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ltc2920-1/ltc2920-2 1 292012f single/dual power supply margining controller the ltc ? 2920 allows power supplies and power supply module output voltages to be precisely adjusted both up and down for automated pcb testing. the power supply output voltage is changed by sourcing or sinking current into the feedback node or voltage adjust pin of the power supply. this allows a system to test the correct operation of electrical components at the upper and/or lower power supply voltage limits specified for a given design (power supply margining). the ltc2920 uses a single resistor to set the voltage margining current. the margining current is adjustable over a 400:1 range. precision margin currents can be supplied to within 0.6v of ground or v cc . the ltc2920-1 is a single margining controller. the ltc2920-2 has two independently controllable margining channels. each channel has its own control pin and current setting resistor. the ltc2920-2 can be used to symmetri- cally margin two power supplies, or asymmetrically mar- gin a single power supply. both the ltc2920-1 and ltc2920-2 feature a trimmed on- board voltage reference. typical power supply margining accuracy is better than 0.4%. n automated pcb production testing n automated preventative maintenance testing n dc/dc converter module margining , ltc and lt are registered trademarks of linear technology corporation. n margin voltage precision <0.4% n 400:1 current programming range n symmetric/asymmetric high and low voltage margining n single control pin per supplyhigh, float, low n single current setting resistor per supply n wide v cc compliance 2.2v < v cc < 6v n wide output compliance 0.6v < v margin < (v cc C 0.6v) n single in 5-pin thinsot tm (ltc2920-1) n dual in 8-pin msop (ltc2920-2) thinsot is a trademark of linear technology corporation. power one i5s013ze-a +v in +v out ? out ? in 1 2 5 6 trim 7 system controller three-state ltc2920-1 i m1 gnd v cc r s1 in1 r set1 10k 1% 2920-1/2 ta01 3.3v at 4a 2 f 0.1 f 150 33 f 48v + + 3.3v quarter brick with 5% voltage margining 5% +v out in1 1ms/div 2920-1/2 ta01a ?% logic hi logic float logic low nom descriptio u typical applicatio u applicatio s u features
ltc2920-1/ltc2920-2 2 292012f order part number s5 part marking t jmax = 125 c, q ja = 250 c/w ltd7 ltd8 ltc2920-1cs5 ltc2920-1is5 absolute axi u rati gs w ww u package/order i for atio uu w (note 1) electrical characteristics the l denotes the specifications which apply over the full operating temperature range, c rs1 = c rs2 = 20pf, otherwise specifications are at t a = 25 c. supply voltage (v cc ) ................................C 0.3v to 6.5v input voltages (in1, in2, r s1 , r s2 )................. C 0.3v to (v cc + 0.3v) output voltages (i m1 , i m2 ) ........... C 0.3v to (v cc + 0.3v) top view s5 package 5-lead plastic sot-23 1 2 3 v cc gnd i m1 5 4 in1 r s1 order part number ms8 part marking ltb6 lta4 ltc2920-2cms8 ltc2920-2ims8 t jmax = 125 c, q ja = 200 c/w 1 2 3 4 r s2 in2 in1 r s1 8 7 6 5 v cc i m2 gnd i m1 top view ms8 package 8-lead plastic msop operating temperature range ltc2920-1c/ltc2920-2c ....................... 0 c to 70 c ltc2920-1i/ltc2920-2i .................... C 40 c to 85 c storage temperature range ................. C 65 c to 150 c lead temperature (soldering, 10 sec).................. 300 c consult ltc marketing for parts specified with wider operating temperature ranges. symbol parameter conditions min typ max units supplies v cc supply operating range (note 2) l 2.3 6 v i cc(source) supply current while sourcing max i im r set1 = r set2 = 15k, l 6ma in1 = in2 < v il i cc(q) quiescent supply current r set1 = r set2 = 200k, l 0.23 1 ma in1 = in2 v il current margining outputs i m1 , i m2 i imlow low range i margin current r set1 , r set2 tied to gnd, l 5 167 m a sourcing or sinking in1, in2 > v ih or in1, in2 < v il , (note 4) i imhigh high range i margin current r set1 , r set2 tied to v cc , l 0.15 2 ma sourcing or sinking in1, in2 > v ih or in1, in2 < v il , (note 4) v m i m1 , i m2 output voltage compliance (note 3) l 0.55 v cc C 0.55 v
