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ltc3108-1 1 31081fa typical application description ultralow voltage step-up converter and power manager the ltc ? 3108-1 is a highly integrated dc/dc converter ideal for harvesting and managing surplus energy from extremely low input voltage sources such as tegs (thermoelectric generators), thermopiles and small solar cells. the step-up topology operates from input voltages as low as 20mv. using a small step-up transformer, the ltc3108-1 provides a complete power management solution for wireless sens- ing and data acquisition. the 2.2v ldo powers an external microprocessor, while the main output is programmed to one of four ? xed voltages to power a wireless transmitter or sensors. the power good indicator signals that the main output voltage is within regulation. a second output can be enabled by the host. a storage capacitor provides power when the input voltage source is unavailable. extremely low quiescent current and high ef? ciency design ensure the fastest possible charge times of the output reservoir capacitor. the ltc3108-1 is functionally equivalent to the ltc3108 except for its unique ? xed v out options. the ltc3108-1 is available in a small, thermally enhanced 12-lead (3mm 4mm) dfn package and a 16-lead ssop package. wireless remote sensor application powered from a peltier cell features applications n operates from inputs of 20mv n complete energy harvesting power management system - selectable v out of 2.5v, 3v, 3.7v or 4.5v - ldo: 2.2v at 3ma - logic controlled output - reserve energy output n power good indicator n uses compact step-up transformers n small 12-lead (3mm 4mm) dfn or 16-lead ssop packages n remote sensors and radio power n surplus heat energy harvesting n hvac systems n industrial wireless sensing n automatic metering n building automation n predictive maintenance 31081 ta01a c1 thermoelectric generator 20mv to 500mv c2 sw vs2 vs1 v out2 pgood 2.2v 470f pgd vldo vstore + v out v out2_en ltc3108-1 vaux gnd 0.1f 6.3v 5.25v 3v 1f 1nf 220f 1:100 330pf sensors rf link p 2.2f + + + v out charge time l , lt, ltc, ltm, linear technology and the linear logo are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. v in (mv) time (sec) 10 1 100 1000 0 31081 ta01b 0 100 150 250 50 200 300 350 400 v out = 3v c out = 470f 1:100 ratio 1:50 ratio 1:20 ratio
ltc3108-1 2 31081fa absolute maximum ratings sw voltage ..................................................C0.3v to 2v c1 voltage ....................................................C0.3v to 6v c2 voltage (note 5) .........................................C8v to 8v v out2 , v out2_en ...........................................C0.3v to 6v vaux .................................................... 15ma into vaux (note 1) 12 11 10 9 8 7 13 gnd 1 2 3 4 5 6 sw c2 c1 v out2_en vs1 vs2 vaux vstore v out v out2 vldo pgd top view de package 12-lead (4mm s 3mm) plastic dfn t jmax = 125c, ja = 43c/w exposed pad (pin 13) is gnd, must be soldered to pcb (note 4) gn package 16-lead plastic ssop narrow 1 2 3 4 5 6 7 8 top view 16 15 14 13 12 11 10 9 gnd vaux vstore v out v out2 vldo pgd gnd gnd sw c2 c1 v out2_en vs1 vs2 gnd t jmax = 125c, ja = 110c/w pin configuration electrical characteristics parameter conditions min typ max units minimum start-up voltage using 1:100 transformer turns ratio, vaux = 0v 20 50 mv no-load input current using 1:100 transformer turns ratio; v in = 20mv, v out2_en = 0v; all outputs charged and in regulation 3ma input voltage range using 1:100 transformer turns ratio l v startup 500 mv the l denotes the speci? cations which apply over the full operating junction temperature range, otherwise speci? cations are for t a = 25c (note 2). vaux = 5v, unless otherwise noted. order information lead free finish tape and reel part marking* package description temperature range ltc3108ede-1#pbf ltc3108ede-1#trpbf 31081 12-lead (4mm 3mm) plastic dfn C40c to 125c ltc3108ide-1#pbf ltc3108ide-1#trpbf 31081 12-lead (4mm 3mm) plastic dfn C40c to 125c ltc3108egn-1#pbf ltc3108egn-1#trpbf 31081 16-lead plastic ssop C40c to 125c ltc3108ign-1#pbf ltc3108ign-1#trpbf 31081 16-lead plastic ssop C40c to 125c consult ltc marketing for parts speci? ed for other ? xed output voltages or wider operating temperature ranges. *the temperature grade is identi? ed by a label on the shipping container. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel speci? cations, go to: http://www.linear.com/tapeandreel/ vs1, vs2, vaux, v out , pgd ........................C0.3v to 6v vldo, vstore ............................................C0.3v to 6v operating junction temperature range (note 2) ................................................. C40c to 125c storage temperature range .................. C65c to 125c
