Electrical Specifications Subject to Change LTC3108 Ultralow Voltage Step-Up Converter and Power Manager FEATURES n n n n n n DESCRIPTION Operates from Inputs of 20mV Complete Energy Harvesting Power Management System - Selectable VOUT of 2.35V, 3.3V, 4.1V or 5V - LDO: 2.2V at 3mA - Logic Controlled Output - Reserve Energy Output Power Good Indicator Ultralow IQ: 6μA Uses Compact Step-Up Transformers Small 12-Lead (4mm × 3mm) DFN or 16-Lead SSOP Packages The LTC®3108 is a highly integrated DC/DC converter ideal for harvesting and managing surplus energy from extremely low input voltage sources such as TEG (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 provides a complete power management solution for wireless sensing and data acquisition. The 2.2V LDO powers an external microprocessor, while the main output is programmed to one of four fixed 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 efficiency design ensure the fastest possible charge times of the output reservoir capacitor. APPLICATIONS n n n n n n n Remote Sensors and Radio Power Surplus Heat Energy Harvesting HVAC Industrial Wireless Sensing Automatic Metering Building Automation Indoor Light Energy Harvesting The LTC3108 is available in a small, thermally enhanced 12-lead (4mm × 3mm) DFN package and a 16-lead SSOP package. 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. TYPICAL APPLICATION Wireless Remote Sensor Application Powered From a Peltier Cell VOUT Charge Time 1nF TEG C1 VSTORE LTC3108 330pF 100 PGOOD PGD 2.2V VLDO SW μP 2.2μF SENSORS VOUT VS2 3.3V + 470μF VS1 VOUT = 3.3V COUT = 470μF 0.1F 6.3V VOUT2 C2 20mV TO 500mV + + 220μF 1000 5V RF LINK TIME (sec) 1:100 + 10 1 1:100 Ratio 1:50 Ratio 1:20 Ratio VOUT2_EN VAUX GND 3108 TA01a 0 0 1μF 50 100 150 200 250 300 350 400 VIN (mV) 3108 TA01b 3108p 1 LTC3108 ABSOLUTE MAXIMUM RATINGS (Note 1) SW Voltage ..................................................–0.3V to 2V C1 Voltage....................................................–0.3V to 6V C2 Voltage.......................................................–8V to 8V VOUT2, VOUT2_EN ...........................................–0.3V to 6V VAUX....................................................15mA into VAUX VS1, VS2, VAUX, VOUT, PGD ........................–0.3V to 6V VLDO, VSTORE ............................................–0.3V to 6V Operating Temperature (Note 2).............. –40°C to 85°C Storage Temperature Range.................. –65°C to 125°C PIN CONFIGURATION TOP VIEW TOP VIEW VAUX 1 12 SW VSTORE 2 11 C2 VOUT 3 VOUT2 4 VLDO PGD 13 GND 10 C1 GND 1 16 GND VAUX 2 15 SW VSTORE 3 14 C2 VOUT 4 13 C1 9 VOUT2_EN VOUT2 5 12 VOUT2_EN 5 8 VS1 VLDO 6 11 VS1 6 7 VS2 PGD 7 10 VS2 GND 8 9 DE PACKAGE 12-LEAD (4mm s 3mm) PLASTIC DFN GND GN PACKAGE 16-LEAD PLASTIC SSOP NARROW TJMAX = 125°C, θJA = 43°C/W, θJC = 4.3°C/W EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB (NOTE 4) TJMAX = 125°C, θJA = 110°C/W, θJC = 40°C/W ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3108EDE#PBF LTC3108EDE#TRPBF LFJM 12-Lead (4mm × 3mm) Plastic DFN –40°C to 85°C LTC3108IDE#PBF LTC3108IDE#TRPBF LFJM 12-Lead (4mm × 3mm) Plastic DFN –40°C to 85°C LTC3108EGN#PBF LTC3108EGN#TRPBF 3108 16-Lead Plastic SSOP –40°C to 85°C LTC3108IGN#PBF LTC3108IGN#TRPBF 3108 16-Lead Plastic SSOP –40°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified 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 specifications, go to: http://www.linear.com/tapeandreel/ 3108p 2 LTC3108 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the specified operating junction temperature range of –40°C to 85°C, otherwise specifications are at TA = 25°C. VAUX = 5V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX Minimum Start-Up Voltage No-Load Input Current Using 1:100 Transformer Turns Ratio 20 50 Using 1:100 Transformer Turns Ratio; VIN = 20mV, VOUT2_EN = 0V; All Outputs Charged and in Regulation 3 Input Voltage Range Using 1:100 Transformer Turns Ratio l VSTARTUP Output Voltage VS1 = VS2 = GND VS1 = VAUX, VS2 = GND VS1 = GND, VS2 = VAUX VS1 = VS2 = VAUX l 2.30 3.234 4.018 4.90 VOUT Quiescent Current VOUT = 3.3V, VOUT2_EN = 0V VAUX Quiescent Current No Load, All Outputs Charged LDO Output Voltage 0.5mA Load LDO Load Regulation For 0mA to 2mA Load LDO Line Regulation For VAUX from 2.5V to 5V LDO Dropout Voltage ILDO = 2mA l 2.156 l UNITS mV mA 500 2.350 3.300 4.100 5.000 mV 2.40 3.366 4.182 5.10 V V V V 0.4 μA 6 μA 2.2 2.244 V 0.5 1 % 0.01 0.02 % 100 200 mV 4 6 mA LDO Current Limit l VOUT Current Limit l 4.5 7 mA VSTORE Current Limit l 4.5 7 mA VAUX Clamp Voltage Current into VAUX = 5mA l 5.25 5.5 VSTORE Leakage Current VSTORE = 5V VOUT2 Leakage Current VOUT2 = 0V, VOUT2_EN = 0V l VS1, VS2 Threshold Voltage VS1, VS2 Input Current 3 5 0.4 VS1 = VS2 = 5V V 0.1 μA 1 μA 0.85 1.2 V 0.01 0.1 μA PGOOD Threshold (Rising) Measured Relative to the VOUT Voltage –7 % PGOOD Threshold (Falling) Measured Relative to the VOUT Voltage –9 % PGOOD VOL Sink Current = 100μA 0.3 0.5 V PGOOD VOH Source Current = 0 2.2 2.3 V 2.1 PGOOD Pull-Up Resistance VOUT2_EN Threshold Voltage 1 VOUT2_EN Rising l 0.4 1 MΩ 1.3 V VOUT2_EN Pull-Down Resistance 5 MΩ VOUT2 Turn-On Time 5 μs 0.15 μs VOUT2 Turn-Off Time (Note 3) l VOUT2 Current Limit 0.2 0.3 0.5 A VOUT2 Current Limit Response Time (Note 3) 350 ns VOUT2 P-Channel MOSFET On-Resistance VOUT = 3.3V (Note 3) 1.3 Ω N-Channel MOSFET On-Resistance C2 = 5V (Note 3) 0.5 Ω 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 LTC3108E is guaranteed to meet performance specifications from 0°C to 85°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3108I is guaranteed to meet specifications over the full –40°C to 85°C operating temperature range. Note 3: Specification 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 43°C/W. 3108p 3 LTC3108 TYPICAL PERFORMANCE CHARACTERISTICS IOUT and Efficiency vs VIN, 1:50 Ratio Transformer IOUT and Efficiency vs VIN, 1:20 Ratio Transformer EFFICIENCY IOUT (μA) 50 1500 40 1200 30 1000 20 60 EFFICIENCY 50 40 IOUT 900 30 600 20 300 10 EFFICIENCY (%) 1500 1800 EFFICIENCY (%) 2000 60 IOUT (μA) 3000 2500 TA = 25°C, unless otherwise noted. IOUT 500 10 0 400 0 100 0 200 300 VIN (mV) 0 100 200 300 VIN (mV) 3108 G01 IOUT and Efficiency vs VIN, 1:100 Ratio Transformer 60 12 50 10 3108 G02 IOUT vs VIN and Source Resistance, 1:20 Ratio Input Resistance vs VIN (VOUT Shorted) 1200 10000 1Ω 2Ω 3Ω 5Ω 10Ω 15Ω 600 30 400 20 200 10 0 100 0 200 VIN (mV) 300 8 1:50 RATIO 1000 1:20 RATIO IVOUT (μA) IOUT (μA) 40 EFFICIENCY EFFICIENCY (%) 800 INPUT RESISTANCE (Ω) IOUT 1000 6 4 100 1:100 RATIO 2 0 400 0 20 40 60 IOUT vs VIN and Source Resistance, 1:100 Ratio 1000 100 100 20 40 60 80 100 120 140 160 180 200 VIN OPEN CIRCUIT (mV) 3108 G06 300 3108 G05 1.5" SQUARE 100 10 1 150 200 250 100 VIN OPEN CIRCUIT (mV) IOUT vs dT and TEG Size, 1:100 Ratio IOUT (μA) IOUT (μA) 1 50 1000 1Ω 2Ω 3Ω 5Ω 10Ω 15Ω IVOUT (μA) 1000 1Ω 2Ω 3Ω 5Ω 10Ω 15Ω 10 80 100 120 140 160 180 200 VIN (mV) 3108 G04 3108 G03 IOUT vs VIN and Source Resistance, 1:50 Ratio 10 0 400 0 0.625" SQUARE 10 20 40 60 80 100 120 140 160 180 200 VIN OPEN CIRCUIT (mV) 3108 G07 1 0.1 1 10 dT ACROSS TEG (°C) 100 3108 G08 3108p 4 LTC3108 TYPICAL PERFORMANCE CHARACTERISTICS Resonant Switching Waveforms TA = 25°C, unless otherwise noted. LDO Dropout Voltage LDO Load Regulation 0.00 VIN = 20mV 1:100 RATIO TRANSFORMER 0.20 0.18 0.16 DROP IN VLDO (%) SW PIN 50mV/DIV DROPOUT VOLTAGE (V) –0.25 C1 PIN 2V/DIV C2 PIN 2V/DIV –0.50 –0.75 10μs/DIV 0.14 0.12 0.10 0.08 0.06 0.04 3108 G09 0.02 0.00 –1.00 0 0.5 1 1.5 2 2.5 LDO LOAD (mA) 3 3.5 0 4 0.5 1 1.5 2 2.5 LDO LOAD (mA) 3 3.5 VOUT and PGD Response During a Step Load Start-Up Voltage Sequencing VIN = 50mV 1:100 RATIO TRANSFORMER COUT = 220μF CSTORE = 470μF CLDO = 2.2μF CH2, VOUT 1V/DIV CH3, VLDO 1V/DIV 10sec/DIV VOUT Ripple 50mA LOAD STEP COUT = 220μF CH1 VSTORE 1V/DIV 4 3108 G11 3108 G10 30μA LOAD COUT = 220μF CH2 VOUT 1V/DIV 20mV/ DIV CH1 PGD 1V/DIV 3108 G12 5ms/DIV 3108 G13 100ms/DIV Enable Input and VOUT2 LDO Step Load Response 3108 