutdown by pulling the
Enable pin below its 1 V threshold. This will tristate both, the high side driver as well as the low
side driver. Alternatively, the output can be
shutdown by pulling the soft-start pin below 0.3V.
In shutdown by this method, the high side driver
is turned off, and the low side driver is turned on.
Thus, in this method, the output voltage can be
actively discharged through the synchronous
FET. Normal operation is resumed by cycling the
voltage at the Soft Start pin.
Over-Current Protection
The over current protection is performed by
sensing current through the RDS(on) of low side
MOSFET. This method enhances the converter’s
efficiency and reduces cost by eliminating a
current sense resistor. As shown in figure 7, an
external resistor (ROCSet) is connected between
OCSet pin and the switch node (SW) which sets
the current limit set point.
An internal current source sources current (IOCSet
) out of the OCSet pin. This current is a function
of the switching frequency and hence, of Rt.
11/05/10
I OCSet (μA ) =
1400
.......... .......... ...............( 2)
R t (kΩ)
Table 1. shows IOCSet at different switching
frequencies. The internal current source
develops a voltage across ROCSet. When the low
side MOSFET is turned on, the inductor current
flows through the Q2 and results in a voltage at
OCSet which is given by:
VOCSet = ( IOCSet ∗ ROCSet ) − ( RDS(on) ∗ I L ) ...........(3)
Fig. 7. Connection of over current sensing resistor
An over current is detected if the OCSet pin goes
below ground. Hence, at the current limit
threshold, VOCset=0. Then, for a current limit
setting ILimit, ROCSet is calculated as follows:
ROCSet =
R
DS (on)
* I Limit
IOCSet
........................ (4)
An overcurrent detection trips the OCP
comparator, latches OCP signal and cycles the
soft start function in hiccup mode.
The hiccup is performed by shorting the soft-start
capacitor to ground and counting the number of
switching cycles. The Soft Start pin is held low
until 4096 cycles have been completed. The
OCP signal resets and the converter recovers.
After every soft start cycle, the converter stays in
this mode until the overload or short circuit is
removed.
The OCP circuit starts sampling current typically
160 ns after the low gate drive rises to about 3V.
This delay functions to filter out switching noise.
13
IR3831MPbF
Thermal Shutdown
Temperature sensing is provided inside IR3831.
The trip threshold is typically set to 140oC. When
trip threshold is exceeded, thermal shutdown
turns off both MOSFETs and discharges the soft
start capacitor.
Automatic restart is initiated when the sensed
temperature drops within the operating range.
There is a 20oC hysteresis in the thermal
shutdown threshold.
Power Good Output
The IC continually monitors the output voltage via
Feedback (Fb pin). The feedback voltage is
compared to a fixed voltage. As soon as the Fb
voltage reaches 0.515 V, the Power Good signal
flags. This pin is open drain and it needs to be
externally pulled high. High state indicates that
output is in regulation. Fig 8. shows the timing
diagram for the PGood function.
TIMING DIAGRAM OF PGOOD FUNCTION
Fig.8a IR3831 Non-Tracking Operation
Fig.8b IR3831 Tracking Operation
11/05/10
14
IR3831MPbF
Minimum on time Considerations
Maximum Duty Ratio Considerations
The minimum ON time is the shortest amount of
time for which the Control FET may be reliably
turned on, and this depends on the internal
timing delays. For the IR3831, the typical
minimum on-time is specified as 50 ns.
Any design or application using the IR3831 must
ensure operation with a pulse width that is higher
than this minimum on-time and preferably higher
than 100 ns. 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.
A fixed off-time of 200 ns maximum is specified
for the IR3831. This provides an upper limit on
the operating duty ratio at any given switching
frequency. It is clear, that higher the switching
frequency, the lower is the maximum duty ratio at
which the IR3831 can operate. To allow some
margin, the maximum operating duty ratio in any
application using the IR3831 should still
accommodate about 250 ns off-time. Fig 9.
shows a plot of the maximum duty ratio v/s the
switching frequency, with 250 ns off-time.
