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Electrical Specifications Subject to Change
LTC3108 Ultralow Voltage Step-Up Converter and Power Manager FEATURES
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DESCRIPTION
The LTC(R)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. The LTC3108 is available in a small, thermally enhanced 12-lead (4mm x 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.
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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: 6A Uses Compact Step-Up Transformers Small 12-Lead (4mm x 3mm) DFN or 16-Lead SSOP Packages
APPLICATIONS
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Remote Sensors and Radio Power Surplus Heat Energy Harvesting HVAC Industrial Wireless Sensing Automatic Metering Building Automation Indoor Light Energy Harvesting
TYPICAL APPLICATION
Wireless Remote Sensor Application Powered From a Peltier Cell
1nF 1:100 + TEG C1 330pF C2 20mV TO 500mV SW VSTORE LTC3108 VOUT2 PGOOD PGD 2.2V VLDO 5V 1000
VOUT Charge Time
VOUT = 3.3V COUT = 470F
+
220F
+
0.1F 6.3V 100 TIME (sec) P 2.2F SENSORS
10
VS2
VOUT
3.3V
+
470F
RF LINK
1 1:100 Ratio 1:50 Ratio 1:20 Ratio 0 50 100 150 200 250 300 350 400 VIN (mV)
3108 TA01b
VS1 VAUX
VOUT2_EN GND 1F
3108 TA01a
0
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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).............. -40C to 85C Storage Temperature Range.................. -65C to 125C
PIN CONFIGURATION
TOP VIEW GND VAUX VSTORE VOUT VOUT2 VLDO PGD 1 2 3 4 5 6 13 GND 12 SW 11 C2 10 C1 9 8 7 VOUT2_EN VS1 VS2 VAUX VSTORE VOUT VOUT2 VLDO PGD GND 1 2 3 4 5 6 7 8 TOP VIEW 16 GND 15 SW 14 C2 13 C1 12 VOUT2_EN 11 VS1 10 VS2 9 GND
DE PACKAGE 12-LEAD (4mm 3mm) PLASTIC DFN TJMAX = 125C, JA = 43C/W, JC = 4.3C/W EXPOSED PAD (PIN 13) IS GND, MUST BE SOLDERED TO PCB (NOTE 4)
GN PACKAGE 16-LEAD PLASTIC SSOP NARROW TJMAX = 125C, JA = 110C/W, JC = 40C/W
ORDER INFORMATION
LEAD FREE FINISH LTC3108EDE#PBF LTC3108IDE#PBF LTC3108EGN#PBF LTC3108IGN#PBF TAPE AND REEL LTC3108EDE#TRPBF LTC3108IDE#TRPBF LTC3108EGN#TRPBF LTC3108IGN#TRPBF PART MARKING* LFJM LFJM 3108 3108 PACKAGE DESCRIPTION 12-Lead (4mm x 3mm) Plastic DFN 12-Lead (4mm x 3mm) Plastic DFN 16-Lead Plastic SSOP 16-Lead Plastic SSOP TEMPERATURE RANGE -40C to 85C -40C to 85C -40C to 85C -40C to 85C
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/
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LTC3108 ELECTRICAL CHARACTERISTICS
PARAMETER Minimum Start-Up Voltage No-Load Input Current Input Voltage Range Output Voltage CONDITIONS Using 1:100 Transformer Turns Ratio Using 1:100 Transformer Turns Ratio; VIN = 20mV, VOUT2_EN = 0V; All Outputs Charged and in Regulation Using 1:100 Transformer Turns Ratio VS1 = VS2 = GND VS1 = VAUX, VS2 = GND VS1 = GND, VS2 = VAUX VS1 = VS2 = VAUX VOUT = 3.3V, VOUT2_EN = 0V No Load, All Outputs Charged 0.5mA Load For 0mA to 2mA Load For VAUX from 2.5V to 5V ILDO = 2mA
l l l l l l l l
The l denotes the specifications which apply over the specified operating junction temperature range of -40C to 85C, otherwise specifications are at TA = 25C. VAUX = 5V, unless otherwise noted.