G14 Running on Storage Capacitor CH3 VSTORE 1V/DIV CSTORE = 470μF VOUT LOAD = 100μA CH2, VOUT 1V/DIV CH2, VOUT2 1V/DIV CH4, VLDO 1V/DIV CH1, VIN 50mV/DIV CH1 VOUT2_EN 1V/DIV 1ms/DIV 3108 G16 5sec/DIV 3108 G17 10mA LOAD ON VOUT2 COUT = 220μF 3108p 5 LTC3108 PIN FUNCTIONS (DFN/SSOP) VAUX (Pin 1/Pin 2): Output of the Internal Rectifier Circuit and VCC for the IC. Bypass VAUX with at least 1μF 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. VOUT (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. VOUT2 (Pin 4/Pin 5): Switched Output of the Converter. Connect this pin to a switched load. This output is open until VOUT2_EN is driven high, then it is connected to VOUT through a 1.3Ω P-channel switch. If not used, this pin should be left open or tied to VOUT. 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 2.2μF 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 VOUT is within 7% of its programmed value, PGD will be pulled up to VLDO through a 1MΩ resistor. If VOUT drops 9% below its programmed value PGD will go low. This pin can sink up to 100μA. VS1 (Pin 8/Pin 11): VOUT Select Pin 1. Connect this pin to ground or VAUX to program the output voltage (see Table 1). VOUT2_EN (Pin 9/Pin 12): Enable Input for VOUT2. VOUT2 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 Rectifier 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 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 VOUT GND GND 2.35V GND VAUX 3.3V VAUX GND 4.1V VAUX VAUX 5V VS2 (Pin 7/Pin 10): VOUT Select Pin 2. Connect this pin to ground or VAUX to program the output voltage (see Table 1). 3108p 6 LTC3108 BLOCK DIAGRAM (DFN Package Shown) LTC3108 VOUT2 1.3Ω VOUT2 ILIM VOUT2_EN SYNC RECTIFY REFERENCE 1.2V VREF 5M C1 1:100 VIN VOUT C1 VOUT CIN COUT 5.25V C2 + – C2 SW OFF ON VOUT SW VHOLD VS1 CHARGE CONTROL VS2 0.5Ω VOUT PROGRAM VREF VLDO 1M – + VAUX 1μF PGD PGOOD VSTORE VOUT GND (SSOP) VBEST VREF EXPOSED PAD (DFN) LDO CSTORE VLDO 3108 BD 2.2V 2.2μF OPERATION (Refer to the Block Diagram) The LTC3108 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 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 measurements and transmit data. The LTC3108 can also be used to trickle charge a standard capacitor, supercapacitor or rechargeable battery, using energy harvested from a Peltier or photovoltaic cell. 3108p 7 LTC3108 OPERATION Oscillator Synchronous Rectifiers The LTC3108 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 20kHz to 200kHz. 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. Once VAUX exceeds 2V, synchronous rectifiers in parallel with each of the internal diodes take over the job of rectifying the input voltage, improving efficiency. Charge Pump and Rectifier The AC voltage produced on the secondary winding of the transformer is boosted and rectified using an external charge pump capacitor (from the secondary winding to pin C1) and the rectifiers internal to the LTC3108. The rectifier circuit feeds current into the VAUX pin, providing charge to the external VAUX capacitor and the other outputs. VAUX The active circuits within the LTC3108 are powered from VAUX, which should be bypassed with a 1μF capacitor. Larger capacitor values will reduce the ripple on VAUX but increase the time it takes for VAUX to rise and the other outputs to become active. Once VAUX exceeds 2.5V, the main VOUT 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. Low Dropout Linear Regulator (LDO) The LTC3108 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 VOUT. This enables it to become active as soon as VAUX has charged to 2.3V, while the VOUT storage capacitor is still charging. In the event of a step load on the LDO output, current can come from the main VOUT capacitor if VAUX drops below VOUT. The LDO requires a 2.2μF 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 typical. VOUT The main output voltage on VOUT is charged from the VAUX supply, and is user programmed to one of four regulated voltages using the voltage select pins VS1 and VS2, according 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 VOUT GND GND 2.35V GND VAUX 3.3V VAUX GND 4.1V VAUX VAUX 5V Voltage Reference 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 VOUT has reached the proper value, the charging current is turned off. The LTC3108 includes a precision, micropower reference, for accurate regulated output voltages. This reference becomes active as soon as VAUX exceeds 2V. The internal programmable resistor divider sets VOUT, eliminating the need for very high value external resistors that are susceptible to board leakage. 