ltc2920-1/ltc2920-2 3 292012f electrical characteristics the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. symbol parameter conditions min typ max units i imaccuracy low range current accuracy 100 m a ? i m ? 167 m a, (note 6) c-grade l 3 7.5 % i-grade l 313 % 30 m a ? i m ? < 100 m a, (note 6) c-grade l 511 % i-grade l 515 % 5 m a ? i m ? < 30 m a, (note 6) c-grade l 520 % i-grade l 525 % high range current accuracy 1.5ma ? i m ? 2ma, (note 7) c-grade l 3 7.5 % i-grade l 311 % 600 m a ? i m ? 1.5ma, (note 7) c-grade l 511 % i-grade l 515 % 150 m a ? i m ? 600 m a, (note 7) c-grade l 515 % i-grade l 520 % i oz i m1 , i m2 leakage current l 100 na c im equivalent capacitance at i m1 , i m2 v in = v off , (note 5) 10 pf v in = v il , high range, (note 5) 2 nf v in = v il , low range, (note 5) 30 pf control inputs in1, in2 v ih control voltage for i m current sinking v cc < 2.5v l 2.1 v v cc 3 2.5v l 2.4 v v il control voltage for i m current sourcing l 0.6 v v off control voltage for i m current off l 1.1 1.4 v v oz control voltage when left floating 1.2 v r in in1, in2 input resistance l 51220 k w i flt maximum allowed leakage at in1, in2 l C10 10 m a for i m current off switching characteristics v in(delayon) i m1 , i m2 turn-on time v in transitions from v off to l 15 100 m s v ih or v il v in(delayoff) i m1 , i m2 turn-off time v in transitions from l 15 100 m s v ih or v il to v off i m(on) i m1 rise time ? i m ? 5% to 95%, (note 5) 5 m s i m(off) i m1 fall time ? i m ? 95% to 5%, (note 5) 0.3 m s note 1: absolute maximum ratings are those values beyond which the life of a device may be impaired. note 2: v cc must always be above the maximum of i m1 and i m2 less 0.2v. see preventing potential power supply overvoltages in the applications information section. note 3: v m compliance is the voltage range within which i m1 and i m2 are guaranteed to be sourcing or sinking current. i m accuracy will vary within this range. note 4: consult ltc marketing for parts specified with wider i m current limits. note 5: determined by design, not production tested. note 6: ? 1 C (i m C r s ) ? ? 100%; v cc 4v: 0.58 v m (v cc C 1.1); v cc > 4v: 0.58 v m (v cc C 1.4); c rs 20pf note 7: ? 1 C (i m ? r s / 30) ? ? 100%; 0.79 v m (v cc C 0.6); c rs 20pf
ltc2920-1/ltc2920-2 4 292012f pi fu ctio s uuu v cc (pin 1/pin 8): power supply input. all internal circuits are powered from this pin. v cc should be connected to a low noise power supply voltage between 2.2v and 6v and should be bypassed with at least a 0.1 m f capacitor to the gnd pin in close proximity to the ltc2920. current sourced out of the i m pins comes from the v cc pin. note that v cc must come up no later than the time the controlled power supply turns on or damage to the load may result. see preventing potential power supply over- voltages in the applications information section for power sequencing considerations. in certain applications, it may be necessary to further isolate v cc by adding a resistor in series with its power source. see v cc power filtering in the applications information section. gnd (pin 2/pin 6): ground. all internal circuits are re- turned to the gnd pin. connect this ground pin to the ground of the power supply(s) being margined. current sunk into the i m pins of the ltc2920 is returned to ground through this pin. r s1 (pin 4/pin 4): i m1 current set input. the r s1 pin is used to set the margining current which is sourced out of or sunk into the i m1 pin. the r s1 pin must be connected to either v cc or ground with an external resistor r set with a value between 6k and 200k. connecting r set to ground sets the current at the i m1 pin with a multiplier of 1. connecting r set to v cc sets the current at the i m1 pin with a multiplier of 30. if r set is connected to ground, ? 1v will appear at the r s1 pin. if r set is connected to v cc , ? (v cc C 1v) will appear at the r s1 pin. in either case, the current through r set will be ? 1v/r set . (s5 package/ms8 package) i m1 (pin 3/pin 5): i m1 current output. this pin should be connected to the power supply feedback pin or voltage adjust pin. (see the applications information section for further details.) current is either sourced out of or sunk into this pin. the direction of the current is controlled by the in1 pin. the amount of current flowing into or out of the i m1 pin is controlled by the r s1 pin. in1 (pin 5/pin 3): i m1 control pin. this pin is a 3-level input pin which controls the i m1 pin. if the in1 pin is pulled above v ih , current is sunk into the i m1 pin. if the in1 pin is pulled below v il , current is sourced from the i m1 pin. if the in1 pin is left floating, or held between 1.1v and 1.4v, the i m1 pin is a high impedance output. internally, the in1 pin is connected to a 1.2v voltage source by an internal ~10k resistor. the ltc2920 has an internal rc circuit to sup- press noise entering from this pin. ltc2920-2 only r s2 (na/pin 1): i m2 current set input. sets the current for i m2 . see r s1 . i m2 (na/pin 7): i m2 current output. this pin is the second margin current output for the ltc2920. see i m1 . in2 (na/pin 2): i m2 control pin. this pin controls the current at the i m2 pin. see in1.