MIN TYP 20 3 VSTARTUP 2.30 3.234 4.018 4.90 2.350 3.300 4.100 5.000 0.4 6 2.156 2.2 0.5 0.01 100 3 4 4.5 4.5 5 5.25 0.1 1
l
MAX 50
UNITS mV mA
500 2.40 3.366 4.182 5.10
mV V V V V A A
VOUT Quiescent Current VAUX Quiescent Current LDO Output Voltage LDO Load Regulation LDO Line Regulation LDO Dropout Voltage LDO Current Limit VOUT Current Limit VSTORE Current Limit VAUX Clamp Voltage VSTORE Leakage Current VOUT2 Leakage Current VS1, VS2 Threshold Voltage VS1, VS2 Input Current PGOOD Threshold (Rising) PGOOD Threshold (Falling) PGOOD VOL PGOOD VOH PGOOD Pull-Up Resistance VOUT2_EN Threshold Voltage VOUT2_EN Pull-Down Resistance VOUT2 Turn-On Time VOUT2 Turn-Off Time VOUT2 Current Limit VOUT2 Current Limit Response Time VOUT2 P-Channel MOSFET On-Resistance N-Channel MOSFET On-Resistance
2.244 1 0.02 200 6 7 7 5.5
V % % mV mA mA mA V A A
Current into VAUX = 5mA VSTORE = 5V VOUT2 = 0V, VOUT2_EN = 0V
0.4
0.85 0.01 -7 -9 0.3
1.2 0.1
V A % %
VS1 = VS2 = 5V Measured Relative to the VOUT Voltage Measured Relative to the VOUT Voltage Sink Current = 100A Source Current = 0 VOUT2_EN Rising
l
0.5 2.3 1.3
V V M V M s s
2.1 0.4
2.2 1 1 5 5
(Note 3)
l
0.15 0.2 0.3 350 1.3 0.5 with statistical process controls. The LTC3108I is guaranteed to meet specifications over the full -40C to 85C 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 43C/W. 0.5
A ns
(Note 3) VOUT = 3.3V (Note 3) C2 = 5V (Note 3)
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 0C to 85C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation
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LTC3108 TYPICAL PERFORMANCE CHARACTERISTICS
IOUT and Efficiency vs VIN, 1:20 Ratio Transformer
3000 2500 2000 IOUT (A) 1500 1000 IOUT 500 0 0 100 200 VIN (mV) 300 10 0 400
3108 G01
TA = 25C, unless otherwise noted.
IOUT and Efficiency vs VIN, 1:50 Ratio Transformer
60 1800 1500 1200 IOUT (A) IOUT 900 600 300 0 0 100 200 VIN (mV) 300 30 20 10 0 400
3108 G02
60 EFFICIENCY 50 40 EFFICIENCY (%)
EFFICIENCY
50 40 30 20 EFFICIENCY (%)
IOUT and Efficiency vs VIN, 1:100 Ratio Transformer
1200 IOUT 1000 800 IOUT (A) 600 400 200 0 0 100 200 VIN (mV) 300 EFFICIENCY 50 INPUT RESISTANCE () 40 30 20 10 0 400
3108 G03
Input Resistance vs VIN (VOUT Shorted)
60 12 10 8 6 4 2 0 20 40 60 80 100 120 140 160 180 200 VIN (mV)
3108 G04
IOUT vs VIN and Source Resistance, 1:20 Ratio
10000 1 2 3 5 10 15
1:50 RATIO 1:20 RATIO
1000 IVOUT (A) 100 10
EFFICIENCY (%)
1:100 RATIO
50
150 200 250 100 VIN OPEN CIRCUIT (mV)
300
3108 G05
IOUT vs VIN and Source Resistance, 1:50 Ratio
1000 1000
IOUT vs VIN and Source Resistance, 1:100 Ratio
1 2 3 5 10 15 1000
IOUT vs dT and TEG Size, 1:100 Ratio
1.5" SQUARE 100 IOUT (A)
100 IOUT (A) IVOUT (A)
100
10
1
1 2 3 5 10 15 20 40 60 80 100 120 140 160 180 200 VIN OPEN CIRCUIT (mV)
3108 G06
0.625" SQUARE 10
10
1
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
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LTC3108 TYPICAL PERFORMANCE CHARACTERISTICS
Resonant Switching Waveforms
VIN = 20mV 1:100 RATIO TRANSFORMER C1 PIN 2V/DIV C2 PIN 2V/DIV SW PIN 50mV/DIV 10s/DIV
3108 G09
TA = 25C, unless otherwise noted.