3108p 8 LTC3108 OPERATION In a typical application, a storage capacitor (typically a few hundred microfarads) is connected to VOUT. As soon as VAUX exceeds 2.5V, the VOUT 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 4mA. PGOOD A power good comparator monitors the VOUT voltage. The PGD pin is an open-drain output with a weak pull-up (1MΩ) to the LDO voltage. Once VOUT has charged to within 7% of its regulated voltage, the PGD output will go high. If VOUT 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 configuration with other circuitry. VOUT2 VOUT2 is an output that can be turned on and off by the host, using the VOUT2_EN pin. When enabled, VOUT2 is connected to VOUT through a 1.3Ω P-channel MOSFET switch. This output, controlled by a host processor, can be used to power external circuits such as sensors and amplifiers, that do not have a low power sleep or shutdown capability. VOUT2 can be used to power these circuits only when they are needed. Minimizing the amount of decoupling capacitance on VOUT2 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/transmitter. A small VOUT2 capacitor will also minimize the energy that will be wasted in charging the capacitor every time VOUT2 is enabled. VOUT2 has a soft-start time of about 5μs to limit capacitor charging current and minimize glitching of the main output when VOUT2 is enabled. It also has a current limiting circuit that limits the peak current to 0.3A typical. The VOUT2 enable input has a typical threshold of 1V with 100mV of hysteresis, making it logic-compatible. If VOUT2_EN (which has an internal pull-down resistor) is low, VOUT2 will be off. Driving VOUT2_EN high will turn on the VOUT2 output. Note that while VOUT2_EN is high, the current limiting circuitry for VOUT2 draws an extra 8μA of quiescent current from VOUT. This added current draw has a negligible effect on the application and capacitor sizing, since the load on the VOUT2 output, when enabled, is likely to be orders of magnitude higher than 8μA. VSTORE The VSTORE output can be used to charge a large storage capacitor or rechargeable battery after VOUT has reached regulation. Once VOUT 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 VOUT, VOUT2 and LDO outputs. If VAUX drops below VSTORE, the LTC3108 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 VOUT 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 VOUT. Any pulse load on VOUT must be handled by the VOUT capacitor. Short-Circuit Protection All outputs of the LTC3108 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. 3108p 9 LTC3108 OPERATION 5.0 VSTORE (V) 2.5 0 3.0 2.0 PGD (V) 1.0 VOLTAGE (V) 0 5.0 2.5 VOUT (V) 0 3.0 2.0 VLDO (V) 1.0 0 5.0 VAUX (V) 2.5 0 0 10 20 30 40 50 60 70 80 TIME (ms) 3108 F01a Figure 1. Output Voltage Sequencing (with VOUT Programmed for 3.3V) 3108p 10 LTC3108 APPLICATIONS INFORMATION Introduction 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. The LTC3108 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 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 during 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. 