=
M a x Duty Cycle
D
Fs
Vout
Vin × Fs
In any application that uses the IR3831, the
following condition must be satisfied:
t on (min) ≤ t on
∴ t on (min) ≤
Vout
t on(min)
The minimum output voltage for the IR3831 is
limited to Vout(min) = 0.6 V.
∴ Vin × Fs ≤
95
90
85
80
75
70
65
60
55
250
Vout
Vin × Fs
∴ Vin × Fs ≤
Max D uty C ycle (% )
t on =
450
650
850
1050
1250
1450
1650
S w itching Frequency (kH z )
Fig. 9. Maximum duty cycle v/s switching
frequency.
Vout (min)
∴ Vin × Fs ≤
t on (min)
0.6 V
= 6 × 10 6 V/s
100 ns
Furthermore, for the IR3831, especially for active
bus termination applications, it is strongly
recommended to use a switching frequency of
400 kHz to obtain clean and jitter free operation
in sourcing as well as sinking modes. Therefore,
the maximum input voltage that may be stepped
down to 0.6V at 400 kHz without jitter or pulse
skipping is 15 V.
11/05/10
15
IR3831MPbF
When an external resistor divider is connected to
the output as shown in figure 10.
Equation (5) can be rewritten as:
Application Information
Design Example:
The following example is a typical application for
IR3831. The application circuit is shown on page
22.
Vin = 12 V ( 13.2V max)
Vo = 0.75 V
Io = 8 A
⎛ Vp ⎞
⎟ ..................................(8)
R9 = R8 ∗ ⎜
⎜ V o−Vp ⎟
⎠
⎝
For low voltage applications, such as this
design, it is often advisable to eliminate the bias
resistor R9 from Fb to ground. For the calculated
value of R8 see feedback compensation section.
ΔVo ≤ 22.5mV
Fs = 400 kHz
Enabling the IR3831
As explained earlier, the precise threshold of
the Enable lends itself well to implementation of
a UVLO for the Bus Voltage.
V in
IR3831
Enable
R1
R2
For a typical Enable threshold of VEN = 1.2 V
Vin(min) *
R2
= VEN = 1.2 .......... (5)
R1 + R 2
R 2 = R1
VEN
.......... (6)
Vin( min ) − VEN
For a Vin (min)=10.2V, R1=4.99K and R2=750 ohm
is a good choice.
Programming the frequency
For Fs = 400 kHz, select Rt = 35.7 kΩ, using
Table. 1.
Output Voltage Programming
Output voltage is programmed by the tracking
reference voltage at Vp and external voltage
divider. The divider is ratioed such that the
voltage at the Fb pin is equal to the voltage at the
Vp pin pin when the output is at its desired value.
The output voltage is defined by using the
following equation:
⎛ R ⎞
Vo = Vp ∗ ⎜⎜1 + 8 ⎟⎟ ...................................(7)
⎝ R9 ⎠
11/05/10
Fig. 10. Typical application of the IR3831 for
programming the output voltage
Further, the tracking reference Vp may be itself
derived from some master reference by means
of a resistive divider as shown in Fig. 9. This is
common in active bus termination circuits such
as Voltage Tracking Termination (VTT) where
the tracking reference Vp may be obtained as
half of the master reference VDDQ which forms
the input to one or more memory banks.
In this design,
VDDQ=1.5V
Rp1=Rp2=1.5 kΩ
Vp=0.75V
Soft-Start Programming
The soft-start timing can be programmed by
selecting the soft-start capacitance value. From
(1), for a desired start-up time of the converter,
the soft start capacitor can be calculated by
using:
C SS (μF) = Tstart ( ms ) × 0.02857 .......... (9)
Where Tstart is the desired start-up time (ms).
For tracking applications the output is generally
required to track Vp even at start-up. Hence, it is
necessary to ensure that the SS pin is already
up to 3 V before the tracking reference signal is
applied to the Vp pin. This can be done by
choosing a small value for the soft-start
capacitor to ensure that the voltage at the SS pin
rises to 3 V quickly. A 0.022 uF capacitor is
chosen for this purpose.