ltc3108-1 3 31081fa note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. note 2: the ltc3108-1 is tested under pulsed load conditions such that t j t a . the ltc3108-1e is guaranteed to meet speci? cations from 0c to 85c junction temperature. speci? cations over the C40c to 125c operating junction temperature range are assured by design, characterization and correlation with statistical process controls. the ltc3108-1i is guaranteed over the full C40c to 125c operating junction temperature range. note that the maximum ambient temperature is determined by speci? c operating conditions in conjunction with board layout, the rated thermal package thermal resistance and other environmental factors. the junction temperature (t j ) is calculated from the ambient temperature (t a ) and power dissipation (p d ) according to the formula: t j = t a + (p d ? ja c/w), where ja is the package thermal impedance. note 3: speci? cation is guaranteed by design and not 100% tested in production. note 4: failure to solder the exposed backside of the package to the pc board ground plane will result in a thermal resistance much higher than 43c/w. note 5: the absolute maximum rating is a dc rating. under certain conditions in the applications shown, the peak ac voltage on the c2 pin may exceed 8v. this behavior is normal and acceptable because the current into the pin is limited by the impedance of the coupling capacitor. electrical characteristics the l denotes the speci? cations which apply over the full operating junction temperature range, otherwise speci? cations are for t a = 25c (note 2). vaux = 5v, unless otherwise noted. parameter conditions min typ max units output voltage vs1 = vs2 = gnd vs1 = vaux, vs2 = gnd vs1 = gnd, vs2 = vaux vs1 = vs2 = vaux l l l l 2.45 2.94 3.626 4.41 2.50 3.00 3.70 4.50 2.55 3.06 3.774 4.59 v v v v v out quiescent current v out = 3.7v, v out2_en = 0v 0.2 a vaux quiescent current no load, all outputs charged 6 9 a ldo output voltage 0.5ma load l 2.134 2.2 2.266 v ldo load regulation for 0ma to 2ma load 0.5 1 % ldo line regulation for vaux from 2.5v to 5v 0.05 0.2 % ldo dropout voltage i vldo = 2ma l 100 200 mv ldo current limit vldo = 0v l 411 ma v out current limit v out = 0v l 2.8 4.5 7 ma vstore current limit vstore = 0v l 2.8 4.5 7 ma vaux clamp voltage current into vaux = 5ma l 5 5.25 5.55 v vstore leakage current vstore = 5v 0.1 0.3 a v out2 leakage current v out2 = 0v, v out2_en = 0v 0.1 a vs1, vs2 threshold voltage l 0.4 0.85 1.2 v vs1, vs2 input current vs1 = vs2 = 5v 0.01 0.1 a pgd threshold (rising) measured relative to the v out voltage C7.5 % pgd threshold (falling) measured relative to the v out voltage C9 % pgd v ol sink current = 100a 0.15 0.3 v pgd v oh source current = 0 2.1 2.2 2.3 v pgd pull-up resistance 1m v out2_en threshold voltage v out2_en rising l 0.4 1 1.3 v v out2_en pull-down resistance 5m v out2 turn-on time 5s v out2 turn-off time (note 3) 0.15 s v out2 current limit v out = 3.7v l 0.15 0.3 0.45 a v out2 current limit response time (note 3) 350 ns v out2 p-channel mosfet on-resistance v out = 3.7v (note 3) 1.3 n-channel mosfet on-resistance c2 = 5v (note 3) 0.5
ltc3108-1 4 31081fa typical performance characteristics i vout and ef? ciency vs v in , 1:20 ratio transformer input resistance vs v in (v out charging) i vout vs v in and source resistance, 1:20 ratio t a = 25c, unless otherwise noted. i vout and ef? ciency vs v in , 1:50 ratio transformer i vout and ef? ciency vs v in , 1:100 ratio transformer v in (mv) 0 i vout (a) efficiency (%) 2500 3000 3500 2000 1500 100 200 400 300 500 500 0 1000 4000 50 60 70 40 30 10 0 20 80 31081 g01 i vout (v out = 4v) efficiency (v out = 4v) i vout (v out = 0v) c1 = 10nf v in (mv) 0 i vout (a) efficiency (%) 2000 2400 2800 1600 1200 100 200 400 300 500 400 0 800 3200 50 60 70 40 30 10 0 20 80 31081 g02 c1 = 4.7nf i vout (v out = 4v) efficiency (v out = 4v) i vout (v out = 0v) v in (mv) 0 i vout (a) efficiency (%) 1000 1200 800 600 100 200 400 300 500 200 0 400 1400 50 60 70 40 30 10 0 20 31081 g03 c1 = 1nf i vout (v out = 4v) efficiency (v out = 4v) i vout (v out = 0v) v in (mv) 0 input resistance () 5 6 7 4 3 100 200 400 300 500 1 0 2 8 9 10 31081 g04 1:20 ratio 1:50 ratio 1:100 ratio v in open-circuit (mv) i vout (a) 100 10 1000 10000 0 31081 g05 0 200 300 500 100 400 600 700 800 1 2 5 10 c1 = 10nf v in (mv) i in (ma) 31081 g00 1000 100 10 1 10 100 1000 1:50 ratio, c1 = 4.7n 1:100 ratio, c1 = 1n 1:20 ratio, c1 = 10n i in vs v in , (v out = 0v)
ltc3108-1 5 31081fa ldo load regulation typical performance characteristics i vout vs dt and teg size, 1:100 ratio resonant switching waveforms t a = 25c, unless otherwise noted. ldo dropout voltage i vout vs v in and source resistance, 1:50 ratio i vout vs v in and source resistance, 1:100 ratio v in open-circuit (mv) i vout (a) 100 10 1000 10000 0 31081 g06 0 200 300 500 100 400 600 700 800 1 2 5 10 c1 = 4.7nf v in open-circuit (mv) i vout (a) 100 1000 10 31081 g07 0 100 200 300 400 500 1 2 5 10 c1 = 1nf dt across teg (c) i vout (a) 100 10 1000 10000 0 31081 g08 0.1 10 1 100 v out = 0v 40mm teg 15mm teg 1:50 ratio 1:100 ratio 1:50 ratio 1:100 ratio ldo load (ma) 0 C1.00 drop in vldo (%) C0.75 C0.50 0.5 1 1.5 2 32.5 3.5 C0.25 0.00 4 31081 g10 ldo load (ma) 0 0.00 dropout voltage (v) 0.04 0.08 0.12 0.5 1 1.5 2 32.5 3.5 0.16 0.20 0.02 0.06 0.10 0.14 0.18 4 31081 g11 10s/div c1 pin 2v/div c2 pin 2v/div sw pin 50mv/ div 31081 g09 v in = 20mv 1:100 ratio transformer
ltc3108-1 6 31081fa v out and pgd response during a step load v out ripple ldo step load response enable input and v out2 running on storage capacitor 5ms/div 31081 g13 ch2 v out , 1v/div ch1 pgd, 1v/div 50ma load step c out = 220f 100ms/div 31081 g14 20mv/ div 30a load c out = 220f 1ms/div 31081 g16 ch2 v out2 1v/div ch1 v out2_en 1v/div 10ma load on v out2 c out = 220f 5sec/div 31081 g17 ch2, v out 1v/div ch1, v in 50mv/div ch3 vstore 1v/div ch4, v ldo 1v/div cstore = 470f v out load = 100a v ldo 20mv/div i ldo 5ma/div 200s/div 31081 g15 0ma to 3ma load step c ldo = 2.2f start-up voltage sequencing 10sec/div 31081 g12 ch1 vstore 1v/div ch2, v out 1v/div ch3, v ldo 1v/div v in = 50mv 1:100 ratio transformer c out = 220f cstore = 470f c ldo = 2.2f typical performance characteristics t a = 25c, unless otherwise noted.