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 source’s ESR and the peak primary switching current (which can reach hundreds of milliamps). The time constant of the filter 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 junctions, sandwiched between two parallel ceramic plates. Although Peltier cells are often used as coolers by applying 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 temperature 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 this manner, a Peltier cell is referred to as a thermoelectric generator (TEG). The PGD signal can be used to enable a sleeping microprocessor or other circuitry when VOUT reaches regulation, indicating that enough energy is available for a burst. Input Voltage Sources The LTC3108 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. 10 RS = 2Ω MAX POUT 0.2 1 VOC 0.1 0.1 0 MAXIMUM POUT (mW) VOPEN_CKT (V) 0.3 0.01 0 5 15 10 ΔT (°C) 20 25 3108 F02 Figure 2. Typical Performance of a Peltier Cell Acting as a Thermoelectric Generator 3108p 11 LTC3108 APPLICATIONS INFORMATION The low voltage capability of the LTC3108 design allows it to operate from a TEG with temperature differentials as low as 1°C, making it ideal for harvesting energy in applications in which a temperature difference exists between two surfaces or between a surface and the ambient temperature. The internal resistance (ESR) of most cells is in the range of 1Ω to 5Ω, allowing for reasonable power transfer. The curves in Figure 2 show the opencircuit output voltage and maximum power transfer for a typical Peltier cell (with an ESR of 2Ω) over a 20°C range of temperature differential. TEG Load Matching The LTC3108 was designed to present a minimum input resistance (load) in the range of 2Ω to 10Ω, 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 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. Thermopile Generator Thermopile generators (also called powerpile generators) are made up of a number of series-connected thermocouples 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 temperature of a flame. Typical examples are the Honeywell CQ200 and Q313. These devices have an internal series resistance of less than 3Ω, 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. Because of the higher output voltages possible with a thermopile generator, a much lower transformer turns ratio can be used (typically 1:20, depending on the application). Photovoltaic Cell The LTC3108 converter can also operate from a single photovoltaic (solar) cell operating in indoor lighting, where the available power is orders of magnitude less than in direct sunlight. This capability allows the LTC3108 to power circuits at light levels far too low for even very low input voltage boost converters. Peltier Cell (TEG) Suppliers Non-Boost Applications 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. The LTC3108 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. Table 3. Peltier Cell Manufacturers Any source whose peak voltage exceeds 2.5V AC or 5V DC can be connected to the C1 input through a currentlimiting resistor where it will be rectified/peak detected. In these applications the C2 and SW pins are not used and can be grounded or left open. Fujitaka www.fujitaka.com/pub/peltier/english/thermoelectric_power.html FerroTec www.ferrotec.com/products/thermal/modules Melcor www.melcor.com/tec.html Micropelt www.micropelt.com Nextreme www.nextreme.com TE Technology www.tetech.com/Peltier-Thermoelectric-Cooler-Modules.html Tellurex www.tellurex.com 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 100Ω/V should be used to limit the maximum current into the VAUX shunt regulator. 3108p 12 LTC3108 APPLICATIONS INFORMATION output current capability. Refer to the Typical Applications schematic examples for the recommended value for a given turns ratio. 