ltc2920-1/ltc2920-2 5 292012f typical perfor a ce characteristics uw i margin (ma) 0 0.5 1 1.5 2 2.5 i cc (ma) 2920-1/2 g01 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 1 channel 2 channels i margin ( a) 020 40 60 80 100 120 140 160 180 i cc ( a) 2920-1/2 g02 1800 1600 1400 1200 1000 800 600 400 200 0 1 channel 2 channels v margin (v) 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 error (%) 2920-1/2 g03 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0.15 (ma) 0.3 0.5 1 2 v cc = 5v high range v margin (v) 0 0.5 1 1.5 2 2.5 error (%) 2920-1/2 g04 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0.15 (ma) 0.3 0.5 1 2 v cc = 2.5v high range 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 v margin (v) error (%) 2920-1/2 g05 6 5 4 3 2 1 0 5 ( a) 20 50 166.7 100 v cc = 5v low range v margin (v) 0 0.5 1 1.5 2 2.5 error (%) 2920-1/2 g06 6 5 4 3 2 1 0 5 ( a) 20 50 166.7 100 v cc = 2.5v low range i cc vs i margin high range sourcing current i cc vs i margin low range sourcing current i margin error vs v margin i margin error vs v margin i margin error vs v margin i margin error vs v margin 2920-1/2 g07 100% 100% source r set = 20k 0% sink 1 s/div high range v in(delayon) ends high range low range 2920-1/2 g08 100% 100% r set = 20k 0% 100ns/div high range low range high range low range source sink v in(delayoff) ends i margin rise time i margin fall time
ltc2920-1/ltc2920-2 6 292012f overview power supply voltage margining in high reliability pcb manufacturing and test, it is desir- able to test system functionality and performance at the upper and/or lower power supply voltage limits allowed for a given design (known as power supply margining). doing so can greatly improve the lifetime reliability of a system. the ltc2920 provides a means of power supply voltage margin testing which is: ? flexible ? easy to design ? requires very little pcb board space symmetric/asymmetric power supply margining any one ltc2920 channel requires only a single external resistor to symmetrically margin both above and below the nominal power supply voltage. the ltc2920-2 can be used to symmetrically margin two different power sup- plies. in cases where the design calls for margining one applicatio s i for atio wu u u voltage above the nominal power supply voltage and a different voltage below the nominal, the ltc2920-2 can be used. one channel is used for margining above the nomi- nal power supply voltage, and the other channel is used to margin below the nominal voltage. voltage margining power supplies using a feedback pin one common power supply architecture supported by the ltc2920 is a power supply with a feedback pin and two feedback resistors. even complicated switching power supplies can be typically modeled as a simple amplifier with a reference voltage and a two resistor feedback network (figure 1.) 2920-1/2 f01 r g v ref v psout + r f i fb + figure 1 in1 irng range detection ltc2920-2 only in2 2920-1/2 bd i m2 i m1 r s2 input detection current setting output control i program sink off source v mok v m compliance voltage reference thermal shutdown uvlo high range low range r set1 r s1 connect to v cc for high range or to gnd for low range v cc fu ctio al block diagra uu w
ltc2920-1/ltc2920-2 7 292012f knowing the value of the resistors r f and r g , and the voltage of v ref , v psout can be calculated by: v psout = v ref ? [1+ (r f /r g )] since the op amp keeps its inverting terminal equal to the noninverting terminal, the voltage at the inverting terminal between r f and r g is v ref . knowing the current flowing in the feedback resistor network, v psout can be also calculated by: v psout = v ref + (i fb ? r f ) this is the voltage on one side of r f , plus the voltage across r f . this equation is helpful in understanding how the ltc2920 changes the power supply output voltage. figure 2 shows the simplified model with the ltc2920 added. power supply module voltage margining another method of accomplishing voltage margining is useful for power supply brick modules with voltage adjust pins. typically, the power supply manufacturer will design the power supply to be adjusted up or down, using external resistors connected to the trim pin. the values of these resistors are usually calculated by the design engi- neer using two different equations supplied by the manu- facturer. there is usually one equation for trimming the voltage up, and another equation for trimming the voltage down. in most cases, the power supply module is treated like a black box and very little information is given on how the trimming is accomplished from an internal circuit standpoint. traditionally such power supply modules are margined by calculating the two resistors, and alternately connecting each to v cc or ground with analog switches or relays. figure 3 shows how the ltc2920 can be used in these applications as well. using the ltc2920 for these