16
IR3831MPbF
Bootstrap Capacitor Selection
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 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 (C6). The operation of the
circuit is as follows: When the lower MOSFET is
turned on, the capacitor node connected to SW is
pulled down to ground. The capacitor charges
towards Vcc through the internal bootstrap diode,
which has a forward voltage drop VD. The voltage
Vc across the bootstrap capacitor C6 is
approximately given as
Vc ≅ Vcc − VD ........................................ (10)
When the upper MOSFET turns on in the next
cycle, the capacitor node connected to SW rises
to the bus voltage Vin. However, if the value of C6
is appropriately chosen, the voltage Vc across C6
remains approximately unchanged and the
voltage at the Boot pin becomes
VBoot ≅ Vin + Vcc − VD ........................................ (11)
D=
Vo
................................ (13)
Vin
Where:
D is the Duty Cycle
IRMS is the RMS value of the input capacitor
current.
Io is the output current.
For Io=8 A and D = 0.0625, the IRMS = 1.94 A
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 3x10uF 16V ceramic capacitors
ECJ-3YX1C106K from Panasonic. In addition to
these, although not mandatory, a 330uF 25V
SMD capacitor EEV-FK1E331P from Panasonic
may be used as a bulk capacitor, and is
recommended if the input power supply is not
located close to the converter.
Inductor Selection
The inductor is selected based on output power,
operating frequency and efficiency requirements.
A low inductor value results in a smaller size and
faster response to a load transient but poor
efficiency and high output noise due to large
ripple current. Generally, the selection of the
inductor value can be reduced to the desired
maximum ripple current in the inductor (Δi ). 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:
Δi
1
; Δt = D ∗
Δt
Fs
.......... .......... ... (14)
Vo
L = (Vin − Vo ) ∗
Vin ∗ Δi * Fs
Vin − Vo = L ∗
Fig. 11. Bootstrap circuit to generate
Vc voltage
A bootstrap capacitor of value 0.1uF is suitable
for most applications.
Input Capacitor Selection
The ripple current generated during the on time of
the upper MOSFET should be provided by the
input capacitor. The RMS value of this ripple is
expressed by:
IRMS = Io ∗ D ∗ (1 − D) ........................(12)
11/05/10
Where: Vin = Maximum input voltage
Vo = Output Voltage
Δi = Inductor ripple current
F s = Switching frequency
Δt = Turn on time
D = Duty cycle
If Δi ≈ 35%(Io), then the output inductor is
calculated to be 0.63μH. Select L=0.6 μH.
The MPL104-0R6 from Delta provides a compact,
low profile inductor suitable for this application.
17
IR3831MPbF
Output Capacitor Selection
The voltage ripple and transient requirements
determine the output capacitors type and values.
The criteria is normally based on the value of the
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
ΔVo = ΔVo( ESR ) + ΔVo( ESL ) + ΔVo(C )
⎛ V − Vo ⎞
ΔVo( ESL ) = ⎜⎜ in
⎟⎟ * ESL
L
⎝
⎠
voltage
ΔI L = Inductor
ripple
2 ∗ π Lo ∗ Co
................................ (16)
Phase
Gain
0 dB
00
-40dB/decade
ripple
current
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 IR3831
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. Eight of the Panasonic ECJ2FB0J226ML (22uF, 6.3V, 3mOhm) capacitors is
a good choice.
Feedback Compensation
The IR3831 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 provide a closed-loop transfer
function with the highest 0 dB crossing frequency
and adequate phase margin (greater than 45o).
11/05/10
1
.......... .......... ..... (15)
ΔI L
8 * C o * Fs
ΔVo = Output
FLC =
Figure 12 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.
ΔVo( ESR ) = ΔI L * ESR
ΔVo(C ) =
The output LC filter introduces a double pole,
–40dB/decade gain slope above its corner
resonant frequency, and a total phase lag of 180o
(see figure 12). The resonant frequency of the LC
filter is expressed as follows:
-1800
FLC Frequency
FLC Frequency
Fig. 12. Gain and Phase of LC filter
The IR3831 uses a voltage-type error amplifier
with high-gain (110dB) and wide-bandwidth. The
output of the error 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 Fig. 13.