ltc3108-1 7 31081fa vaux (pin 1/pin 2): output of the internal recti? er cir- cuit and v cc for the ic. bypass vaux with at least 1f of capacitance. an active shunt regulator clamps vaux to 5.25v (typical). vstore (pin 2/pin 3): output for the storage capacitor or battery. a large capacitor may be connected from this pin to gnd for powering the system in the event the input voltage is lost. it will be charged up to the maximum vaux clamp voltage. if not used, this pin should be left open or tied to vaux. v out (pin 3/pin 4): main output of the converter. the voltage at this pin is regulated to the voltage selected by vs1 and vs2 (see table 1). connect this pin to an energy storage capacitor or to a rechargeable battery. v out2 (pin 4/pin 5): switched output of the converter. connect this pin to a switched load. this output is open until v out2_en is driven high, then it is connected to v out through a 1.3 p-channel switch. if not used, this pin should be left open or tied to v out . the peak current in this output is limited to 0.3a typical. vldo (pin 5/pin 6): output of the 2.2v ldo. connect a 2.2f or larger ceramic capacitor from this pin to gnd. if not used, this pin should be tied to vaux. pgd (pin 6/pin 7): power good output. when v out is within 7.5% of its programmed value, pgd will be pulled up to vldo through a 1m resistor. if v out drops 9% below its programmed value pgd will go low. this pin can sink up to 100a. vs2 (pin 7/pin 10): v out select pin 2. connect this pin to ground or vaux to program the output voltage (see table 1). vs1 (pin 8/pin 11): v out select pin 1. connect this pin to ground or vaux to program the output voltage (see table 1). v out2_en (pin 9/pin 12): enable input for v out2 . v out2 will be enabled when this pin is driven high. there is an internal 5m pull-down resistor on this pin. if not used, this pin can be left open or grounded. c1 (pin 10/pin 13): input to the charge pump and recti? er circuit. connect a capacitor from this pin to the secondary winding of the step-up transformer. c2 (pin 11/pin 14): input to the n-channel gate drive circuit. connect a capacitor from this pin to the secondary winding of the step-up transformer. sw (pin 12/pin 15): drain of the internal n-channel switch. connect this pin to the primary winding of the transformer. gnd (pins 1, 8, 9, 16) ssop only: ground gnd (exposed pad pin 13) dfn only: ground. the dfn exposed pad must be soldered to the pcb ground plane. it serves as the ground connection, and as a means of conducting heat away from the die. table 1. regulated voltage using pins vs1 and vs2 vs2 vs1 v out gnd gnd 2.5v gnd vaux 3v vaux gnd 3.7v vaux vaux 4.5v pin functions (dfn/ssop)
ltc3108-1 8 31081fa block diagram operation the ltc3108-1 is designed to use a small external step-up transformer to create an ultralow input voltage step-up dc/dc converter and power manager. it is ideally suited for low power wireless sensors and other applications in which surplus energy harvesting is used to generate system power because traditional battery power is inconvenient or impractical. the ltc3108-1 is designed to manage the charging and regulation of multiple outputs in a system in which the average power draw is very low, but there may be periodic pulses of higher load current required. this is typical of wireless sensor applications, where the quiescent power draw is extremely low most of the time, except for transmit bursts when circuitry is powered up to make measure- ments and transmit data. the ltc3108-1 can also be used to trickle charge a standard capacitor, supercapacitor or rechargeable battery, using energy harvested from a peltier or photovoltaic cell. (refer to the block diagram) 31081 bd c1 c2 5m sw 5.25v 1.2v v ref sw v out vstore vldo off on v out2 v out2 v out v out program c out pgood v out2_en v out vs1 vs2 pgd vstore c1 c in v in vldo c store 1f 1:100 c2 sync rectify reference v out 2.2v charge control vaux + C + C ilim ltc3108-1 1.3 0.5 1m exposed pad (dfn) 2.2f gnd (ssop) v ref ldo v ref v best
ltc3108-1 9 31081fa operation oscillator the ltc3108-1 utilizes a mosfet switch to form a resonant step-up oscillator using an external step-up transformer and a small coupling capacitor. this allows it to boost input voltages as low as 20mv high enough to provide multiple regulated output voltages for powering other circuits. the frequency of oscillation is determined by the inductance of the transformer secondary winding and is typically in the range of 10khz to 100khz. for input voltages as low as 20mv, a primary-secondary turns ratio of about 1:100 is recommended. for higher input voltages, this ratio can be lower. see the applications information section for more information on selecting the transformer. charge pump and recti? er the ac voltage produced on the secondary winding of the transformer is boosted and recti? ed using an external charge pump capacitor (from the secondary winding to pin c1) and the recti? ers internal to the ltc3108-1. the recti? er circuit feeds current into the vaux pin, provid- ing charge to the external vaux capacitor and the other outputs. vaux the active circuits within the ltc3108-1 are powered from vaux, which should be bypassed with a 1f capacitor. larger capacitor values are recommended when using turns ratios of 1:50 or 1:20 (refer to the typical applica- tion examples). once vaux exceeds 2.5v, the main v out is allowed to start charging. an internal shunt regulator limits the maximum voltage on vaux to 5.25v typical. it shunts to gnd any excess current into vaux when there is no load on the converter or the input source is generating more power than is required by the load. voltage reference the ltc3108-1 includes a precision, micropower refer- ence, for accurate regulated output voltages. this reference becomes active as soon as vaux exceeds 2v. synchronous recti? ers once vaux exceeds 2v, synchronous recti? ers in parallel with each of the internal diodes take over the job of rectify- ing the input voltage, improving ef? ciency. low dropout linear regulator (ldo) the ltc3108-1 includes a low current ldo to provide a regulated 2.2v output for powering low power processors or other low power ics. the ldo is powered by the higher of vaux or v out . this enables it to become active as soon as vaux has charged to 2.3v, while the v out storage capacitor is still charging. in the event of a step load on the ldo output, current can come from the main v out capacitor if vaux drops below v out . the ldo requires a 2.2f ceramic capacitor for stability. larger capacitor values can be used without limitation, but will increase the time it takes for all the outputs to charge up. the ldo output is current limited to 4ma minimum. v out the main output voltage on v out is charged from the vaux supply, and is user programmed to one of four regulated voltages using the voltage select pins vs1 and vs2, ac- cording to table 2. although the logic threshold voltage for vs1 and vs2 is 0.85v typical, it is recommended that they be tied to ground or vaux. table 2. regulated voltage using pins vs1 and vs2 vs2 vs1 v out gnd gnd 2.5v gnd vaux 3v vaux gnd 3.7v vaux vaux 4.5v when the output voltage drops slightly below the regulated value, the charging current will be enabled as long as vaux is greater than 2.5v. once v out has reached the proper value, the charging current is turned off. the internal programmable resistor divider sets v out , eliminating the need for very high value external resistors that are susceptible to board leakage.
