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IR3721
DATA SHEET
Power Monitor IC with Analog Output
FEATURES
Accurate TruePowerTM 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
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 VK to VO and connecting an RC filter to DI. The IR3721 uses Patent Pending TruePowerTM 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
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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 .... -10C to 150C Storage Temperature Range .......... -65C to 150C ESD Rating ............HBM Class 2 JEDEC Standard MSL Rating ..................................................Level 2 Reflow Temperature ..................................... 260C
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%, 0oC TJ 125oC, 0.5 Vo 1.8 V, and operation in the typical application circuit. See notes following table. PARAMETER BIAS SUPPLY VDD Turn-on Threshold, VDDUP VDD Turn-off Threshold, VDDDN VDD UVLO Hysteresis VDD Operating Current, ICC VOLTAGE REFERENCE VRT Voltage RT resistance range CONVERTER Vo common mode range Duty Ratio Accuracy TEST CONDITION MIN TYP MAX 3.10 DI output low when off 2.4 75 350 RT = 25.5k Note 1 1.452 1.493 25.5 450 1.535 UNIT V V mV A V k 1.8 2.5 V %
0.5 VDCR=20 mV, VO=1V, RT=25.5k, RCS1+RCS2=600 Tj=65C, Note 1 VDCR=20 mV, VO=1V, RT=25.5k, RCS1+RCS2=600 , Note 1 435 -0.5 -250 0.5 1250 512
Duty Ratio Accuracy
4
%
Sampling frequency, fCLK Comparator Offset CS pin input current, ICS 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
2000
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IR3721
DATA SHEET
BLOCK DIAGRAM
VDD VK VO VCS VRT VO result out DI
IREF IREF
TruePowerTM 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 RT 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
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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 RT. 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 VOmin. 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 VRT pin while the pin current is monitored and used to set the amplitude of the current monitor switched current source IREF. Connect this pin to GND through a precision resistor network RT. 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.
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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 IL flowing through the inductor. Power is measured with the aid of International Rectifier's proprietary TruePowerTM circuit. Current is converted to a duty ratio that appears at the DI pin. The duty ratio of the DI pin is IL DCR R T DIDUTYRATIO = (R CS1 + R CS2 ) VR 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 IL. If VO is applied to VK 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 PFS that can be measured is the product of full-scale voltage and full scale current.
IL Vin L DCR VDD VK RCS1 CCS1 CCS2 RCS2 VRT RT GND VO VCS Vo
IR3721
DI
Power
Figure 1 Functional Description Diagram
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IR3721
DATA SHEET
THERMAL COMPENSATION FOR INDUCTOR DCR CURRENT SENSING
The positive temperature coefficient of the inductor DCR can be compensated if RT varies inversely proportional to the DCR. DCR of a copper coil, as a function of temperature, is approximated by
DCR (T ) = DCR (TR ) (1 + (T - TR ) TCRCu )
R th (T ) = R th (T0
) e 1 1 - T T 0
Equation 3
Equation 2 TR is some reference temperature, usually 25 C, and TCRCu is the resistive temperature coefficient of copper, usually assumed to be 0.39 %/C near room temperature. Note that equation 2 is linearly increasing with temperature and has an offset of DCR(TR) at the reference temperature. If RT incorporates a negative temperature coefficient thermistor then temperature effects of DCR can be minimized. Consider a circuit of two resistors and a thermistor as shown below.
where Rth(T) is the thermistor resistance at some temperature T, Rth(T0) is the thermistor resistance at the reference temperature T0, and is the material constant provided by the thermistor manufacturer. Kelvin degrees are used in the exponential term of equation 3. If RS is large and RP is small, the curvature of the equivalent network resistance can be reduced from the curvature of the thermistor alone. Although the exponential equation 3 can never compensate linear equation 2 at all temperatures, a spreadsheet can be constructed to minimize error over the temperature interval of interest. The equivalent resistance RT of the network shown as a function of temperature is
RT (T ) = R s +
Rs
1 1 1 + R p R th (T )
Equation 4
Rp
Rth
using Rth(T) from equation 3. Equation 2 may be rewritten as a new function of temperature using equations 2 and 4 as follows:
Figure 2 RT Network
IFS ( T ) =
If Rth is an NTC thermistor then the value of the network will decrease as temperature increases. Unfortunately, most thermistors exhibit far more variation with temperature than copper wire. One equation used to model thermistors is
VR (R CS1 + R CS2 ) R T (T ) DCR( T )
Equation 5
With Rs and Rp as additional free variables, use a spreadsheet to solve equation 5 for the desired full scale current while minimizing the IFS(T) variation over temperature.