This method requires that the output capacitor
should have enough ESR to satisfy stability
requirements. In general the output capacitor’s
ESR generates a zero typically at 5kHz to 50kHz
which is essential for an acceptable phase
margin.
The ESR zero of the output capacitor is
expressed as follows:
FESR =
1
.......... .......... ....... (17)
2 ∗ π*ESR*C o
18
IR3831MPbF
VOUT
Z IN
C POLE
R3
C4
R8
Zf
Fb
R9
Gain(dB)
E/A
Comp
Ve
To cancel one of the LC filter poles, place the
zero before the LC filter resonant frequency pole:
Fz = 75%FLC
VREF
Fz = 0.75*
H(s) dB
FZ
F
POLE
Frequency
Fig. 13. Type II compensation network
and its asymptotic gain plot
The transfer function (Ve/Vo) is given by:
Ve
1 + sR3C4
Z
= H(s) = − f = −
.....(18)
ZIN
sR8C4
Vo
The (s) indicates that the transfer function varies
as a function of frequency. This configuration
introduces a gain and zero, expressed by:
H(s) =
Where:
Vin = Maximum Input Voltage
Vosc = Oscillator Ramp Voltage
Fo = Crossover Frequency
FESR = Zero Frequency of the Output Capacitor
FLC = Resonant Frequency of the Output Filter
R8 = Feedback Resistor
R3
......................................(19)
R8
1
Fz =
............................(20)
2π * R3 * C4
First select the desired zero-crossover frequency
(Fo):
Fo > FESR and Fo ≤ (1/5 ~ 1/10) * Fs
Vosc * Fo * FESR * R8
2
Vin * FLC
...........................(21)
FP =
.....................................(22)
1
.................................(23)
C4 * CPOLE
2π * R3 *
C4 + CPOLE
The pole sets to one half of the switching
frequency which results in the capacitor CPOLE:
CPOLE =
1
π*R3*Fs −
1
C4
≅
1
......................(24)
π*R3*Fs
For a general solution for unconditional stability
for any type of output capacitors, and a wide
range of ESR values, we should implement local
feedback with a type III compensation network.
The typically used compensation network for
voltage-mode controller is shown in figure 14.
Again, the transfer function is given by:
Ve
Z
= H(s) = − f
Vo
ZIN
By replacing Zin and Zf according to figure 14,
the transfer function can be expressed as:
H(s) = −
11/05/10
2π Lo * Co
Use equations (20), (21) and (22) to calculate
C4.
One more capacitor is sometimes added in
parallel with C4 and R3. This introduces one
more pole which is mainly used to suppress the
switching noise.
The additional pole is given by:
Use the following equation to calculate R3:
R3 =
1
(1 + sR3C4 )[1 + sC7 (R8 + R10 )]
⎡
⎛ C * C3 ⎞⎤
⎟⎥(1 + sR10C7 )
sR8 (C4 + C3 )⎢1 + sR3 ⎜⎜ 4
⎟
⎝ C4 + C3 ⎠⎦⎥
⎣⎢
19
....(25)
IR3831MPbF
VOUT
ZIN
C3
C7
R3
R10
C4
R8
Zf
Fb
R9
Gain(dB)
E/A
Comp
Ve
FZ2
FP2
FP3
Frequency
Fig.14. Type III Compensation network and
its asymptotic gain plot
The compensation network has three poles and
two zeros and they are expressed as follows:
FP1 = 0 ..................................................................(26)
FP 2 =
1
...............................................(27)
2π * R10 * C7
1
FP3 =
FZ1
≅
1
...............(28)
2π * R3 * C3
⎛ C * C3 ⎞
⎟
2π * R3 ⎜⎜ 4
⎟
⎝ C4 + C3 ⎠
1
=
.............................................(29)
2π * R3 * C4
FZ 2 =
1
1
≅
..........(30)
2π * C7 * (R8 + R10 ) 2π * C7 * R8
Cross over frequency is expressed as:
Fo = R3 * C7 *
Vin
1
*
................................(31)
Vosc 2π * Lo * Co
Based on the frequency of the zero generated by
the output capacitor and its ESR, relative to
crossover frequency, the compensation type can
be different. The table below shows the
compensation types and location of the
crossover frequency.