LDO Load Regulation
0.00 0.20 0.18 DROPOUT VOLTAGE (V) -0.25 DROP IN VLDO (%) 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 -1.00 0 0.5 1 1.5 2 2.5 LDO LOAD (mA) 3 3.5 4
3108 G10
LDO Dropout Voltage
-0.50
-0.75
0.00 0 0.5 1 1.5 2 2.5 LDO LOAD (mA) 3 3.5 4
3108 G11
Start-Up Voltage Sequencing
VIN = 50mV 1:100 RATIO TRANSFORMER COUT = 220F CSTORE = 470F CLDO = 2.2F CH1 VSTORE 1V/DIV CH2, VOUT 1V/DIV CH3, VLDO 1V/DIV CH2 VOUT 1V/DIV CH1 PGD 1V/DIV
VOUT and PGD Response During a Step Load
50mA LOAD STEP COUT = 220F
VOUT Ripple
30A LOAD COUT = 220F
20mV/ DIV
10sec/DIV
3108 G12
5ms/DIV
3108 G13
100ms/DIV
3108 G14
LDO Step Load Response
Enable Input and VOUT2
CH3 VSTORE 1V/DIV CH2, VOUT2 1V/DIV CH2, VOUT 1V/DIV CH4, VLDO 1V/DIV CH1 VOUT2_EN 1V/DIV 1ms/DIV 10mA LOAD ON VOUT2 COUT = 220F
3108 G16
Running on Storage Capacitor
CSTORE = 470F VOUT LOAD = 100A
CH1, VIN 50mV/DIV 5sec/DIV
3108 G17
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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 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. 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 1.3 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 2.2F 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 VOUT is within 7% of its programmed value, PGD will be pulled up to VLDO through a 1M resistor. If VOUT drops 9% below its programmed value PGD will go low. This pin can sink up to 100A. VS2 (Pin 7/Pin 10): VOUT Select Pin 2. Connect this pin to ground or VAUX to program the output voltage (see Table 1).
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 GND GND VAUX VAUX VS1 GND VAUX GND VAUX VOUT 2.35V 3.3V 4.1V 5V
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LTC3108 BLOCK DIAGRAM
(DFN Package Shown)
LTC3108 1.3 ILIM VOUT2_EN SYNC RECTIFY C1 VIN CIN C2 C2 SW SW VOUT VHOLD 0.5 CHARGE CONTROL VREF VLDO 1M 1:100 C1 5.25V REFERENCE 1.2V VREF VOUT2 VOUT2
OFF ON
5M VOUT VOUT COUT
+ -
VS1 VS2 VOUT PROGRAM
- +
VAUX 1F VOUT GND (SSOP) VBEST VREF LDO VLDO
PGD PGOOD VSTORE CSTORE
3108 BD
EXPOSED PAD (DFN)
2.2V 2.2F
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.