ltc3108-1 10 31081fa in a typical application, a storage capacitor (typically a few hundred microfarads) is connected to v out . as soon as vaux exceeds 2.5v, the v out capacitor will be allowed to charge up to its regulated voltage. the current available to charge the capacitor will depend on the input voltage and transformer turns ratio, but is limited to about 4.5ma typical. pgood a power good comparator monitors the v out voltage. the pgd pin is an open-drain output with a weak pull-up (1m) to the ldo voltage. once v out has charged to within 7.5% of its regulated voltage, the pgd output will go high. if v out drops more than 9% from its regulated voltage, pgd will go low. the pgd output is designed to drive a microprocessor or other chip i/o and is not intended to drive a higher current load such as an led. pulling pgd up externally to a voltage greater than vldo will cause a small current to be sourced into vldo. pgd can be pulled low in a wire-or con? guration with other circuitry. v out2 v out2 is an output that can be turned on and off by the host, using the v out2_en pin. when enabled, v out2 is connected to v out through a 1.3 p-channel mosfet switch. this output, controlled by a host processor, can be used to power external circuits such as sensors and ampli? ers, that do not have a low power sleep or shutdown capability. v out2 can be used to power these circuits only when they are needed. minimizing the amount of decoupling capacitance on v out2 will allow it to be switched on and off faster, allowing shorter burst times and, therefore, smaller duty cycles in pulsed applications such as a wireless sensor/transmit- ter. a small v out2 capacitor will also minimize the energy that will be wasted in charging the capacitor every time v out2 is enabled. v out2 has a soft-start time of about 5s to limit capacitor charging current and minimize glitching of the main output when v out2 is enabled. it also has a current limiting circuit that limits the peak current to 0.3a typical. the v out2 enable input has a typical threshold of 1v with 100mv of hysteresis, making it logic-compatible. if v out2_en (which has an internal pull-down resistor) is low, v out2 will be off. driving v out2_en high will turn on the v out2 output. note that while v out2_en is high, the current limiting cir- cuitry for v out2 draws an extra 8a of quiescent current from v out . this added current draw has a negligible effect on the application and capacitor sizing, since the load on the v out2 output, when enabled, is likely to be orders of magnitude higher than 8a. vstore the vstore output can be used to charge a large storage capacitor or rechargeable battery after v out has reached regulation. once v out has reached regulation, the vstore output will be allowed to charge up to the vaux voltage. the storage element on vstore can be used to power the system in the event that the input source is lost, or is unable to provide the current demanded by the v out , v out2 and ldo outputs. if vaux drops below vstore, the ltc3108-1 will automatically draw current from the storage element. note that it may take a long time to charge a large capacitor, depending on the input energy available and the loading on v out and vldo. since the maximum current from vstore is limited to a few milliamps, it can safely be used to trickle-charge nicd or nimh rechargeable batteries for energy storage when the input voltage is lost. note that the vstore capacitor cannot supply large pulse currents to v out . any pulse load on v out must be handled by the v out capacitor. short-circuit protection all outputs of the ltc3108-1 are current limited to protect against short-circuits to ground. output voltage sequencing a timing diagram showing the typical charging and voltage sequencing of the outputs is shown in figure 1. note: time not to scale. operation
ltc3108-1 11 31081fa figure 1. output voltage sequencing with v out programmed for 3v (time not to scale) operation 31081 f01a time (ms) voltage (v) 3.0 2.0 1.0 0 0 70 20 40 10 30 50 60 80 3.0 2.0 1.0 0 5.0 5.0 2.5 2.5 0 0 5.0 2.5 0 vstore (v) pgd (v) v out (v) vldo (v) vaux (v)
ltc3108-1 12 31081fa introduction the ltc3108-1 is designed to gather energy from very low input voltage sources and convert it to usable output voltages to power microprocessors, wireless transmitters and analog sensors. such applications typically require much more peak power, and at higher voltages, than the input voltage source can produce. the ltc3108-1 is designed to accumulate and manage energy over a long period of time to enable short power bursts for acquiring and transmitting data. the bursts must occur at a low enough duty cycle such that the total output energy dur- ing the burst does not exceed the average source power integrated over the accumulation time between bursts. for many applications, this time between bursts could be seconds, minutes or hours. the pgd signal can be used to enable a sleeping micro- processor or other circuitry when v out reaches regulation, indicating that enough energy is available for a burst. input voltage sources the ltc3108-1 can operate from a number of low input voltage sources, such as peltier cells, photovoltaic cells or thermopile generators. the minimum input voltage required for a given application will depend on the transformer turns ratio, the load power required, and the internal dc resistance (esr) of the voltage source. lower esr will allow the use of lower input voltages, and provide higher output power capability. applications information figure 2. typical performance of a peltier cell acting as a thermoelectric generator refer to the i in vs v in curves in the typical performance characteristics section to see what input current is required for the source for a given input voltage. for a given transformer turns ratio, there is a maximum recommended input voltage to avoid excessively high secondary voltages and power dissipation in the shunt regulator. it is recommended that the maximum input voltage times the turns ratio be less than 50. note that a low esr bulk decoupling capacitor will usually be required across the input source to prevent large voltage droop and ripple caused by the sources esr and the peak primary switching current (which can reach hundreds of milliamps). the time constant of the ? lter capacitor and the esr of the voltage source should be much longer than the period of the resonant switching frequency. peltier cell (thermoelectric generator) a peltier cell (also known as a thermoelectric cooler) is made up of a large number of series-connected p-n junc- tions, sandwiched between two parallel ceramic plates. although peltier cells are often used as coolers by apply- ing a dc voltage to their inputs, they will also generate a dc output voltage, using the seebeck effect, when the two plates are at different temperatures. the polarity of the output voltage will depend on the polarity of the tem- perature differential between the plates. the magnitude of the output voltage is proportional to the magnitude of the temperature differential between the plates. when used in 31081 f02 1000 100 10 1 1 10 100 dt (c) teg v open_circuit (mv) teg maximum p out ideal (mw) 1 100 10 0.1 v oc max p out (ideal) teg: 30mm 127 couples r = 2