11/05/10
FESR vs Fo
Output
Capacitor
Type II
FLC<FESR<Fo<Fs/2
Electrolytic
Tantalum
Type III
FLC<Fo<FESR
Tantalum
Ceramic
VREF
H(s) dB
FZ1
Compensator
Type
The higher the crossover frequency, the
potentially faster the load transient response.
However, the crossover frequency should be low
enough to allow attenuation of switching noise.
Typically, the control loop bandwidth or crossover
frequency 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.
For this design we have:
Vin=12V
Vo=0.75V
Vosc=1.8V
Vp=0.75V
Lo=0.6 uH
Co=8x22uF, ESR=3mOhm each
It must be noted here that the value of the
capacitance used in the compensator design
must be the small signal value. For instance, the
small signal capacitance of the 22uF capacitor
used in this design is 12uF at 0.75 VDC bias and
400 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 (16) to compute the small signal
Co.
These result to:
FLC=20.97 kHz
FESR=4.4 MHz
Fs/2=200 kHz
Select crossover frequency: Fo= 60 kHz
Since FLC<Fo<Fs/2<FESR, TypeIII is selected to
place the pole and zeros.
20
IR3831MPbF
Detailed calculation of compensation TypeIII
Desired Phase Margin Θ = 70o
FZ2 = Fo
1 − sin Θ
= 10.58 kHz
1 + sin Θ
FP2 = Fo
1 + sin Θ
= 340.28 kHz
1 − sin Θ
Select: FZ1 = 0.5 * FZ2 = 5.29 kHz and
FP3 = 0.5* Fs = 200 kHz
Select: C7 = 2.2nF
Calculate R3, C3 and C4 :
2π * Fo * Lo * Co * Vosc
;R3 = 1.48 kΩ
C7 * Vin
R3 =
Select: R3 = 1.47 kΩ
1
C4 =
; C4 = 20.47nF, Select: C4 = 22 nF
2π * FZ1 * R 3
C3 =
1
; C3 = 541.34pF, Select: C3 = 560 pF
2π * FP3 * R3
Programming the Current-Limit
The Current-Limit threshold can be set by
connecting a resistor (ROCSET) from the SW pin
to the OCSet pin. The resistor can be calculated
by using equation (4). This resistor ROCSET must
be placed close to the IC.
The RDS(on) has a positive temperature
coefficient and it should be considered for the
worst case operation.
ISET = IL (critical) =
ROCSet∗ IOCSet
.......................(32)
RDS(on)
RDS(on) = 8.7mΩ * 1.5 = 13.05 mΩ
ISET ≅ I o(LIM) = 8 A *1.5 = 12 A
(50% over nominal output current)
IOCSet = 39.22μA (at Fs = 400kHz)
ROCSet = 3.99kΩ Select R7 = 4.02 kΩ
Setting the Power Good Threshold
Power Good threshold is internally set at 0.515 V.
When the voltage at the FB pin exceeds the
threshold, PGood is asserted.
The PGood is an open drain output. Hence, it is
necessary to use a pull up resistor RPG from
PGood pin to Vcc. The value of the pull-up
resistor must be chosen such as to limit the
current flowing into the PGood pin, when the
output voltage is not in regulation, to less than 5
mA. A typical value used is 4.7kΩ.
