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ir3721 data sheet page 1 of 16 www.irf.com 09/15/08 power monitor ic with analog output features ? accurate truepower tm 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 inductors 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 package order quantity IR3721Mtrpbf 10 lead dfn (3x3 mm body) 3000 piece reel * IR3721Mpbf 10 lead dfn (3x3 mm body) 121 piece tube * samples only downloaded from: http:///
ir3721 data sheet page 2 of 16 www.irf.com 09/15/08 absolute maximum ratings absolute maximum rating s (referenced to gnd) vdd:.................................................................3.9v all other analog and di gital pins ......................3.9v operating junction temper ature .... -10c to 150c storage temperature rang e .......... -65c to 150c esd rating ............hbm cl ass 2 jedec standard msl rating ..................................................level 2 reflow temper ature ..................................... 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 t he device at these or any other conditions beyond those indicated in the operational sections of the specificati ons are not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. electrical specifications unless otherwise specified, these spec ifications 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. notes: 1. guaranteed by design parameter test condition min typ max unit bias supply vdd turn-on threshold, vdd up 3.10 v vdd turn-off threshold, vdd dn 2.4 v vdd uvlo hysteresis di output low when off 75 mv vdd operating current, icc 350 450 a voltage reference v rt voltage r t = 25.5k ? 1.452 1.493 1.535 v r t resistance range note 1 25.5 k ? converter vo common mode range 0.5 1.8 v duty ratio accuracy v dcr =20 mv, v o =1v, r t =25.5k ? , r cs1 +r cs2 =600 ? t j =65c, note 1 2.5 % duty ratio accuracy v dcr =20 mv, v o =1v, r t =25.5k ? , r cs1 +r cs2 =600 ? , note 1 4 % sampling frequency, f clk 435 512 589 khz comparator offset -0.5 +0.5 mv cs pin input current, i cs di output low -250 +250 na digital output vk pin voltage range 0.5 1.8 v di source resistance 1250 2000 3000 downloaded from: http:///
ir3721 data sheet page 3 of 16 www.irf.com 09/15/08 block diagram di v rt vdd ir3721 vk vcs vo truepower ? result out vcs i ref i ref result out/ vo gnd ic pin description name number i/o level description vcs 1 analog current sensing input, connect through resistor to sensing node vo 2 analog current sensing reference connect to output voltage vrt 3 analog r t thermistor network from this pin to gnd programs thermal monitor gnd 4 bias return and signal reference vdd 5 3.3v ic bias supply gnd 6 connect to pin 4 gnd 7 connect to pin 4 di 8 analog power monitor output; connect to output filter vk 9 1.8v connect to fixed voltage or vo, multiplied by di to become analog output vdd 10 3.3v connect to pin 5 base pad connect to pin 4 downloaded from: http:///
ir3721 data sheet page 4 of 16 www.irf.com 09/15/08 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 cu rrent 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. 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 inductors 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 out put voltage then the output of the di driven rc filter is proportional to power. 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. downloaded from: http:///
ir3721 data sheet page 5 of 16 www.irf.com 09/15/08 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 rectifiers proprietary truepower? circuit. current is converted to a duty ratio t hat appears at the di pin. the duty ratio of the di pin is ? ) + ( ? = r t 2cs 1cs l ratio duty v r r r dcr i di 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. r t r cs2 gnd vo c cs2 di power r cs1 c cs1 l dcr v rt vdd ir3721 vk vcs i l vo vin figure 1 functional description diagram downloaded from: http:///
ir3721 data sheet page 6 of 16 www.irf.com 09/15/08 thermal compensation fo r inductor dcr current sensing the positive temperature coefficient of the inductor dcr can be compensated if r t varies inversely proportional to the dcr. dcr of a copper coil, as a function of temperature, is approximated by ) ?) (+(?) ( =)( cu r r tcr t t t dcr t dcr - 1 equation 2 t r is some reference temperature, usually 25 c, and tcr cu 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 temperat ure and has an offset of dcr(t r ) at the reference temperature. if r t incorporates a negative temperature coefficient thermistor then temperatur e effects of dcr can be minimized. consider a circ uit of two resistors and a thermistor as shown below. rs rth rp figure 2 r t network 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 ? ?? ? ? ?? ? ? ?? ? ? ?? ? ?) ( =)( 0 11 0 tt th th e t r t r - equation 3 where r th (t) is the thermistor resistance at some temperature t, r th (t 0 ) is the thermistor resistance at the reference temperature t 0 , and is the material constant provided by the thermistor manufacturer. kelvin degrees are used in the exponential term of equation 3. if r s is large and r p 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 r t of the network shown as a function of temperature is )( + + =)( t r r r t r th p s t 1 1 1 equation 4 using r th (t) from equation 3. equation 2 may be rewritten as a new function of temperature using equations 2 and 4 as follows: () )( + ? )( =)( t dcr r r t r v t i 2cs 1cs t r fs 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 i fs (t) variation over temperature. downloaded from: http:///