ltc3108-1 13 31081fa this manner, a peltier cell is referred to as a thermoelectric generator (teg). the low voltage capability of the ltc3108-1 design allows it to operate from a teg with temperature differentials as low as 1c, making it ideal for harvesting energy in applications in which a temperature difference exists between two surfaces or between a surface and the am- bient temperature. the internal resistance (esr) of most cells is in the range of 1 to 5, allowing for reasonable power transfer. the curves in figure 2 show the open- circuit output voltage and maximum power transfer for a typical peltier cell (with an esr of 2) over a 20c range of temperature differential. teg load matching the ltc3108-1 was designed to present a minimum input resistance (load) in the range of 2 to 10, depending on input voltage and transformer turns ratio (as shown in the typical performance characteristics curves). for a given turns ratio, as the input voltage drops, the input resistance increases. this feature allows the ltc3108-1 to optimize power transfer from sources with a few ohms of source resistance, such as a typical teg. note that a lower source resistance will always provide more output current capability by providing a higher input voltage under load. peltier cell (teg) suppliers peltier cells are available in a wide range of sizes and power capabilities, from less than 10mm square to over 50mm square. they are typically 2mm to 5mm in height. a list of peltier cell manufacturers is given in table 3. table 3. peltier cell manufacturers fujitaka www.fujitaka.com/pub/peltier/english/thermoelectric_power.html ferrotec www.ferrotec.com/products/thermal/modules laird technologies www.lairdtech.com marlow industries www.marlow.com micropelt www.micropelt.com nextreme www.nextreme.com te technology www.tetech.com/peltier-thermoelectric-cooler-modules.html tellurex www.tellurex.com kryotherm www.kryothermusa.com applications information table 4. recommended teg part numbers by size manufacturer 15mm 15mm 20mm 20mm 30mm 30mm 40mm 40mm cui inc. (distributor) cp60133 cp60233 cp60333 cp85438 ferrotec 9501/031/030 b 9501/071/040 b 9500/097/090 b 9500/127/100 b fujitaka fph13106nc fph17106nc fph17108ac fph112708ac kryotherm tgm-127-1.0-0.8 lcb-127-1.4-1.15 laird technology pt6.7.f2.3030.w6 pt8.12.f2.4040.ta.w6 marlow industries rc3-8-01 rc6-6-01 rc12-8-01ls tellurex c2-15-0405 c2-20-0409 c2-30-1505 c2-40-1509 te technology te-31-1.0-1.3 te-31-1.4-1.15 te-71-1.4-1.15 te-127-1.4-1.05
ltc3108-1 14 31081fa thermopile generator thermopile generators (also called powerpile generators) are made up of a number of series-connected thermo- couples enclosed in a metal tube. they are commonly used in gas burner applications to generate a dc output of hundreds of millivolts when exposed to the high tem- perature of a ? ame. typical examples are the honeywell cq200 and q313. these devices have an internal series resistance of less than 3, and can generate as much as 750mv open-circuit at their highest rated temperature. for applications in which the temperature rise is too high for a solid-state thermoelectric device, a thermopile can be used as an energy source to power the ltc3108-1. because of the higher output voltages possible with a thermopile generator, a lower transformer turns ratio can be used (typically 1:20, depending on the application). photovoltaic cell the ltc3108-1 converter can also operate from a single photovoltaic cell (also known as a pv or solar cell) at light levels too low for other low input voltage boost convert- ers to operate. however, many variables will affect the performance in these applications. light levels can vary over several orders of magnitude and depend on light- ing conditions (the type of lighting and indoor versus outdoor). different types of light (sunlight, incandescent, ? uorescent) also have different color spectra, and will produce different output power levels depending on which type of photovoltaic cell is being used (monocrystalline, polycrystalline or thin-? lm). therefore, the photovoltaic cell must be chosen for the type and amount of light avail- able. note that the short-circuit output current from the cell must be at least a few milliamps in order to power the ltc3108-1 converter non-boost applications the ltc3108-1 can also be used as an energy harvester and power manager for input sources that do not require boosting. in these applications the step-up transformer can be eliminated. any source whose peak voltage exceeds 2.5v ac or 5v dc can be connected to the c1 input through a current- limiting resistor where it will be recti? ed/peak detected. in these applications the c2 and sw pins are not used and can be grounded or left open. examples of such input sources would be piezoelectric transducers, vibration energy harvesters, low current generators, a stack of low current solar cells or a 60hz ac input. a series resistance of at least 100/v should be used to limit the maximum current into the vaux shunt regulator. component selection step-up transformer the step-up transformer turns ratio will determine how low the input voltage can be for the converter to start. using a 1:100 ratio can yield start-up voltages as low as 20mv. other factors that affect performance are the dc resistance of the transformer windings and the inductance of the windings. higher dc resistance will result in lower ef? ciency. the secondary winding inductance will deter- mine the resonant frequency of the oscillator, according to the following formula. frequency = 1 2? ? l(sec)? c hz where l is the inductance of the transformer secondary winding and c is the load capacitance on the secondary winding. this is comprised of the input capacitance at pin c2, typically 30pf, in parallel with the transformer secondary windings shunt capacitance. the recommended resonant frequency is in the range of 10khz to 100khz. see table 5 for some recommended transformers. table 5. recommended transformers vendor part number coilcraft www.coilcraft.com lpr6235-752sml (1:100 ratio) lpr6235-253pml (1:20 ratio) lpr6235-123qml (1:50 ratio) wrth www.we-online s11100034 (1:100 ratio) s11100033 (1:50 ratio) s11100032 (1:20 ratio) applications information