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 efficiency. The secondary winding inductance will determine the resonant frequency of the oscillator, according to the following formula. Frequency = 1 2 • π • 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 winding’s shunt capacitance. The recommended resonant frequency is in the range of 20kHz to 200kHz. See Table 4 for some recommended transformers. Table 4. Recommended Transformers VENDOR PART NUMBER Coilcraft www.coilcraft.com LPR6235-752SML (1:100 Ratio) LPR6235-253PML (1:20 Ratio) LPR6235-123QML (1:50 Ratio) C1 Capacitor The charge pump capacitor that is connected from the transformer’s secondary winding to the C1 pin has an effect on converter input resistance and maximum output current capability. Generally, a minimum value of 1nF is recommended when operating from very low input voltages 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 VOUT and VSTORE Capacitor For pulsed load applications, the VOUT 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 VOUT by VS1 and VS2. COUT (mF) ≥ ILOAD(mA) • t PULSE (sec) Δ VOUT Note that there must be enough energy available from the input voltage source for VOUT 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 (thousands of microfarads or even Farads), to provide holdup 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 VOUT), so ensure that the holdup capacitor has a working voltage rating of at least 5.5V at the temperature for which it will be used. The VSTORE capacitor can be sized using the following: ⎡6µA + IQ + ILDO + (IBURST • t • f)⎤⎦ • TSTORE C STORE ≥ ⎣ 5 . 25 − VOUT Where 6μA is the quiescent current of the LTC3108, IQ is the load on VOUT in between bursts, ILDO is the load on the LDO between bursts, IBURST 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 VOUT is the output voltage required. Note that for a programmed output voltage of 5V, the VSTORE capacitor cannot provide any beneficial storage time. 3108p 13 LTC3108 APPLICATIONS INFORMATION To minimize losses and capacitor charge time, all capacitors used for VOUT and VSTORE should be low leakage. See Table 5 for recommended storage capacitors. Due to the very low input voltage the circuit may operate from, the connections to VIN, the primary of the transformer and the SW and GND pins of the LTC3108 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 efficiency and increase capacitor charge time. Table 5. 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/Bussman 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) Also, due to the low charge currents available at the outputs of the LTC3108, any sources of leakage current on the output voltage pins must be minimized. An example board layout is shown in Figure 3. Design Example 1 This design example will explain how to calculate the necessary storage capacitor value for VOUT in pulsed load applications, such as a wireless sensor/transmitter. 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 occurring periodically during a transmit burst. The storage capacitor on VOUT supports the load during the transmit burst, and the long sleep time between bursts allows the LTC3108 to recharge 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. VIN VAUX VSTORE VOUT VOUT VOUT2 VOUT2 VLDO VLDO PGOOD PGD 1 12 2 11 3 10 4 9 5 8 6 7 SW C2 C1 VOUT2_EN VS1 VS2 GND 3108 FO3 VIAS TO GROUND PLANE Figure 3. Example Component Placement for Two-Layer PC Board (DFN Package) 3108p 14 LTC3108 APPLICATIONS INFORMATION the capacitor. A method for calculating the maximum rate at which the load pulses can occur for a given output current from the LTC3108 will also be shown. In this example, VOUT is set to 3.3V, and the maximum allowed voltage droop during a transmit burst is 10%, or 0.33V. 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 VOUT is: 40mA • 1ms = 121µF 0 . 33V addition, use the value of 150μF for the VOUT capacitor. The maximum transmit rate (neglecting the duration of the transmit burst, which is typically very short) is then given by: t= 150 µF • 0 . 33V = 1 . 5 sec or f MAX = 0 . 666Hz (50 µA − 17µA) Therefore, in this application example, the circuit can support a 1ms transmit burst every 1.5 seconds. 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. 