applica- tions can save a significant amount of pcb real estate and cost. applicatio s i for atio wu u u r set 2920-1/2 f02 r g v ref v psout + r f i margin i fb i rg i m r s ltc2920 + figure 2. simplified power supply model ltc2920 i m r s power module sense v in v o trim v in + v o + v o v psout r set r system 2920-1/2 f03 i margin sense + figure 3. margining a power supply module power supply module design considerations there are usually practical limits to v o + . for instance, v o + usually has upper and lower voltage limits specified by the power module manufacturer. a common value is 10% above and 20% below the rated output voltage of the power supply module. this limit includes v margin plus any voltage drop across r system . see the manufacturers power supply module specifications for details. see the selecting the r set resistor section of this datasheet for instructions on how to choose r set in module applica- tions. again in this circuit, the op amp will keep the voltage at its inverting input at v ref . if we add or subtract current at this node, the delta current will always be added or subtracted from i fb , and never i rg . ( i margin is used rather than a signed i margin value to emphasize the fact that current is added or subtracted at the feedback pin.) because of this, the voltage across r f will be: v rf = (i fbnom i margin ) ? r f or v rf = (i fbnom ? r f ) (i margin ? r f ) and finally v psout = v ref + (i fbnom ? r f ) (i margin ? r f ) note that the delta voltage v margin depends only on i margin and r f , not r g or v ref .
ltc2920-1/ltc2920-2 8 292012f since d v psout will appear on r f as noted in the overview section, margin current i margin can be calculated by: i margin = d v psout /r f example: if d v psout = 0.165v and r f = 10k: i margin = 0.165/10k = 16.5 m a if i margin is between 5 m a and 167 m a, use the ltc2920s low current range. r set is then calculated by: r set = 1v/i margin = 1v/16.5 m a = 60.6k in this case, r set would be connected between the r s pin and ground. if i margin is between 150 m a and 2ma, use the ltc2920s high current range. r set is then calculated by: r set = 1v/(i margin /30) or simply: r set = 30v/i margin applicatio s i for atio wu u u example: if the value of the feedback resistor r f is 500 w in the example above then: d v psout = 0.05 ? 3.3v = 0.165v i margin = 0.165v/500 w = 330 m a r set = 30v/i margin = 30v/330 m a = 90.1k in this case, r set would be connected between the r s pin and v cc . if i margin is less than 5 m a, or greater than 2ma, it will be necessary to adjust both power supply feedback resistors r f and r g . again, this is usually a simple process. it is easy to calculate the magnitude of the change by dividing the i margin current calculated above by the desired new selecting the r set resistor selecting r set with an existing power supply containing a feedback pin and two feedback resistors calculating the value of the current setting resistor, r set , for a power supply with a feedback pin is straight forward. when the ltc2920 is being added to an existing power supply design, the power supply feedback resistors r f and r g have already been selected. by knowing r f , the power supply output voltage, v psout , and the amount to margin, %change, r set can be calculated. first, the margining voltage d v psout can be calculated by knowing the percentage of the power supply voltage v psout change desired. d v psout = %change ? v psout example: if a 3.3v power supply is to be margined by 5%, then: d v psout = 0.05 ? 3.3v = 0.165v figure 6. 3.3v supply with 5% margining (high range) r set = 90k v cc 2920-1/2 f06 r g = 286 v ref = 1.2v v psout = 3.3v + r f = 500 i margin = 330 a i fb = 4.2ma i m r s ltc2920 + r set 2920-1/2 f04 r g v ref v psout + r f i margin i fb i m r s ltc2920 + figure 4. simplified power supply model r set = 60.6k 2920-1/2 f05 r g = 5.76k v ref = 1.2v v psout = 3.3v + r f = 10k i margin = 16.5 a i fb = 210 a i m r s ltc2920 + figure 5. 3.3v supply with 5% margining (low range)
ltc2920-1/ltc2920-2 9 292012f i margin current. select a new i margin current that is within one of the two ltc2920s i margin ranges, then calculate the scaling factor: i factor = i margin(old) /i margin(new) the new feedback resistors would then be: r f(new) = r f(old) ? i factor r g(new) = r g(old) ? i factor and r set can then be calculated as descibed above. warning in some cases, adjusting the feedback resistors on a switching supply might require recompensating the power supply. please refer to the applications information sup- plied with the power supply for further information. applicatio s i for atio wu u u between the trim pin and the power supply positive voltage output or the trim pin and the negative power supply output (ground). the polarity of the voltage trim and trim resistor configuration are chosen by the manufacturer. the equations describing the