ltc3108-1 15 31081fa c1 capacitor the charge pump capacitor that is connected from the transformers secondary winding to the c1 pin has an ef- fect on converter input resistance and maximum output current capability. generally, a minimum value of 1nf is recommended when operating from very low input volt- ages using a transformer with a ratio of 1:100. too large a capacitor value can compromise performance when operating at low input voltage or with high resistance sources. for higher input voltages and lower turns ratios, the value of the c1 capacitor can be increased for higher output current capability. refer to the typical applications schematic examples for the recommended value for a given turns ratio. squegging certain types of oscillators, including transformer-coupled oscillators such as the resonant oscillator of the ltc3108-1, can exhibit a phenomenon called squegging. this term refers to a condition that can occur which blocks or stops the oscillation for a period of time much longer than the period of oscillation, resulting in bursts of oscillation. an example of this is the blocking oscillator, which is designed to squegg to produce bursts of oscillation. squegging is also encountered in rf oscillators and regenerative receivers. in the case of the ltc3108-1, squegging can occur when a charge builds up on the c2 gate coupling capacitor, such that the dc bias point shifts and oscillation is extinguished for a certain period of time, until the charge on the capacitor bleeds off, allowing oscillation to resume. it is dif? cult to predict when and if squegging will occur in a given ap- plication. while squegging is not harmful, it reduces the average output current capability of the ltc3108-1. squegging can easily be avoided by the addition of a bleeder resistor in parallel with the coupling capacitor on the c2 pin. resistor values in the range of 100k to 1m are suf? cient to eliminate squegging without having any negative impact on performance. for the 330pf capacitor used for c2 in most applications, a 499k bleeder resistor is recommended. see the typical applications schematics for an example. using external charge pump recti? ers the synchronous charge pump recti? ers in the ltc3108-1 (connected to the c1 pin) are optimized for operation from very low input voltage sources, using typical transformer step-up ratios between 1:100 and 1:50, and typical c1 charge pump capacitor values less than 10nf. operation from higher input voltage sources (typically 250mv or greater, under load), allows the use of lower transformer step-up ratios (such as 1:20 and 1:10) and larger c1 capacitor values to provide higher output cur- rent capability from the ltc3108. however, due to the resulting increase in recti? er currents and resonant oscil- lator frequency in these applications, the use of external charge pump recti? ers is recommended for optimal performance. in applications where the step-up ratio is 1:20 or less, and the c1 capacitor is 10nf or greater, the c1 pin should be grounded and two external recti? ers (such as 1n4148 or 1n914 diodes) should be used. these are available as dual diodes in a single package. avoid the use of schottky recti? ers, as their lower forward-voltage drop increases the minimum startup voltage. see the typical applications schematics for an example. v out and vstore capacitor for pulsed load applications, the v out capacitor should be sized to provide the necessary current when the load is pulsed on. the capacitor value required will be dictated by the load current, the duration of the load pulse, and the amount of voltage droop the circuit can tolerate. the capacitor must be rated for whatever voltage has been selected for v out by vs1 and vs2. c out (f) i load (ma) ? t pulse (ms) v out (v) note that there must be enough energy available from the input voltage source for v out to recharge the capacitor during the interval between load pulses (to be discussed in the next example). reducing the duty cycle of the load pulse will allow operation with less input energy. the vstore capacitor may be of very large value (thou- sands of microfarads or even farads), to provide holdup applications information
ltc3108-1 16 31081fa at times when the input power may be lost. note that this capacitor can charge all the way to 5.25v (regardless of the settings for v out ), so ensure that the holdup capacitor has a working voltage rating of at least 5.5v at the tem- perature for which it will be used. the vstore capacitor can be sized using the following: c store 6a + i q + i ldo + (i burst ?t?f) [] ?tstore 5.25 ? v out where 6a is the quiescent current of the ltc3108-1, i q is the load on v out in between bursts, i ldo is the load on the ldo between bursts, i burst is the total load during the burst, t is the duration of the burst, f is the frequency of the bursts, tstore is the storage time required and v out is the output voltage required. to minimize losses and capacitor charge time, all capaci- tors used for v out and vstore should be low leakage. see table 6 for recommended storage capacitors. table 6. recommended storage capacitors vendor part number/series avx www.avx.com bestcap series taj and tps series tantalum cap-xx www.cap-xx.com gz series cooper/bussmann www.bussmann.com/3/powerstor.html kr series p series vishay/sprague www.vishay.com/capacitors tantamount 592d 595d tantalum 150crz/153crv aluminum 013 rlc (low leakage) storage capacitors requiring voltage balancing are not recommended due to the current draw of the balancing resistors. pcb layout guidelines due to the rather low switching frequency of the resonant converter and the low power levels involved, pcb layout is not as critical as with many other dc/dc converters. there are, however, a number of things to consider. due to the very low input voltage the circuit may operate from, the connections to v in , the primary of the transformer and the sw and gnd pins of the ltc3108-1 should be designed to minimize voltage drop from stray resistance and able to carry currents as high as 500ma. any small voltage drop in the primary winding conduction path will lower ef? ciency and increase capacitor charge time. also, due to the low charge currents available at the outputs of the ltc3108-1, any sources of leakage current on the output voltage pins must be minimized. an example board layout is shown in figure 3. applications information figure 3. example component placement for two-layer pc board (dfn package) 31081 fo3 v out2 v out v in vias to ground plane vldo pgood gnd 12 11 8 9 10 4 5 3 2 1 v out2_en vs1 vs2 sw c2 c1 v out v out2 vldo pgd vaux vstore 6 7