ir3721 data sheet page 7 of 16 www.irf.com 09/15/08 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 (i l1 + i l2 ). superposition is used to derive the transfer function for multiphase sensing. the voltage on r cs2 due to i l1 is ) || (+ ) || ( ? ? 3 2 1 3 2 1 1 cs cs cs cs cs l r r r r r dcr i likewise, the voltage on rcs2 due to il2 is ) || (+ ) || ( ? ? 1 2 3 1 2 2 2 cs cs cs cs cs l r r r r r dcr i the current through r cs2 due to both inductor currents is i cs . from the two equations above 3 2 3 1 2 1 1 2 2 3 1 1 cs cs cs cs cs cs cs l cs l cs r r r r r r r dcr i r dcr i i + + + = the duty ratio of di is ref t cs dutyratio v r i di ? = if dcr1=dcr2, and rcs1=rcs3, then i cs can be simplified to 2 1 1 2 1 2 cs cs l l cs r r dcr i i i + ?) + ( = and the di duty ratio simplifies to ?) + ( ? ?) + ( = r 2cs 1cs t 2l 1l dutyratio v r2 r r dcr i i di full scale current occurs when di duty ratio becomes one. figure 3 two phase dcr sensing circuit downloaded from: http:///
ir3721 data sheet page 8 of 16 www.irf.com 09/15/08 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 i l through the inductor, r cs2 and c cs2 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, r t can be a fixed resistor. v rt gnd vcs vo l dcr i l r cs2 vo ir3721 vdd bypass cap vdd shunt r t di c cs2 vdd phase 1 power return buck converter vk power figure 4 resistor sensing circuit downloaded from: http:///
ir3721 data sheet page 9 of 16 www.irf.com 09/15/08 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 r t inside the r t 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 i fs 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, i cs , errors to dominate. select r cs2 to be the next higher standard value resistor from (r shunt ? i fs ? r t ) / v rt in order to accommodate full scale current i fs . bypass vcs to vo with capacitor c cs2 . the value of this capacitor limits the bandwidth, but is required because it is the integr ator of the delta sigma modulator. consider selecting the value of c cs2 to place a filter corner frequency at 5 khz, which will reduce sampling ripple by 40 db. dcr current sensing select an r t network resistance between 20k ? and 45.3k ? . consider the r t network of figure 5 for dcr current sensing. 26.1 k ? , 1% 15.0 k ? , 1% 2.00 k ? , 1% murata thermistor ncp15wb473f03rc 47 k ? , 1% figure 5 r t network the resistance of the network above at 25c, r t (25), is 37.58k ? . over temperature r t (t) is multiplied by copper resistance, dcr(25) ? (1+(t-25) ? 0.0039), divided by (dcr(25) ? ( r t (25)) to normalize the results, and plotted as nominal error in figure 6. -5% -4% -3% -2% -1% 0% 1% 2% 3% 4% 5% 02 04 06 08 01 0 0 temperature [c] nominal error figure 6 nominal error vs. temperature note that the error due to temperature compensation at 25c is zero, assuming ideal r t components. at other temperatures the results are over or under reported by the factor in percent indicated. proceed to calculate r sum , defined as the sum of r cs1 plus r cs2 , as follows. r sum =i fs ? dcr(25) ? r t (25) / v rt again, i fs is full scale current and v rt is the reference voltage establishing the current in r t . estimate the capacitance c cs1 with the following equation. downloaded from: http:///
ir3721 data sheet page 10 of 16 www.irf.com 09/15/08 sum cs r dcr l c ?) ( ? > 25 4 1 choose a standard capacitor value larger than indicated by the right hand side of the inequality above. calculate the equivalent resistance r eq . r eq = l / (dcr(25) ? c cs1 ) we now have two equations, r sum = r cs1 + r cs2 and req = (r cs1 ? r cs2 ) / (r cs1 + r cs2 ). calculate r cs1 and r cs2 using the following two equations. ? ? ? ? ? ?? ? ? ? ? ? ? ?? ? ? + ? = 2 r r 4 1 1 r r sum eq sum cs1 - and ? ? ? ? ? ?? ? ? ? ? ? ? ?? ? ? ? = 2 r r 4 1 1 r r sum eq sum 2cs - - 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 c cs2 . the value of this capacitor limits the bandwidth, but is required because it is the integr ator of the delta sigma modulator. consider selecting the value of c cs2 to place a filter corner frequency at 5 khz, which will reduce sampling ripple by 40 db. downloaded from: http:///
ir3721 data sheet page 11 of 16 www.irf.com 09/15/08 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 sh unt) 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. 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. di_filt gnd rs c_vk 0.1uf rth 1 2 rp r1 r_di_filt c_di_filt vout u1 ir3721 cs 1 vo 2 rt 3 gnd_1 4 vcc 5 gnd_4 6 gnd_3 7 di 8 vk 9 vcc_1 10 c_vdd 0.1uf rcs1 l1 1 2 ccs1 vout vdd rcs2 ccs2 0 figure 7 example schematic downloaded from: http:///
ir3721 data sheet page 12 of 16 www.irf.com 09/15/08 figure 8 layer 1 figure 9 layer 2 figure 10 layer 3 figure 11 layer 4 1 2 3 4 6 7 downloaded from: http:///
ir3721 data sheet page 13 of 16 www.irf.com 09/15/08 pcb pad and component placement figure 12 below shows suggested pad and component placement. figure 12 pad placement downloaded from: http:///
ir3721 data sheet page 14 of 16 www.irf.com 09/15/08 solder resist figure 13 below shows suggested solder resist placement. figure 13 solder resist downloaded from: http:///
ir3721 data sheet page 15 of 16 www.irf.com 09/15/08 stencil design figure 14 below shows a suggested stencil design. figure 14 stencil design downloaded from: http:///
ir3721 data sheet page 16 of 16 www.irf.com 09/15/08 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 irs web site. ir world headquarters: 233 kansas st., el segundo, californi a 90245, usa te l: (310) 252-7105 tac fax: (310) 252-7903 visit us at www.irf.com for sales contact information . downloaded from: http:///

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