IR3721
DATA SHEET
Power Monitor IC with
Analog Output
FEATURES
Accurate
TruePower
monitor
•
2.5% static accuracy
•
Minimizes dynamic errors
Minimizes power dissipation
•
5mV - 150mV full scale current range
Versatile
•
Monitors power or current
•
Single buck or multiphase converters
•
Inductor DCR or resistive shunt sensing
Simple add-on to existing converters
10 pin 3x3 DFN lead free package
RoHS compliant
TM
DESCRIPTION
The IR3721 is a versatile power or current monitor IC
for low-voltage DC-DC converters. The IR3721
monitors the inductor current in buck or multiphase
converters using either a current sensing resistor or the
inductor’s winding resistance (DCR). The output (DI) is
a pulse code modulated signal whose duty ratio is
proportional to the inductor current. An analog voltage
that is proportional to power is realized by connecting
V
K
to V
O
and connecting an RC filter to DI.
The IR3721 uses Patent Pending
TruePower
TM
technology to accurately capture highly dynamic power
waveforms typical of microprocessor loads.
TYPICAL APPLICATION CIRCUIT
ORDERING INFORMATION
Device
IR3721MTRPBF
* IR3721MPBF
*
Samples only
Package
10 lead DFN (3x3 mm body)
10 lead DFN (3x3 mm body)
Order Quantity
3000 piece reel
121 piece tube
Page 1 of 16
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09/15/08
IR3721
DATA SHEET
ABSOLUTE MAXIMUM RATINGS
Absolute Maximum Ratings (Referenced to GND)
VDD:.................................................................3.9V
All other Analog and Digital pins ......................3.9V
Operating Junction Temperature .... -10°C to 150°C
Storage Temperature Range .......... -65°C to 150°C
ESD Rating ............HBM Class 2 JEDEC Standard
MSL Rating ..................................................Level 2
Reflow Temperature ..................................... 260°C
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. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.
ELECTRICAL SPECIFICATIONS
Unless otherwise specified, these specifications apply: VDD = 3.3V ± 5%, 0
o
C
≤
T
J
≤
125
o
C, 0.5
≤
Vo
≤
1.8 V, and
operation in the typical application circuit. See notes following table.
PARAMETER
BIAS SUPPLY
VDD Turn-on Threshold, VDD
UP
VDD Turn-off Threshold, VDD
DN
VDD UVLO Hysteresis
VDD Operating Current, ICC
VOLTAGE REFERENCE
V
RT
Voltage
R
T
resistance range
ΔΣ
CONVERTER
Vo common mode range
Duty Ratio Accuracy
TEST CONDITION
MIN
TYP
MAX
3.10
DI output low when off
2.4
75
350
R
T
= 25.5k
Ω
Note 1
1.452
1.493
25.5
450
1.535
UNIT
V
V
mV
μA
V
kΩ
1.8
2.5
V
%
0.5
V
DCR
=20 mV, V
O
=1V,
R
T
=25.5kΩ, R
CS1
+R
CS2
=600
Ω
T
j
=65°C, Note 1
V
DCR
=20 mV, V
O
=1V,
R
T
=25.5kΩ, R
CS1
+R
CS2
=600
Ω,
Note 1
435
-0.5
-250
0.5
1250
512
Duty Ratio Accuracy
4
%
Sampling frequency, f
CLK
Comparator Offset
CS pin input current, I
CS
DIGITAL OUTPUT
VK pin voltage range
DI source resistance
NOTES:
1.
Guaranteed by design
DI output low
589
+0.5
+250
1.8
3000
kHz
mV
nA
V
Ω
2000
Page 2 of 16
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09/15/08
IR3721
DATA SHEET
BLOCK DIAGRAM
VDD
VK
VO
VCS
V
RT
VO
result out
DI
I
REF
I
REF
TruePower™
VCS
result out/
IR3721
GND
IC PIN DESCRIPTION
NAME
VCS
VO
VRT
GND
VDD
GND
GND
DI
VK
VDD
BASE PAD
NUMBER
1
2
3
4
5
6
7
8
9
10
I/O LEVEL
Analog
Analog
Analog
3.3V
DESCRIPTION
Current sensing input, connect through resistor to sensing node
Current sensing reference connect to output voltage
R
T
thermistor network from this pin to GND programs thermal monitor
Bias return and signal reference
IC bias supply
Connect to pin 4
Connect to pin 4
Power Monitor output; connect to output filter
Connect to fixed voltage or VO, multiplied by DI to become analog output
Connect to pin 5
Connect to pin 4
Analog
1.8V
3.3V
Page 3 of 16
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09/15/08
IR3721
DATA SHEET
IC PIN FUNCTIONS
VDD PINS
These pins provide operational bias current to circuits
internal to the IR3721. Bypass them with a high
quality ceramic capacitor to the GND pins.
GND PINS
These pins return operational bias current to system
ground. VO is measured with respect to GND. The
GND pin sinks reference current established by the
external resistor R
T
.
VO PIN
Since this pin measures DCR voltage drop it is critical
that it be Kelvin connected to the buck inductor
output. Power accuracy may be degraded if the
voltage at this pin is below VO
min
.
VCS PIN
A switched current source internal to the IR3721
maintains the average voltage of this pin equal to the
voltage of the VO pin. The average current into this
pin is therefore proportional to buck inductor current.
DI PIN
The Dl pin output has a duty ratio proportional to the
current into VCS, and an amplitude equal to the
voltage at the VK pin. The DI pin is intended to drive
an external low pass filter. The output of this filter is
the product of the current and voltage terms.
VRT PIN
A voltage reference internal to the IR3721 drives the
V
RT
pin while the pin current is monitored and used to
set the amplitude of the current monitor switched
current source I
REF
. Connect this pin to GND through
a precision resistor network R
T
. This network may
include provision for canceling the positive
temperature coefficient of the buck inductor’s DC
resistance (DCR).
VK PIN
The voltage of the VK pin is used to modulate the
amplitude of the DI pin. This is one of the terms used
to determine the product of the multiplier output. If VK
is connected to a fixed voltage then the output of the
multiplier is proportional to current. If VK is connected
to the buck converter output voltage then the output
of the DI driven RC filter is proportional to power.
Page 4 of 16
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09/15/08
IR3721
DATA SHEET
FUNCTIONAL DESCRIPTION
Please refer to the Functional Description Diagram
below. Power flow from the buck converter inductor is
the product of output voltage times the current I
L
flowing through the inductor.
Power is measured with the aid of International
Rectifier’s proprietary TruePower™ circuit. Current is
converted to a duty ratio that appears at the DI pin.
The duty ratio of the DI pin is
I
L
⋅
DCR
R
⋅
T
DI
DUTYRATIO
=
(
R
CS1
+
R
CS2
)
V
R
Τ
Equation 1
The full-scale current that can be measured
corresponds to a duty ratio of one.
The amplitude of the DI pin is the voltage appearing
at pin VK. If a fixed voltage is applied to VK then the
output of the RC filter driven by DI will be proportional
to inductor current I
L
.
If VO is applied to V
K
as shown in the figure then the
output of the DI driven RC network will be
proportional to power. The full-scale voltage that can
be measured is established on the chip to be 1.8V.
The full scale power P
FS
that can be measured is the
product of full-scale voltage and full scale current.
I
L
Vin
L
DCR
VDD
VK
R
CS1
C
CS1
C
CS2
R
CS2
V
RT
R
T
GND
VO
VCS
Vo
IR3721
DI
Power
Figure 1 Functional Description Diagram
Page 5 of 16
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09/15/08
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