ltc3108-1 17 31081fa applications information design example 1 this design example will explain how to calculate the necessary storage capacitor value for v out in pulsed load applications, such as a wireless sensor/transmit- ter. in these types of applications, the load is very small for a majority of the time (while the circuitry is in a low power sleep state), with bursts of load current occur- ring periodically during a transmit burst. the storage capacitor on v out supports the load during the transmit burst, and the long sleep time between bursts allows the ltc3108-1 to recharge the capacitor. a method for calculating the maximum rate at which the load pulses can occur for a given output current from the ltc3108-1 will also be shown. in this example, v out is set to 3v, and the maximum al- lowed voltage droop during a transmit burst is 10%, or 0.3v. the duration of a transmit burst is 1ms, with a total average current requirement of 40ma during the burst. given these factors, the minimum required capacitance on v out is: c out (f) 40ma ? 1ms 0.3v = 133f note that this equation neglects the effect of capacitor esr on output voltage droop. for most ceramic or low esr tantalum capacitors, the esr will have a negligible effect at these load currents. a standard value of 150f or larger could be used for c out in this case. note that the load current is the total current draw on v out , v out2 and vldo, since the current for all of these outputs must come from v out during a burst. current contribution from the holdup capacitor on vstore is not considered, since it may not be able to recharge between bursts. also, it is assumed that the charge current from the ltc3108-1 is negligible compared to the magnitude of the load current during the burst. to calculate the maximum rate at which load bursts can occur, determine how much charge current is available from the ltc3108-1 v out pin given the input voltage source being used. this number is best found empirically, since there are many factors affecting the ef? ciency of the converter. also determine what the total load cur- rent is on v out during the sleep state (between bursts). note that this must include any losses, such as storage capacitor leakage. assume, for instance, that the charge current from the ltc3108-1 is 50a and the total current drawn on v out in the sleep state is 17a, including capacitor leakage. in addition, use the value of 150f for the v out capacitor. the maximum transmit rate (neglecting the duration of the transmit burst, which is typically very short) is then given by: t = 150f ? 0.3v (50a ? 17a) = 1.36sec or f max = 0.73hz therefore, in this application example, the circuit can sup- port a 1ms transmit burst every 1.3 seconds. it can be determined that for systems that only need to transmit every few seconds (or minutes or hours), the average charge current required is extremely small, as long as the sleep current is low. even if the available charge current in the example above was only 10a and the sleep current was only 5a, it could still transmit a burst every 9 seconds. the following formula enables the user to calculate the time it will take to charge the ldo output capacitor and the v out capacitor the ? rst time, from 0v. here again, the charge current available from the ltc3108-1 must be known. for this calculation, it is assumed that the ldo output capacitor is 2.2f. t ldo = 2.2v ? 2.2f i chg ? i ldo if there were 50a of charge current available and a 5a load on the ldo (when the processor is sleeping), the time for the ldo to reach regulation would be 107ms. if v out were programmed to 3v and the v out capacitor was 150f, the time for v out to reach regulation would be: t vout = 3v ? 150f i chg ? i vout ? i ldo + t ldo
ltc3108-1 18 31081fa peltier-powered energy harvester for remote sensor applications 31081 ta02 c1 c2 sw vs2 vs1 cooper bussman pb-5roh104-r or kr-5r5h104-r v out2 v out2 pgood 2.2v 499k c out * pgd vldo vstore + v out v out2_en ltc3108-1 vaux gnd c store 0.1f 6.3v 5.25v 3v 1f 1nf c in 1:100 t1 t1: coilcraft lpr6235-752sml *c out value dependent on the magnitude and duration of the load pulse 330pf t = 1c to 20c sensors xmtr p 2.2f on off 3v + + + thermoelectric generator typical applications if there were 50a of charge current available and 5a of load on v out , the time for v out to reach regulation after the initial application of power would be 11.35 seconds. design example 2 in many pulsed load applications, the duration, magnitude and frequency of the load current bursts are known and ? xed. in these cases, the average charge current required from the ltc3108-1 to support the average load must be calculated, which can be easily done by the following: i chg i q + i burst ?t t where i q is the sleep current on v out required by the ex- ternal circuitry in between bursts (including cap leakage), i burst is the total load current during the burst, t is the duration of the burst and t is the period of the transmit burst rate (essentially the time between bursts). in this example, i q = 5a, i burst = 100ma, t = 5ms and t = one hour. the average charge current required from the ltc3108-1 would be: i chg 5a + 100ma ? 0.005sec 3600sec = 5.14a therefore, if the ltc3108-1 has an input voltage that al- lows it to supply a charge current greater than 5.14a, the application can support 100ma bursts lasting 5ms every hour. it can be determined that the sleep current of 5a is the dominant factor because the transmit duty cycle is so small (0.00014%). note that for a v out of 3v, the average power required by this application is only 15.4w (not including converter losses). note that the charge current available from the ltc3108-1 has no effect on the sizing of the v out capacitor (if it is assumed that the load current during a burst is much larger than the charge current), and the v out capacitor has no effect on the maximum allowed burst rate. applications information