resistor values versus the desired output voltage changes are typically not linear. fortunately, the relationship between trim pin current and output voltage change is typically linear. the current trim equation is usually the same (in magnitude) for changing the output voltage up or down. once the equation for trim current is determined, it is much easier to use than trim resistors. to illustrate this, figure 8 shows a typical resistor trim down curve for a power module. figure 9 shows a typical current trim down curve for the same power module. ltc2920 i m r s power module sense v in v o trim v in + v o + v o v psout r set 2920-1/2 f07 i margin sense + figure 7. using a power module trim pin for voltage margining selecting the r set resistor using voltage trim pins with brick type power supply modules brick power supply modules often have a trim pin which can be used for voltage margining. figure 7 shows a typical connection using the ltc2920 for voltage margin- ing a power supply module. the amount of current necessary to adjust the output voltage of the power supply module is not normally given directly by the manufacturer. however, by using informa- tion that is supplied by the manufacturer, a measurement can be made to determine a simple equation that is useful for power supply module voltage margining. typically, the manufacturer will supply two different equa- tions for selecting trim resistors: one for trimming the output voltage up and a different one for trimming the output voltage down. trim resistors are nominally placed trim voltage (v) 0 0.1 0.2 0.3 0.4 0.5 trim down resistance ( ) 2920-1/2 f08 1m 100k 10k 1k 100 10 1 figure 8. typical trim voltage vs trim resistor curve trim voltage (v) 0 0.1 0.2 0.3 0.4 0.5 trim current ( a) 2920-1/2 f09 300 250 200 150 100 50 0 figure 9. typical trim voltage vs trim current curve
ltc2920-1/ltc2920-2 10 292012f applicatio s i for atio wu u u for any desired v margin : i trim = v margin /k trim r set can now be calculated for the ltc2920. for 5 m a i trim 167 m a: r set = 1v/i trim connect r set between the r s pin and the ltc2920 ground pin. for 167 m a < i trim 2ma: r set = 1v/(i trim /30) connect r set between the r s pin and the ltc2920 v cc pin. if i trim falls outside of this range, the ltc2920 cannot be used for this application. the ltc2920 can source or sink current only when the voltage at the i m pin is between 0.6 and (v cc C 0.6) volts. in order to be sure that the ltc2920 will operate correctly in this application, ensure that the v t node will stay within these limits. to do this, calculate the effective output resistance of the power supply modules trim output pin, r vt (refer to figure 10). using the measurements taken above, the open circuit voltage is: v ref = v tnom to calculate r vt , subtract the untrimmed v tnom and trimmed v ttrim voltages measured above: v tdelta = v tnom C v ttrim the effective trim pin source resistance can then be calculated by: r vt = v tdelta /i trim the voltage at the ltc2920 i margin pin for any i trim can now calculated for both voltage margin directions. refering to figure 10: v tsink = v ref C (r vt ? i trim ) v tsource = v ref + (r vt ? i trim ) note: be sure to use these equations to verify that v tsink and v tsource are within ltc2920 v m voltages specified in even though the manufacturer does not directly supply the equation for the trim current, a simple measurement can be made to calculate an equation for v trim as a function of i trim . to do this, select the trim resistor configuration which places the trim resistor between the trim pin and ground (see figure 10). with the trim resistor connected to ground, note the direction of the power module output voltage change. this is the direction that the power module output voltage will change when the ltc2920 in control pin is high, above v ih . remember that the direction of the voltage trim for this configuration can vary among power modules, even among power modules from the same manufacturer. calculate a resistor value from the manufacturers equa- tion, or select it from a chart (if a chart is supplied by the manufacturer). pick a value near the middle of the trim resistor range. obtain and measure the selected resistor with an ohmmeter or use a precision 0.1% resistor. knowing the correct value of this resistance is critical to obtaining good results. make provisions to connect and disconnect this test resistor between the trim pin and the power supply modules negative output pin. (figure 10.) carefully follow all other manufacturers application notes regarding power supply input voltage, minimum and maximum output voltages, sense pin connections (if any), minimum and maximum current loads, etc. failure to do so may permanently damage the power supply module! apply the specified input voltage to the power supply module. measure the power supply output voltage v ps and the v t voltages before and after connecting the trim resistor. subtract the untrimmed (v psnom ) and trimmed (v pstrim ) power supply output voltages to obtain the trim voltage (v delta ): v delta = v psnom C v pstrim and the trim current: i trim = v trim /r trim calculate the linear current trim constant k trim : k trim = v delta /i trim