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 10μA and the sleep current was only 5μA, it could still transmit a burst every ten seconds. A standard value of 150μF or larger could be used for COUT in this case. Note that the load current is the total current draw on VOUT, VOUT2 and VLDO, since the current for all of these outputs must come from VOUT 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 is negligible compared to the magnitude of the load current during the burst. The following formula enables the user to calculate the time it will take to charge the LDO output capacitor and the VOUT capacitor the first time, from 0V. Here again, the charge current available from the LTC3108 must be known. For this calculation, it is assumed that the LDO output capacitor is 2.2μF. 2 . 2V • 2 . 2 µF t LDO = ICHG − ILDO COUT (µF) ≥ To calculate the maximum rate at which load bursts can occur, determine how much charge current is available from the LTC3108 VOUT pin given the input voltage source being used. This number is best found empirically, since there are many factors affecting the efficiency of the converter. Also determine what the total load current is on VOUT 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 is 50μA and the total current drawn on VOUT in the sleep state is 17μA, including capacitor leakage. In If there were 50μA of charge current available and a 5μA load on the LDO (when the processor is sleeping), the time for the LDO to reach regulation would be 107ms. If VOUT were programmed to 3.3V and the VOUT capacitor was 150μF, the time for VOUT to reach regulation would be: 3 . 3V • 150 µF t VOUT = + t LDO ICHG − I VOUT If there were 50μA of charge current available and 5μA of load on VOUT, the time for VOUT to reach regulation after the initial application of power would be 11.1 seconds. 3108p 15 LTC3108 APPLICATIONS INFORMATION Design Example 2 Therefore, if the LTC3108 has an input voltage that allows it to supply a charge current greater than 5.14μA, the application can support 100mA bursts lasting 5ms every hour. It can be determined that the sleep current of 5μA is the dominant factor because the transmit duty cycle is so small (0.00014%). Note that for a VOUT of 3.3V, the average power required by this application is only 17μW (not including converter losses). In many pulsed load applications, the duration, magnitude and frequency of the load current bursts are known and fixed. In these cases, the average charge current required from the LTC3108 to support the average load must be calculated, which can be easily done by the following: ICHG ≥ IQ + IBURST • t T Note that the charge current available from the LTC3108 has no effect on the sizing of the VOUT capacitor (if it is assumed that the load current during a burst is much larger than the charge current), and the VOUT capacitor has no effect on the maximum allowed burst rate. Where IQ is the sleep current on VOUT required by the external circuitry in between bursts (including cap leakage), IBURST 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, IQ = 5μA, IBURST = 100mA, t = 5ms and T = one hour. The average charge current required from the LTC3108 would be: ICHG ≥ 5µA + 100 mA • 0 . 005 sec = 5 . 14 µA 3600 sec TYPICAL APPLICATIONS Peltier-Powered Energy Harvester for Remote Sensor Applications 1:100 T1 + 1nF VSTORE CIN VOUT2 330pF LTC3108 VLDO SW COOPER BUSSMAN PB-5ROH104-R OR KR-5R5H104-R CSTORE 0.1F 3.3V 6.3V VOUT2 PGOOD PGD C2 ΔT = 1°C TO 20°C + C1 + TEG 5V SENSORS μP 2.2V 2.2μF VOUT XMTR 3.3V + VS2 COUT VS1 VOUT2_EN GND VAUX OFF ON T1: COILCRAFT LPR6235-752SML 1μF 3108 TA02 3108p 16 LTC3108 TYPICAL APPLICATIONS Li-Ion Battery Charger and LDO Powered by a Solar Cell with Indoor Lighting T1 1:20 + 0.047μF C1 VSTORE + SOLAR CELL 220μF VOUT2 330pF – LTC3108 C2 PGD SW VOUT T1: COILCRAFT LPR6235-253PML 2.2V VLDO VS2 VS1 4.1V VLDO VOUT 2.2μF Li-Ion VOUT2_EN VAUX GND 1μF 3108 TA03 Supercapacitor Charger and LDO Powered by a Thermopile Generator HONEYWELL CQ200 THERMOPILE 0.047μF 1:20 C1 VSTORE + 220μF VOUT2 330pF LTC3108 C2 SW PGD PGOOD 2.2V VLDO VOUT VS2 VS1 2.35V + 2.2μF 150mF 2.5V VOUT2_EN VAUX VLDO VOUT GND CAP-XX GZ115F 1μF 3108 TA04 AC Input Energy Havester and Power Manager DC Input