Calculate R10, R8 and R9 :
R10 =
1
; R10 = 215 Ω, Select: R10 = 210 Ω
2π * C7 * FP2
R8 =
1
- R10; R8 = 6.63 kΩ,
2π * C7 * FZ2
Select: R8 = 6.65 kΩ
11/05/10
21
IR3831MPbF
Application Diagram:
Fig. 15. Application circuit diagram for a 12V to 0.75 V, 8 A Point Of Load Converter
Suggested Bill of Materials for the application circuit:
Part Reference
Cin
Quantity
Value
330uF
10uF
0.6uH
22uF
4.99K
750
35.7K
Description
SMD Elecrolytic, Fsize, 25V, 20%
1206, 16V, X7R, 20%
11.5x10x4mm, 20%, 1.7mOhm
0805, 6.3V, X5R, 20%
Thick Film, 0603,1/10 W,1%
Thick Film, 0603,1/10W,1%
Thick Film, 0603,1/10W,1%
Manufacturer
Panasonic
Panasonic - ECG
Delta
Panasonic- ECG
Rohm
Rohm
Rohm
Part Number
EEV-FK1E331P
ECJ-3YX1C106K
MPL104-0R6
ECJ-2FB0J226ML
MCR03EZPFX4991
MCR03EZPFX7500
MCR03EZPFX3572
Lo
Co
R1
R2
Rt
1
3
1
8
1
1
1
ROCSet
1 4.02K
Thick Film, 0603,1/10 W,1%
Rohm
MCR03EZPFX4021
RPG
Css
R3
C3
C4
R8
R10
C7
U1
1
1
1
1
1
1
1
1
1
Thick Film, 0603,1/10W,1%
0603, 25V, X7R, 10%
Thick Film, 0603,1/10W,1%
50V, 0603, NPO, 5%
0603, 50V, X7R, 10%
Thick Film, 0603,1/10W,1%
Thick Film, 0603,1/10W,1%
0603, 50V, X7R, 10%
SupIRBuck PQFN 5x6mm
Rohm
Panasonic - ECG
Rohm
Panasonic- ECG
Panasonic - ECG
Rohm
Panasonic - ECG
Panasonic - ECG
International Rectifier
MCR03EZPFX4701
ECJ-1VB1E223K
MCR03EZPFX1471
ECJ-1VC1H561J
ECJ-1VB1H223K
MCR03EZPFX6651
ERJ-3EKF2100V
ECJ-1VB1H222K
IR3831MPbF
11/05/10
4.7K
0.022uF
1.47k
560pF
22000pF
6.65K
210
2200pF
IR3831
22
IR3831MPbF
TYPICAL OPERATING WAVEFORMS
Vin=12.0V, Vcc=5V, Vo=0.75V, Io=0- ±8A, Room Temperature, No Air Flow
Fig. 16: Start up at 8A, sourcing
current Ch1:Vout, Ch2:VDDQ,
Ch3:SS, Ch4:PGood
Fig. 17: Start up with Prebias, 0A
Load Ch1:Vout, Ch2:VDDQ, Ch3:SS
Fig. 18: Inductor node at 8A, sourcing
current, Ch3:SW, Ch4:Iout
Fig. 19: Inductor node at -3A, sinking
current, Ch3:SW , Ch4:Iout
Fig. 20: Output Voltage Ripple, 8A,
sourcing current, Ch2: Vout
Fig. 21: Short (Hiccup) Recovery
Ch2:Vout, Ch3:VSS , Ch4:PGood
11/05/10
23
IR3831MPbF
TYPICAL OPERATING WAVEFORMS
Vin=12V, Vcc=5V, Vo=0.75V, Room Temperature, No Air Flow
Fig. 22: Tracking 8A, sourcing
current, Ch1:Vout, Ch3:VDDQ,
Ch4:PGood
Fig. 23: Tracking -3A load, sinking
current, Ch1:Vout, Ch2: IL,
Ch3:VDDQ, Ch4:PGood
Fig. 24: Transient Response, 1A/us
-0.5A to +0.5A load , Ch1:Vout, Ch4:Io
11/05/10
24
IR3831MPbF
TYPICAL OPERATING WAVEFORMS
Vin=12V, Vcc=5V, Vo=0.75V, Io=+8A, Room Temperature, No Air Flow
Fig.25: Bode Plot at 8A load (sourcing current) shows a bandwidth of 43kHz and phase margin of
63 degrees
11/05/10
25
IR3831MPbF
Layout Considerations
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 all the connections for the power
components in 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, output capacitors and the IR3831
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 Vin pin of IR3831.