ltc2920-1/ltc2920-2 11 292012f applicatio s i for atio wu u u the i maccuracy specification. if v t does not fall within this range, the ltc2920 cannot be used for this application. figure 10. power module i trim model sense v in v o trim v in + v o + r vt v ps v o i trim r trim 2920-1/2 f10 v t sense + v ref + r set = 20k 2920-1/2 f12 r g = 944k v ref = 1.2v v psout = 3.3v + r f = 1.65k i margin = 50 a i fb = 1.27ma i m r s ltc2920 + figure 12. power supply voltage margin model power supply voltage margining (%) 0 1 2 3 4 56 power supply margined voltage error ? 1 ?actual voltage/expected voltage ? ?100 (%) 2920-1/2 f11 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1% feedback resistor inaccuracy 1% r set resistor inaccuracy 5% ltc2920 i margin inaccuracy figure 11. sources of power supply margined voltage errors for this example). the second error is the power supply initial set point accuracy. in this example the r f resistor has a 1% accuracy error causing a 0.6% initial set point error in the power supply. because the margined power supply voltage is the change in the voltage, v margin , from the power supply initial set point voltage, this error shows up in the margined power supply voltage. when these two errors are combined, the error is: error = ? 1 C (3.4043/3.3825) ? ? 100 = 0.65% the error caused by a 1% inaccuracy in r g will be similar since the dominate error source is the power supply initial set point voltage. errors caused by r f and r g can be a major contributor to voltage margin errors. using 0.1% resistors for both r f and r g is often the best choice for improving both voltage margin accuracy and power supply initial accuracy. accuracy of power supply voltages when margining the accuracy of margined power supply voltages depends on several factors. figure 11 shows the magnitude of the errors discussed in detail below as a function of power supply margining percentage. in a typical feedback model (figure 12), the delta voltage is a function of the margin current, i margin , and the feedback resistor, r f . v margin = i margin ? r f errors in v margin are directly proportional to errors in i margin and errors in r f . a 5% error in i margin will cause a 5% error in v margin . in this example, a 3.3v power supply is margined by 2.5%, or 0.0825v to 3.3825v. with a 5% v margin error, the actual margin voltage is 0.0866v and the actual power supply voltage is 3.3866v. the error in the expected voltage is then: error = ? 1 C (3.3866/3.3825) ? ? 100 = 0.12% similarly, a 1% inaccuracy in the r set resistor would cause only 0.024% error in the expected power supply margined voltage. in effect, i margin errors caused by the r set resistor or the ltc2920 are attenuated by the voltage margining percentage. the accuracy of the r f resistor introduces two errors in the margined supply voltage. the first is the error in v margin (i margin ? r f ). this error is similar in magnitude to the errors described above and is generally quite small (0.024%
ltc2920-1/ltc2920-2 12 292012f applicatio s i for atio wu u u preventing potential power supply overvoltages care must be taken when selecting the power source for the ltc2920. if v cc on the ltc2920 is not powered, and the power supply being margined is on, undesired i m fault current can flow into the i m pin of the ltc2920. this can cause the margined power supply to create an overvoltage condition causing serious damage to power supply and its load. the best solution is to connect the ltc2920 to a power source that is guaranteed to be on when the power supply being margined is on. often this is the input or output voltage of the power supply being margined. see the design guidelines below for the best solution for your application. be sure to follow all other ltc2920 design specifications. at a minimum, the voltage at the v cc pin of the ltc2920 must be maintained above 0.2v below the highest voltage present at the i m1 and i m2 pins. this will keep the i m fault current below 5 m a. the voltage at the i m1 and i m2 pins is normally the voltage at the feedback node of the power supply. see the power supply manufacturers data sheet for this voltage. preventing i m fault current in the ltc2920-1 connecting v cc to the power supply v in or v out of the supply being margined connecting the ltc2920-1 v cc to v in or v out is the best choice and should be used when conditions permit. it requires no external components and provides the best protection from power supply overvoltage. if the power supply being margined has a v in voltage that is within the ltc2920s