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IR3721
DATA SHEET
TYPICAL 2-PHASE DCR SENSING APPLICATION
The IR3721 is capable of monitoring power in a multiphase converter. A Two Phase DCR Sensing Circuit is shown below. The voltage output of any phase is equal to that of any and every other phase because they are electrically connected and monitored at VO as before. Output current is the sum of the two inductor currents (IL1 + IL2). Superposition is used to derive the transfer function for multiphase sensing. The voltage on RCS2 due to IL1 is
I L1 DCR1 (RCS 2 || RCS 3 ) RCS1 + (RCS 2 || RCS 3 )
The duty ratio of DI is
DI DUTYRATIO = ICS RT VREF
If DCR1=DCR2, and RCS1=RCS3, then ICS can be simplified to
ICS = (I L1 + I L 2 ) DCR1 RCS1 + 2RCS 2
and the DI duty ratio simplifies to
DIDUTYRATIO =
(IL1 + IL 2 ) DCR R T (R CS1 + 2R CS2 ) VR
Likewise, the voltage on RCS2 due to IL2 is
I L2 (RCS 2 || RCS1 ) DCR 2 RCS 3 + (RCS 2 || RCS1 )
Full scale current occurs when DI duty ratio becomes one.
The current through RCS2 due to both inductor currents is ICS. From the two equations above
ICS = I L1DCR1RCS 3 + I L 2 DCR2 RCS1 RCS1RCS 2 + RCS1RCS 3 + RCS 2 RCS 3
Figure 3 Two Phase DCR Sensing Circuit
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IR3721
DATA SHEET
RESISTOR SENSING APPLICATION
The Resistor Sensing Circuit shown below is an example of resistive current sensing. Because the voltage on the shunt resistor is directly proportional to the current IL through the inductor, RCS2 and CCS2 do not need to match the L / DCR time constant. Because the value of the shunt resistance does not change with temperature as the inductor DCR does, RT can be a fixed resistor.
IL
Phase 1
DCR L RCS2
SHUNT
VO
Buck Converter
Power Return
VDD VDD
CCS2 VCS VO VK VDD Bypass Cap RT GND
VRT
IR3721
DI
Power
Figure 4 Resistor Sensing Circuit
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IR3721
DATA SHEET
COMPONENT SELECTION GUIDELINES
Use a 0.1 F, 6.3V, X7R ceramic bypass capacitor from VDD to GND and from VK to GND. Filter the DI output with an RC filter to give a stable analog representation of the current or power. Some of the DI source resistance of this filter is internal to the IR3721 and specified in the electrical specifications table. Add twenty thousand to fifty thousand additional ohms externally to minimize resistance variation. As the DI source resistance increases beyond these guidelines, the voltage measurement error caused by non-ideal voltmeter conductance will increase. Select a filter capacitor that limits 512 kHz sampling frequency ripple to an acceptable value. Sampling frequency ripple will appear as an error, but can be reduced 20 dB for each decade that the filter corner frequency is below 512 kHz. Select a capacitor value that achieves the desired balance between low sampling frequency ripple and adequate bandwidth. Resistor current sensing For resistor current sensing select a precision resistor for RT inside the RT resistance range limits specified in the Electrical Specifications table, such as 25.5k and 1% tolerance. Next, select a shunt resistor that will provide the most current sensing voltage while also considering the allowable power dissipation limitations. The DI output will saturate to the VK voltage when full scale current IFS flows through this shunt. Recommended maximum current sensing voltage range is 5 to 150 mV. Maximum sensing voltages less than 5 mV will cause comparator input offset voltage errors to dominate, and voltages larger than 150 mV will cause comparator leakage current, ICS, errors to dominate. Select RCS2 to be the next higher standard value resistor from (RSHUNTIFSRT) / VRT in order to accommodate full scale current IFS. Bypass VCS to VO with capacitor CCS2. The value of this capacitor limits the bandwidth, but is required because it is the integrator of the delta sigma modulator. Consider selecting the value of CCS2 to place a filter corner frequency at 5 kHz, which will reduce sampling ripple by 40 dB. DCR current sensing Select an RT network resistance between 20k and 45.3k. Consider the RT network of Figure 5 for DCR current sensing.
26.1 k, 1%
2.00 k, 1% 15.0 k, 1% Murata Thermistor NCP15WB473F03RC 47 k, 1%
Figure 5 RT network The resistance of the network above at 25C, RT(25), is 37.58k. Over temperature RT(T) is multiplied by copper resistance, DCR(25)(1+(T-25)0.0039), divided by (DCR(25)( RT(25)) to normalize the results, and plotted as nominal error in Figure 6.