Energy Havester and Power Manager RIN RIN > 100Ω / V C1 + – VSTORE VIN VIN > 5V VOUT2 PGD C2 SW CSTORE VOUT2 2.2V 3.3V + VOUT COUT VS1 VOUT2_EN VAUX GND 1μF VOUT2_ENABLE VSTORE AC VIN VIN > 5VP-P VOUT2 - PIEZO - 60Hz PGOOD VLDO VOUT C1 + LTC3108 VS2 RIN CIN RIN > 100Ω/ V 5V VLDO 2.2μF PGD + PGOOD 2.2V VLDO C2 VOUT SW CSTORE VOUT2 LTC3108 5V + VOUT VLDO 2.2μF COUT VS2 VOUT2_EN VS1 VAUX 3108 TA05 5V VOUT2_ENABLE GND 1μF 3108 TA06 3108p 17 LTC3108 PACKAGE DESCRIPTION UE/DE Package 12-Lead Plastic DFN (4mm × 3mm) (Reference LTC DWG # 05-08-1695) 0.70 p0.05 3.60 p0.05 2.20 p0.05 3.30 p0.05 1.70 p 0.05 PACKAGE OUTLINE 0.25 p 0.05 0.50 BSC 2.50 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 p0.10 (2 SIDES) 7 R = 0.115 TYP 0.40 p 0.10 12 R = 0.05 TYP PIN 1 TOP MARK (NOTE 6) 0.200 REF 3.00 p0.10 (2 SIDES) 0.75 p0.05 3.30 p0.10 1.70 p 0.10 6 0.25 p 0.05 PIN 1 NOTCH R = 0.20 OR 0.35 s 45o CHAMFER 1 (UE12/DE12) DFN 0806 REV D 0.50 BSC 2.50 REF 0.00 – 0.05 BOTTOM VIEW—EXPOSED PAD 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 3108p 18 LTC3108 PACKAGE DESCRIPTION GN Package 16-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641) .189 – .196* (4.801 – 4.978) .045 ±.005 16 15 14 13 12 11 10 9 .254 MIN .009 (0.229) REF .150 – .165 .229 – .244 (5.817 – 6.198) .0165 ± .0015 .150 – .157** (3.810 – 3.988) .0250 BSC RECOMMENDED SOLDER PAD LAYOUT 1 .015 ± .004 × 45° (0.38 ± 0.10) .007 – .0098 (0.178 – 0.249) 2 3 4 5 6 7 .0532 – .0688 (1.35 – 1.75) 8 .004 – .0098 (0.102 – 0.249) 0° – 8° TYP .016 – .050 (0.406 – 1.270) NOTE: 1. CONTROLLING DIMENSION: INCHES INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) .008 – .012 (0.203 – 0.305) TYP .0250 (0.635) BSC GN16 (SSOP) 0204 3. DRAWING NOT TO SCALE *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 3108p 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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 19 LTC3108 TYPICAL APPLICATION Dual TEG Energy Harvester Operates from Temperature Differentials of Either Polarity 1nF HOT 1:100 C1 + TEG TEC VSTORE VOUT2 330pF LTC3108 C2 COLD LPR6235-752SML PGD VLDO SW VS2 VS1 VOUT 5V + PGOOD 2.2V 3.3V + VOUT2_EN VAUX CSTORE VOUT2 VLDO VOUT 2.2μF COUT GND OFF ON 1μF VAUX 1nF 1:100 COLD TEC TEG C1 VSTORE + VOUT2 330pF LTC3108 C2 HOT LPR6235-752SML SW VS2 VS1 PGD VLDO VOUT VOUT2_EN VAUX GND 3108 TA07 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC1041 Bang-Bang Controller VIN: 2.8V to 16V; VOUT(MIN) = Adj; IQ = 1.2mA; ISD < 1μA; SO-8 Package LTC1389 Nanopower Precision Shunt Voltage Reference VOUT(MIN) = 1.25V; IQ = 0.8μA; SO-8 Package LT1672/LT1673/ LT1674 Single-/Dual-/Quad-Precision 2μA Rail-to-Rail Op Amps SO-8, SO-14 and MSOP-8 Packages LT3009 3μA IQ, 20mA Linear Regulator VIN: 1.6V to 20V; VOUT(MIN): 0.6V to Adj, 1.2V, 1.5V, 1.8V, 2.5V, 3.3V, 5V to Fixed; IQ = 3μA; ISD < 1μA; 2mm × 2mm DFN-8 and SC70 Packages LTC3525L-3/ LTC3525L-3.3/ LTC3525L-5 400mA (ISW), Synchronous Step-Up DC/DC Converter with Output Disconnect VIN: 0.7V to 4V; VOUT(MIN) = 5VMAX; IQ = 7μA; ISD < 1μA; SC70 Package LTC3588 Piezoelectric Energy Generator with Integrated High Efficiency Buck Converter VIN: 2.6V to 19.2V; VOUT(MIN): Fixed to 2.5V, 3V, 3.3V, 3.6V; IQ = 0.95μA; 3mm × 3mm DFN-10 and MSOP-10E Packages LTC3631 45V, 100mA Synchronous MicroPower Buck Converter VIN: 4.5V to 45V, 60VMAX; VOUT(MIN): 0.8V to Adj, 3.3V Fixed, 5V Fixed; IQ = 12μA; ISD < 1μA; 3mm × 3mm DFN-8 and MSOP-8E Packages LTC3632 45V, 20mA Synchronous MicroPower Buck Converter VIN: 4.5V to 45V, 60VMAX; VOUT(MIN): 0.8V to Adj, 3.3V Fixed, 5V Fixed; IQ = 12μA; ISD < 1μA; 3mm × 3mm DFN-8 and MSOP-8E Packages LTC3642 45V, 50mA Synchronous MicroPower Buck Converter VIN: 4.5V to 45V, 60VMAX; VOUT(MIN): 0.8V to Adj, 3.3V Fixed, 5V Fixed; IQ = 12μA; ISD < 1μA; 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 VIN: 2.6V to 16V; VOUT(MIN) = 40VMAX; IQ = 8.5μA; ISD < 1μA; 2mm × 2mm DFN-8 Package 3108p 20 Linear Technology Corporation LT 1109 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2009