v cc range, connect the ltc2920-1 v cc pin to the power supplies v in (figure 13). if the power supply being margined has a v out voltage that is within the ltc2920s v cc range, connect the ltc2920-1 v cc pin to the power supplies v out (figure 14). make sure the power supply voltage is within the ltc2920s v cc specification when the power supply is being margined! figure 13. connecting ltc2920-1 to v in figure 14. connecting ltc2920-1 to v out 2920-1/2 f13 v cc fb v o v in 2.3v to 6v v out v cc 0.1 f gnd i m ltc2920-1 2920-1/2 f14 v in fb v o v in v out v out 2.3v to 6v v cc gnd i m ltc2920-1 0.1 f figure 15. diode connected v cc connecting v cc to power sources other than the supply being margined if it is not practical to power the ltc2920-1 from the v in or v out of the power supply being margined, connect the v cc pin of the ltc2920-1 using a schottky diode (fig- ure 15). this solution works with power supply feedback voltages of less than 1.5v and i margin currents >30 m a. be sure to account for the diode drop across all temperatures to ensure the ltc2920-1 v cc and v margin specifications are met. 2920-1/2 f15 v in fb v o v in v out <1.5v v power bat54c schottky diode 0.1 f v cc gnd i m ltc2920-1
ltc2920-1/ltc2920-2 13 292012f applicatio s i for atio wu u u preventing i m fault current in the ltc2920-2 connecting v cc to a common v in connecting the ltc2920-2 v cc to v in is the best choice and should be used when conditions permit. it requires no external components and provides the best protection from power supply overvoltage (figure 16). power supply 2, power supply 2 supplies enough voltage to keep the ltc2920 from sinking fault current into the i m1 and i m2 pins. the ltc2920-2 will not operate normally under these conditions but it will not cause overvoltage to occur. connecting v cc to power sources other than the supplies being margined if it is not practical to power the ltc2920-2 from the v in s and/or v out s of the power supplies being margined, connect the v cc pin of the ltc2920-2 using a schottky diode (figure 18). this solution works with power supply feedback voltages less that 1.5v and i margin currents >30 m a. be sure to account for the diode drop across all temperatures to ensure the ltc2920-2 v cc and v margin specifications are met. v cc power supply filtering if the ltc2920 is both powered by and margins a power supply that is marginally stable, oscillations can occur. in these cases, it may be necessary to provide an additional filtering resistor between the ltc2920 and the power supply being margined (see figure 19). the oscillation is most likely to occur when the ltc2920 is sourcing current from the i marging pin. the r byp resistor in combination with the c byp capacitor form a lowpass filter. the value of the filter resistor r byp can be calculated by deciding how much voltage drop across the resistor the application can tolerate and how much current the ltc2920 will sink under worst-case conditions. in the ltc2920 low current range, a safe value for the ltc2920 i cc current is the maximum ltc2920 quiescent current plus 4 times the i margin current. in the high current range, a safe value for the ltc2920 i cc current is the maximum ltc2920 quies- cent current plus 1.2 times the i margin current. example: if the i margin current is 100 m a, then: i ccmax = i q + (4 ? i margin ) = 1ma + (4 ? 100 m a ) = 1.4ma in this example, the power supply voltage is 3.3v. drop- ping 0.5v across r byp will provide a v cc at the ltc2920 of 2.8v. this is well above the ltc2920s minimum v cc i m1 ltc2920-2 bat54c 2920-1/2 f17 v out2 1.8v v out1 3.3v gnd i m2 v cc power supply 2 fb power supply 1 fb out out figure 17. dual diode connected v cc figure 16. connecting v cc to v in v in 2920-1/2 f16 out fb v in out fb v in v cc i m1 i m2 ltc2920-2 gnd connecting v cc to diode ord supplies if the margined power supplies derive their v in from different sources, or if a common v in cannot supply power to the ltc2920-2, power the ltc2920-2 using a diode ord connection (figure 17). note that in this example, power supply 2 has only a 1.8v output. power supply 1 will supply the ltc2920-2 under normal operation condi- tions. if power supply 1 fails, or if it is sequenced up after