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LTC3108 OPERATION
Oscillator 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. 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 1F 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. Voltage Reference The LTC3108 includes a precision, micropower reference, for accurate regulated output voltages. This reference becomes active as soon as VAUX exceeds 2V. Synchronous Rectifiers 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. 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 2.2F 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 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 GND GND VAUX VAUX VS1 GND VAUX GND VAUX VOUT 2.35V 3.3V 4.1V 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 VOUT has reached the proper value, the charging current is turned off. The internal programmable resistor divider sets VOUT, eliminating the need for very high value external resistors that are susceptible to board leakage.
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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 (1M) 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 1.3 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 5s 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 8A 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 8A. 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.
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LTC3108 OPERATION
5.0 2.5 0 3.0 2.0 1.0 0 VOLTAGE (V) 5.0 2.5 0 3.0 2.0 1.0 0 5.0 2.5 0 0 10 20 30 40 TIME (ms)
3108 F01a
VSTORE (V)
PGD (V)
VOUT (V)
VLDO (V)
VAUX (V)
50
60
70
80
Figure 1. Output Voltage Sequencing (with VOUT Programmed for 3.3V)
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LTC3108 APPLICATIONS INFORMATION
Introduction 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. 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.
0.3 RS = 2
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 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).
10
MAXIMUM POUT (mW)
VOPEN_CKT (V)
0.2
MAX POUT
1
0.1
VOC
0.1
0 0 5 15 10 T (C) 20 25
3108 F02
0.01
Figure 2. Typical Performance of a Peltier Cell Acting as a Thermoelectric Generator
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LTC3108 APPLICATIONS INFORMATION
The low voltage capability of the LTC3108 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 ambient 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 opencircuit 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 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 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 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
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 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. 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. Non-Boost Applications 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. 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. 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.
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LTC3108 APPLICATIONS INFORMATION
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 * * L(sec)* C Hz
output current capability. Refer to the Typical Applications schematic examples for the recommended value for a given turns ratio. 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) VOUT
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 Coilcraft www.coilcraft.com PART NUMBER LPR6235-752SML (1:100 Ratio) LPR6235-253PML (1:20 Ratio) LPR6235-123QML (1:50 Ratio)
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:
6A + IQ + ILDO + (IBURST * t * f) * TSTORE C STORE 5 . 25 - VOUT
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
Where 6A 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.