ltc3108-1 19 31081fa supercapacitor charger and ldo powered by a solar cell (uses external charge pump recti? ers) dual output converter and ldo powered by a thermopile generator 31081 ta04 c1 honeywell q313 thermopile c2 sw vs2 vs1 vstore v out2 pgood vldo pgd vldo v out v out2_en ltc3108-1 vaux gnd 2.2f 150f 6.3v 2.2f 4.7nf t1: coilcraft lpr6235-123qml 2.2v 4.5v t1 1:50 330pf 220f + + v out 499k 31081 ta03 c1 solar cell* * 2 " diameter monocrystalline cell light level 900 lux t1: coilcraft lpr6235-253pml c2 sw vs2 vs1 vstore v out2 vldo pgd vldo v out v out2_en ltc3108-1 vaux vaux vaux gnd 2.2f 4f* 4.7f 0.022f 2.2v *taiyo yuden pas1020la3r0405 bas31 3.0v t1 1:20 330pf 220f + C + v out 499k + typical applications illuminance (lux) 100 10 i vout (a) 100 1000 10,000 1000 10,000 10,0000 31081 ta03b incandescent light flourescent light outdoor light (cloudy) i vout vs illuminance (2 " diameter monocrystalline cell)
ltc3108-1 20 31081fa typical applications dc input energy harvester and power manager 31081 ta05 c1 c2 sw vs2 vs1 v out2 v out2 pgood 2.2v c out pgd vldo vldo vstore v out v out v out2_en v out2_enable ltc3108-1 vaux gnd c store 5.25v 3v 2.2f v in v in > 5v r in r in > 100 / v 2.2f + C + + ac input energy harvester and power manager 31081 ta06 c1 c2 sw vs1 vs2 v out2 v out2 pgood 2.2v c out pgd vldo vldo vstore v out v out v out2_en v out2_enable ltc3108-1 vaux gnd c store 5.25v 4.5v 2.2f c in v in v in > 5v p-p - piezo - 60hz r in r in > 100 / v 2.2f ac + +
ltc3108-1 21 31081fa typical applications low pro? le (1.5mm) step-up converter/harvester using 1:10 transformer 31081 ta07 c1 c2 sw vs2 vs1 output can support a 20ma, 10ms load pulse every 0.4s at v in = 150mv v out2 v in 150mv to 600mv v out2 pgood 2.2v 330f s 3 pgd vldo vstore v out v out2_en enable ltc3108-1 vaux gnd 2.2v 10ms 10ms 3v bas31 vaux 10f c in t1 1:10 t1: coilcraft lpr4012-202lml *c res lowers start-up voltage to 135mv typical 330pf 499k 2.2f vldo c res * 390pf 0.068f 0.1f on off 3v at 20ma avx tpsx337m004r0100 + v in (mv) 150 i vout (ma) 4 5 3 2 250 300 400 200 350 450 550 500 600 1 0 6 31081 ta07b v out 3v minimum limit typical i vout vs v in (steady state)
ltc3108-1 22 31081fa package description 4.00 0.10 (2 sides) 3.00 0.10 (2 sides) note: 1. drawing proposed to be a variation of version (wged) in jedec package outline m0-229 2. drawing not to scale 3. all dimensions are in millimeters 4. dimensions of exposed pad on bottom of package do not include mold flash. mold flash, if present, shall not exceed 0.15mm on any side 5. exposed pad shall be solder plated 6. shaded area is only a reference for pin 1 location on the top and bottom of package 0.40 0.10 bottom viewexposed pad 1.70 0.10 0.75 0.05 r = 0.115 typ r = 0.05 typ 2.50 ref 1 6 12 7 pin 1 notch r = 0.20 or 0.35 45 chamfer pin 1 top mark (note 6) 0.200 ref 0.00 C 0.05 (ue12/de12) dfn 0806 rev d 3.30 0.10 0.25 0.05 0.50 bsc 2.50 ref recommended solder pad pitch and dimensions apply solder mask to areas that are not soldered 2.20 0.05 0.70 0.05 3.60 0.05 package outline 1.70 0.05 3.30 0.05 0.50 bsc 0.25 0.05 de/ue package 12-lead plastic dfn (4mm 3mm) (reference ltc dwg # 05-08-1695 rev d) gn16 rev b 0212 12 3 4 5 6 7 8 .229 C .244 (5.817 C 6.198) .150 C .157** (3.810 C 3.988) 16 15 14 13 .189 C .196* (4.801 C 4.978) 12 11 10 9 .016 C .050 (0.406 C 1.270) .015 .004 (0.38 0.10) = 45$ 0 C 8 typ .007 C .0098 (0.178 C 0.249) .0532 C .0688 (1.35 C 1.75) .008 C .012 (0.203 C 0.305) typ .004 C .0098 (0.102 C 0.249) .0250 (0.635) bsc .009 (0.229) ref .254 min recommended solder pad layout .150 C .165 .0250 bsc .0165 .0015 .045 .005 * dimension does not include mold flash. mold flash shall not exceed 0.006" (0.152mm) per side ** dimension does not include interlead flash. interlead flash shall not exceed 0.010" (0.254mm) per side inches (millimeters) note: 1. controlling dimension: inches 2. dimensions are in 3. drawing not to scale 4. pin 1 can be bevel edge or a dimple gn package 16-lead plastic ssop (narrow .150 inch) (reference ltc dwg # 05-08-1641 rev b) please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
ltc3108-1 23 31081fa information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. revision history rev date description page number a 06/12 added vendor information to table 5 14
ltc3108-1 24 31081fa linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com linear technology corporation 2010 lt 0612 rev a ? printed in usa related parts typical application part number description comments ltc3108 ultralow voltage step-up converter and power manager v in : 0.02v to 1v; v out = 2.35v, 3.3v, 4.1v, 5v fixed; i q = 6a; i sd <1a; 3mm 4mm dfn-12 and ssop-16 packages ltc4070 li-ion/polymer low current shunt battery charger system v in : 450na to 50ma; v out(min) : v float + 4v, 4.1v, 4.2v; i q = 300na; 2mm 3mm dfn-8 and msop-8 packages ltc1041 bang-bang controller v in : 2.8v to 16v; v out(min) = adj; i q = 1.2ma; i sd < 1a; so-8 package ltc1389 nanopower precision shunt voltage reference v out(min) = 1.25v; i q = 0.8a; so-8 package lt1672/lt1673/ lt1674 single-/dual-/quad-precision 2a rail-to-rail op amps so-8, so-14 and msop-8 packages lt3009 3a i q , 20ma linear regulator v in : 1.6v to 20v; v out(min) : 0.6v to adj, 1.2v, 1.5v, 1.8v, 2.5v, 3.3v, 5v to fixed; i q = 3a; i sd < 1a; 2mm 2mm dfn-8 and sc70 packages ltc3525l-3/ ltc3525l-3.3/ ltc3525l-5 400ma (i sw ), synchronous step-up dc/dc converter with output disconnect v in : 0.7v to 4v; v out(min) = 5v max ; i q = 7a; i sd < 1a; sc70 package ltc3588-1 piezoelectric energy generator with integrated high ef? ciency buck converter v in : 2.7v to 20v; v out(min) : fixed to 1.8v, 2.5v, 3.3v, 3.6v; i q = 0.95a; 3mm 3mm dfn-10 and msop-10e packages LTC3632 45v, 20ma synchronous micropower buck converter v in : 4.5v to 45v, 60v max ; v out(min) : 0.8v to adj, 3.3v fixed, 5v fixed; i q = 12a; i sd < 1a; 3mm 3mm dfn-8 and msop-8e packages ltc3642 45v, 50ma synchronous micropower buck converter v in : 4.5v to 45v, 60v max ; v out(min) : 0.8v to adj, 3.3v fixed, 5v fixed; i q = 12a; i sd < 1a; 3mm 3mm dfn-8 and msop-8e packages lt8410/ lt8410-1 micropower 25ma/8ma low noise boost converter with integrated schottky diode and output disconnect v in : 2.6v to 16v; v out(min) = 40v max ; i q = 8.5a; i sd < 1a; 2mm 2mm dfn-8 package 31081 ta08 c1 tec hot cold c2 sw vs2 vs1 vstore v out2 v out2 pgood v out vldo c out pgd vldo + v out v out2_en ltc3108-1 vaux gnd 2.2f c store 5.25v vaux 1f 1nf 2.2v 3v 1:100 lpr6235-752sml 330pf on off c1 tec c2 sw vs2 vs1 vstore v out2 pgd vldo + v out v out2_en ltc3108-1 vaux gnd 1nf 1:100 lpr6235-752sml 330pf + + cold hot 499k 499k thermoelectric generator thermoelectric generator dual teg energy harvester operates from temperature differentials of either polarity

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