ltc2920-1/ltc2920-2 14 292012f applicatio s i for atio wu u u figure 19. v cc power filtering figure 20. slowing down v margin r set 2920-1/2 f19 r g c byp 0.1 f v psout = 3.3v i margin = 100 a i m r s v cc gnd ltc2920 + r f v ref = 1.2v + r byp 360 2920-1/2 f20 1.5k c s 0.2 f i margin i m r s 5k v cc 3.3v gnd ltc2920 + 5k v ref 1.21v + voltage. the value of the r byp resistor can then be calcu- lated by: r byp = v rb /i ccmax = 0.5v/1.4ma = 360 w with c byp = 0.1 m f, this will provide a pole at 2870hz. if additional filtering is necessary, the value of c byp can be increased. in this example, if c byp is increased from 0.1 m f to 1 m f, the pole would now be at 287hz. to figure 20, slowing down v margin , a capacitor (c s ) and a resistor (r s ) have been added to the power supply model described in previous applications sections. to choose r s , the voltage at the feedback pin of the power supply must be known. refer to the power supply manufacturers data sheet for this voltage. the voltage at the i m pin must be within specified limits of the ltc2920, including the voltage drop across r s . in the example below, the power supply feedback pin voltage is 1.21v, i margin is 100 m a and v cc is 3.3v. to maintain ltc2920 current accuracy, the voltage at the i m pin must be between 0.58v and (v cc C 1) or 2.3v (in the low current range). a reasonable value for the voltage drop across r s is 0.5v. the value of r s is then: r s = v rs /i margin = 0.5v/100 m a = 5k assuming the desired rc time constant is 1ms, c s is calculated by: c s = t rc /r s = 1ms/5k = 0.2 m f note: when c s and r s are used, an additional pole and a zero are added to the power supply feedback loop. it is beyond the scope of this data sheet to predict the behavior of all power supplies but, in general, as long as the smaller of the two feedback resistors is no larger than 2 ? r s , the effect on the power supply stability should be minimal. the larger r s is with respect to the two feedback resistors, the less effect it will have. i m1 ltc2920-2 bat54c schottky diode v power 2920-1/2 f18 i m2 v cc power supply 2 fb power supply 1 fb out out figure 18. diode connected to v cc controlling i margin turn on and turn off times designers of power supply voltage margining circuits often need to ensure that power supply voltages do not overshoot or undershoot (the desired margining voltage) when the margining current is enabled or disabled. the ltc2920 i margin current sourced or sinked at the i m pin(s) is reasonably well behaved (see the typical perfor- mance characteristics curves). the differences in speed between the various curves is caused by the relative impedance differences within the ltc2920. if slower turn on and turn off times are desired, a resistor- capacitor network can be used at the i m pin(s). referring thermal shutdown this ic includes overtemperature protection that is in- tended to protect the device during momentary overload conditions. junction temperature will exceed 125 c when overtemperature protection is active. continuous opera- tion above the specified maximum operating junction temperature may result in device degradation or failure.
ltc2920-1/ltc2920-2 15 292012f package descriptio u 1.50 ?1.75 (note 4) 2.80 bsc 0.30 ?0.45 typ 5 plcs (note 3) datum ? 0.09 ?0.20 (note 3)
ltc2920-1/ltc2920-2 16 292012f linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 l fax: (408) 434-0507 l www.linear.com ? linear technology corporation 2003 lt/tp 0104 1k ? printed in usa related parts part number description comments ltc1329-10/ltc1329-50 micropower 8-bit i out dac in so-8 10 m a/50 m a sourcing, pulse mode or spi input ltc1426 micropower dual 6-bit pwm dac pulse mode or pushbutton input ltc1427-50 smbus micopower 10-bit i out dac in so-8 50 m a sourcing, C15v to (v cc C 1.3v) compliance ltc1428-50 micropower 8-bit i out dac in so-8 50 m a sinking, pulse mode or spi input ltc1663 micropower 10-bit v out dac 2-wire interface, rail-to-rail output, sot-23 or msop ltc2900-1/ltc2900-2 quad voltage monitors in msop 16 user-selectable combinations, 1.5% threshold accuracy ltc2901-1/ltc2901-2 quad voltage monitors with watchdog 16 user-selectable combinations, adjustable rst and watchdog timers ltc2902-1/ltc2902-2 quad voltage monitors with rst disable 16 selectable combinations, rst disable for margining, tolerance selec t ltc2921/ltc2922 power supply tracker with remote sensing three (ltc2921) or five (ltc2922) remote sense switches ltc2923 power supply tracking controller controls two supplies without series fets or a third supply with a series fet i th tg c osc 68pf c c 150pf 51pf 100pf r c 10k c ss 0.1 f sense sense + ltc1435a 1000pf + 4.7 f d b cmdsh-3 c b 0.1 f m2 si4412dy d1 mbrs140t3 l1 4.7 h m1 si4412dy r sense 0.025 + c in 22 f 35v 2 v in 4.5v to 28v + c out 100 f 6.3v 2 r1 3.57k r2 2k v out 3.3v 4.5a pgnd bg c osc v in ltc2920 gnd i m r s in system controller three-state gnd v osense boost 2920-1/2 ta03 sgnd run/ss sw intv cc 21.5k v cc v cc c byp 0.1 f r b 500 3.3v supply with 0.165v (5%) voltage margining 12v supply with 5% margining ltc2920-1 i m1 v cc i in1 r s1 1 5 3 gnd 2 4 system controller three-state gnd r set 188.3k gnd v in sw shdn fb v in 5v 4 51 3 d1 l1 10 h 2 r1 113k lt1930 2920-1/2 ta02 c2 4.7 f c3* 10pf r s 113k c b 0.1 f r2 13.3k c1 2.2 f v out 12v 300ma margin 5% c1: taiyo yuden x5r lmk212bj225mg c2: taiyo yuden x5r emk316bj475ml d1: on semiconductor mbr0520 l1: sumida cr43-100 *optional c s 0.01 f shdn r b 1k typical applicatio s u

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