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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.
Table 5. Recommended Storage Capacitors
VENDOR AVX www.avx.com Cap-XX www.cap-xx.com Cooper/Bussman www.bussmann.com/3/PowerStor.html Vishay/Sprague www.vishay.com/capacitors PART NUMBER/SERIES BestCap Series TAJ and TPS Series Tantalum GZ Series KR Series P Series Tantamount 592D 595D Tantalum 150CRZ/153CRV Aluminum 013 RLC (Low Leakage)
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. 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
VIN
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.
VAUX VOUT VOUT2 VLDO PGOOD VSTORE VOUT VOUT2 VLDO PGD
1 2 3 4 5 6
12 11 10 9 8 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)
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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: COUT (F) 40mA * 1ms = 121F 0 . 33V addition, use the value of 150F 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 F * 0 . 33V = 1 . 5 sec or f MAX = 0 . 666Hz (50 A - 17A)
Therefore, in this application example, the circuit can support a 1ms transmit burst every 1.5 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 ten seconds. 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 2.2F . 2 . 2V * 2 . 2 F t LDO = ICHG - ILDO 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 VOUT were programmed to 3.3V and the VOUT capacitor was 150F the time for VOUT to reach regulation would be: , 3 . 3V * 150 F t VOUT = + t LDO ICHG - I VOUT If there were 50A of charge current available and 5A of load on VOUT, the time for VOUT to reach regulation after the initial application of power would be 11.1 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. A standard value of 150F 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. 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 50A and the total current drawn on VOUT in the sleep state is 17A, including capacitor leakage. In
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LTC3108 APPLICATIONS INFORMATION
Design Example 2 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 Therefore, if the LTC3108 has an input voltage that allows 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 VOUT of 3.3V, the average power required by this application is only 17W (not including converter losses). 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 = 5A, IBURST = 100mA, t = 5ms and T = one hour. The average charge current required from the LTC3108 would be: ICHG 5A + 100 mA * 0 . 005 sec = 5 . 14 A 3600 sec
TYPICAL APPLICATIONS
Peltier-Powered Energy Harvester for Remote Sensor Applications
1:100 T1 + TEG
1nF C1 330pF C2 LTC3108 SW
VSTORE
5V
+
+
CIN
VOUT2 PGD VLDO
COOPER BUSSMAN PB-5ROH104-R OR KR-5R5H104-R CSTORE 0.1F 3.3V 6.3V VOUT2 SENSORS P 2.2F XMTR
PGOOD 2.2V
T = 1C TO 20C
VOUT VS2 VS1 VAUX T1: COILCRAFT LPR6235-752SML 1F VOUT2_EN GND
3.3V
+
COUT
OFF ON
3108 TA02
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LTC3108 TYPICAL APPLICATIONS
Li-Ion Battery Charger and LDO Powered by a Solar Cell with Indoor Lighting
T1 1:20 0.047F C1 330pF C2 SW T1: COILCRAFT LPR6235-253PML VS2 VS1 VAUX LTC3108 VSTORE VOUT2 PGD VLDO VOUT VOUT2_EN GND 4.1V 2.2V VOUT Li-Ion VLDO 2.2F
+
SOLAR CELL
+
220F
-
1F
3108 TA03
Supercapacitor Charger and LDO Powered by a Thermopile Generator
HONEYWELL CQ200 THERMOPILE 0.047F 1:20 C1 330pF C2 SW VS2 VS1 VAUX 1F
3108 TA04
VSTORE VOUT2 LTC3108 PGD VLDO VOUT VOUT2_EN GND CAP-XX GZ115F 2.35V PGOOD 2.2V VLDO 2.2F
+
220F
+
VOUT 150mF 2.5V
DC Input Energy Havester and Power Manager
RIN RIN > 100 / V C1 VSTORE VOUT2 C2 SW VS2 LTC3108 VLDO VOUT 3.3V PGD
AC Input Energy Havester and Power Manager
RIN CIN RIN > 100/ V
5V
+
VOUT2 PGOOD 2.2V VLDO 2.2F CSTORE AC VIN VIN > 5VP-P - PIEZO - 60Hz
C1