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 Vcc should be close to their
respective pins. It is important to place the
feedback components including feedback
resistors and compensation components close to
Fb and Comp pins.
The connection between the OCSet resistor and
the Sw pin should not share any trace with the
connection between the bootstrap capacitor and
the Sw pin. Instead, it is recommended to use a
Kelvin connection of the trace from the OCSet
resistor and the trace from the bootstrap
Vin
capacitor at the Sw pin. PGnd
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
Vout
interfere
with the more sensitive
analog control
AGnd
function. These two grounds must be connected
together on the PC board layout at a single point.
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 26 illustrates the
implementation of the layout guidelines outlined
above, on a 4 layer board.
Vin
PGnd
Vout
AGnd
Enough copper &
minimum length
ground path between
Input and Output
PGnd
Compensation parts
should be placed as close
as possible to
the Comp pin.
Vin
Vin
Resistors Rt and
Rocset should be
placed as close as
possible to their pins.
All bypass caps
should be placed as
close as possible to
their connecting
pins.
PGnd
AGnd
AGnd
Vout
Vout
Fig. 26a. IR3831 layout considerations –
Top Layer
11/05/10
26
IR3831MPbF
PGnd
Feedback
trace should
be kept
away form
noise
sources
Fig. 26b. IR3831 layout considerations –
Bottom Layer
Analog
Ground
plane
Power
Vin
Ground
Plane
Single point
connection between
AGND & PGND,
should be close to
the SupIRBuck, kept
away from noise
sources.
AGnd
Fig. 26c. IR3831 layout considerations – Mid
Layer 1
Use separate traces
for connecting Boot
cap and Rocset to the
switch node and with
the minimum length
traces. Avoid big
loops.
Fig. 26d. IR3831 layout considerations – Mid
Layer 2
11/05/10
27
IR3831MPbF
PCB Metal and Components Placement
Lead lands (the 11 IC pins) width should be equal to nominal part lead width. The
minimum lead to lead spacing should be ≥ 0.2mm to minimize shorting.
Lead land length should be equal to maximum part lead length + 0.3 mm outboard
extension. The outboard extension ensures a large and inspectable toe fillet.
Pad lands (the 4 big pads other than the 11 IC pins) length and width should be equal to
maximum part pad length and width. However, the minimum metal to metal spacing
should be no less than 0.17mm for 2 oz. Copper; no less than 0.1mm for 1 oz. Copper
and no less than 0.23mm for 3 oz. Copper.
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Solder Resist
It is recommended that the lead lands are Non Solder Mask Defined (NSMD). The solder
resist should be pulled away from the metal lead lands by a minimum of 0.025mm to ensure
NSMD pads.
The land pad should be Solder Mask Defined (SMD), with a minimum overlap of the solder
resist onto the copper of 0.05mm to accommodate solder resist mis-alignment.
Ensure that the solder resist in-between the lead lands and the pad land is ≥ 0.15mm due to
the high aspect ratio of the solder resist strip separating the lead lands from the pad land.
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IR3831MPbF
Stencil Design
•
•
The Stencil apertures for the lead lands should be approximately 80% of the area of
the lead lads. Reducing the amount of solder deposited will minimize the
occurrences of lead shorts. If too much solder is deposited on the center pad the part
will float and the lead lands will be open.
The maximum length and width of the land pad stencil aperture should be equal to
the solder resist opening minus an annular 0.2mm pull back to decrease the
incidence of shorting the center land to the lead lands when the part is pushed into
the solder paste.
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IR3831MPbF
BOTTOM VIEW
IR WORLD HEADQUARTERS: 233 Kansas St., El Segundo, California 90245, USA Tel: (310) 252-7105
TAC Fax: (310) 252-7903
This product has been designed and qualified for the Consumer market
Visit us at www.irf.com for sales contact information
Data and specifications subject to change without notice. 11/10
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