5% 4% 3% 2% Nominal error 1% 0% -1% -2% -3% -4% -5% 0 20 40 60 80 100 Temperature [C]
Figure 6 Nominal error vs. Temperature Note that the error due to temperature compensation at 25C is zero, assuming ideal RT components. At other temperatures the results are over or under reported by the factor in percent indicated. Proceed to calculate RSUM, defined as the sum of RCS1 plus RCS2, as follows. RSUM=IFSDCR(25) RT(25) / VRT Again, IFS is full scale current and VRT is the reference voltage establishing the current in RT. Estimate the capacitance CCS1 with the following equation.
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IR3721
DATA SHEET
4 L DCR ( 25 ) R SUM Choose a standard capacitor value larger than indicated by the right hand side of the inequality above. C CS 1 >
Calculate the equivalent resistance Req. Req = L / (DCR(25) CCS1) We now have two equations, RSUM = RCS1 + RCS2 and Req = (RCS1 RCS2) / (RCS1 + RCS2). Calculate RCS1 and RCS2 using the following two equations.
1 + 1 - 4 R eq R SUM R CS1 = R SUM 2 and
R CS2
1 - 1 - 4 R eq R SUM = R SUM 2
Use the next higher standard 1% value than indicated in the equations above. This will insure that full scale current can be measured. Bypass VCS to VO with capacitor CCS2. The value of this capacitor limits the bandwidth, but is required because it is the integrator of the delta sigma modulator. Consider selecting the value of CCS2 to place a filter corner frequency at 5 kHz, which will reduce sampling ripple by 40 dB.
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IR3721
DATA SHEET
LAYOUT GUIDELINES
Refer to figures 7 through 11 for guidance laying out the IR3721. These guidelines also apply to resistive current sensing. The following guidelines will minimize sources of noise and error, which is required because millivolt level signals correspond to amps of inductor current. 1. Place the capacitor Ccs2 close to the VO and VCS pins of the IR3721. Treat VO and VCS as a differential signal pair back to the IC as shown in the elliptical area designated #1 of figure 8. 2. Sense the inductor (or shunt) Kelvin style at its terminals. Route the leads back as a differential pair. Refer to area #2 of figure 8. 3. Route signal VOUT back to the IC VK pin on its own dedicated trace. Refer to area #3 of figure 8. 4. Place the thermistor near the inductor. Refer to area #4 of figure 8. Route the thermistor leads back to the rest of the network using differential traces. Mount the rest of the thermistor network consisting of Rs, Rp, and R1 close to the IC.
1
5. Use an isolated dedicated ground plane connected only to components associated with the IR3721 that connect to GND as shown in figure 9. Connect this dedicated ground plane at one location only to the ground of the monitored voltage. The thermally relived via in figure eight illustrates this connection. 6. Bypass IC VDD pin 5 to GND pin 4 with a high quality 0.1 F ceramic capacitor. Refer to area #6 of figure 8. 7. Bypass the IC VK pin to GND with a high quality 0.1 F ceramic capacitor. Refer to area #7 of figure 8.
L1
2
VOUT
Rcs1
Ccs1 VOUT Rcs2 Ccs2
1 2 3 4 5 U1 CS VCC_1 VO VK RT DI GND_1 GND_3 VCC GND_4 10 9 8 7 6
R_DI_FILT
DI_FILT
Rs
IR3721 VDD C_VDD Rth 0.1uF
2
C_DI_FILT C_VK 0.1uF
Rp
1
R1 GND
0
Figure 7 Example schematic
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IR3721
DATA SHEET
2
3
4
1 6 7
Figure 8 Layer 1
Figure 10 Layer 3
Figure 9 Layer 2
Figure 11 Layer 4
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IR3721
DATA SHEET
PCB PAD AND COMPONENT PLACEMENT
Figure 12 below shows suggested pad and component placement.
Figure 12 Pad placement
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IR3721
DATA SHEET
SOLDER RESIST
Figure 13 below shows suggested solder resist placement.
Figure 13 Solder resist
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IR3721
DATA SHEET
STENCIL DESIGN
Figure 14 below shows a suggested stencil design.
Figure 14 Stencil design
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IR3721
DATA SHEET
PACKAGE INFORMATION
3 X 3 MM 10L DFN LEAD FREE
Data and specifications subject to change without notice. This product has been designed and qualified for the Consumer market. 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.
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