VSTORE VOUT2 PGD LTC3108
5V
+
VOUT2 PGOOD 2.2V 5V VLDO 2.2F CSTORE
+ -
VIN VIN > 5V
C2 SW VS2 VS1 VAUX
VLDO VOUT
+
VOUT COUT
+
VOUT COUT
VS1 VAUX
VOUT2_EN GND
3108 TA05
VOUT2_ENABLE
VOUT2_EN GND
3108 TA06
VOUT2_ENABLE
1F
1F
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LTC3108 PACKAGE DESCRIPTION
UE/DE Package 12-Lead Plastic DFN (4mm x 3mm)
(Reference LTC DWG # 05-08-1695)
0.70 0.05 3.30 0.05 1.70 0.05 PACKAGE OUTLINE
3.60 0.05 2.20 0.05
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 4.00 0.10 (2 SIDES) R = 0.05 TYP 3.30 0.10 PIN 1 TOP MARK (NOTE 6) 3.00 0.10 (2 SIDES) 1.70 0.10 PIN 1 NOTCH R = 0.20 OR 0.35 45 CHAMFER 0.75 0.05 6 0.25 1 0.05 0.50 BSC 2.50 REF 0.00 - 0.05 BOTTOM VIEW--EXPOSED PAD
(UE12/DE12) DFN 0806 REV D
7
R = 0.115 TYP
0.40 12
0.10
0.200 REF
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
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LTC3108 PACKAGE DESCRIPTION
GN Package 16-Lead Plastic SSOP (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1641)
.045 .005
.189 - .196* (4.801 - 4.978) 16 15 14 13 12 11 10 9
.009 (0.229) REF
.254 MIN
.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 x 45 (0.38 0.10)
.007 - .0098 (0.178 - 0.249) 0 - 8 TYP .0532 - .0688 (1.35 - 1.75)
23
4
56
7
8
.004 - .0098 (0.102 - 0.249)
.016 - .050 (0.406 - 1.270)
NOTE: 1. CONTROLLING DIMENSION: INCHES INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) 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
.008 - .012 (0.203 - 0.305) TYP
.0250 (0.635) BSC
GN16 (SSOP) 0204
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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 TEG COLD 1:100 + TEC 330pF C2 SW VS2 VS1 VAUX LTC3108 C1 VSTORE VOUT2 PGD VLDO VOUT VOUT2_EN GND OFF ON 1F VAUX 1nF 1:100 COLD TEG HOT + TEC 330pF C2 SW VS2 VS1 VAUX LTC3108 C1 VSTORE VOUT2 PGD VLDO VOUT VOUT2_EN GND 5V
+
VOUT2 PGOOD 2.2V 3.3V VLDO 2.2F VOUT COUT CSTORE
LPR6235-752SML
+
LPR6235-752SML
3108 TA07
RELATED PARTS
PART NUMBER LTC1041 LTC1389 LT1672/LT1673/ LT1674 LT3009 LTC3525L-3/ LTC3525L-3.3/ LTC3525L-5 LTC3588 LTC3631 LTC3632 LTC3642 LT8410/ LT8410-1 DESCRIPTION Bang-Bang Controller Nanopower Precision Shunt Voltage Reference Single-/Dual-/Quad-Precision 2A Rail-to-Rail Op Amps 3A IQ, 20mA Linear Regulator 400mA (ISW), Synchronous Step-Up DC/DC Converter with Output Disconnect Piezoelectric Energy Generator with Integrated High Efficiency Buck Converter 45V, 100mA Synchronous MicroPower Buck Converter 45V, 20mA Synchronous MicroPower Buck Converter 45V, 50mA Synchronous MicroPower Buck Converter MicroPower 25mA/8mA Low Noise Boost Converter with Integrated Schottky Diode and Output Disconnect COMMENTS VIN: 2.8V to 16V; VOUT(MIN) = Adj; IQ = 1.2mA; ISD < 1A; SO-8 Package VOUT(MIN) = 1.25V; IQ = 0.8A; SO-8 Package SO-8, SO-14 and MSOP-8 Packages 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 = 3A; ISD < 1A; 2mm x 2mm DFN-8 and SC70 Packages VIN: 0.7V to 4V; VOUT(MIN) = 5VMAX; IQ = 7A; ISD < 1A; SC70 Package
VIN: 2.6V to 19.2V; VOUT(MIN): Fixed to 2.5V, 3V, 3.3V, 3.6V; IQ = 0.95A; 3mm x 3mm DFN-10 and MSOP-10E Packages VIN: 4.5V to 45V, 60VMAX; VOUT(MIN): 0.8V to Adj, 3.3V Fixed, 5V Fixed; IQ = 12A; ISD < 1A; 3mm x 3mm DFN-8 and MSOP-8E Packages VIN: 4.5V to 45V, 60VMAX; VOUT(MIN): 0.8V to Adj, 3.3V Fixed, 5V Fixed; IQ = 12A; ISD < 1A; 3mm x 3mm DFN-8 and MSOP-8E Packages VIN: 4.5V to 45V, 60VMAX; VOUT(MIN): 0.8V to Adj, 3.3V Fixed, 5V Fixed; IQ = 12A; ISD < 1A; 3mm x 3mm DFN-8 and MSOP-8E Packages VIN: 2.6V to 16V; VOUT(MIN) = 40VMAX; IQ = 8.5A; ISD < 1A; 2mm x 2mm DFN-8 Package
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20 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 1109 * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2009

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