LM2758 Switched Capacitor Flash LED Driver in micro SMD General Description Features LM2758 is an integrated low-noise, high-current switched capacitor DC/DC converter with a regulated current sink. The device is capable of driving loads up to 700 mA from a singlecell Li-Ion battery. Maximum efficiency is achieved over the input voltage range by actively selecting the proper gain based on the LED forward voltage and current requirements. One external low-power resistor sets the desired current for Indicator, Torch and Flash modes. To protect the devices and the flash LED, internal Time-out circuitry turns off the LM2758 in case of a faulty prolonged Flash mode. Internal soft-start circuitry limits the amount of inrush current during start-up. LM2758 is offered in National’s tiny 12-bump thin micro SMD package. ■ Up to 700 mA Output Current ■ Ultra-Small Solution Size ■ ■ ■ ■ ■ ■ —No Inductor, Only 4 Capacitors and a resistor Required —1.514 mm x 1.996 mm x 0.6 mm thin micro SMD package 90% Peak Efficiency Indicator, Torch and Flash Modes Time-Out Circuitry Limits Flash Duration to 814 msec. (typ.) Adaptive 1x and 1.5x Gains for Maximum Efficiency True Shutdown Internal Soft-Start Eliminates Inrush Current Applications ■ Camera Flash in Mobile Phones ■ Flash for Digital Cameras Typical Application Circuit 30045301 © 2009 National Semiconductor Corporation 300453 www.national.com LM2758 Switched Capacitor Flash LED Driver in micro SMD October 9, 2009 LM2758 Connection Diagram 12-Bump Thin Micro SMD 1.514mm x 1.996mm x 0.6mm NS Package Number TLA12 30045302 Pin Descriptions Pin Name C1 VIN B2 CPOUT Description Supply voltage connection. Charge pump regulated output. A 2.2 µF ceramic capacitor is required from CPOUT to GND. Connect flash LED anode to this pin. B1 C1+ D2 C1− C2 C2+ D1 C2− A2 SGND Analog and control ground for charge pump. This pin should be connected directly to a low impedance ground plane. C3 LED− Regulated current source output. Connect flash LED cathode to this pin. A1 EN1 B3 EN2 The EN1 and EN2 pins are used to select the modes (Torch, Indictor, Flash), as well as to put the part into Shutdown mode. A3 ISET LED current programming resistor pin. A resistor connected between this pin and GND are used to set Torch, Flash and Indicator currents. D3 PGND Power ground for the charge pump and the current source. This pin should be connected directly to a low impedance ground plane. Flying capacitor pins. A 1 µF ceramic capacitor should be connected from C1+ to C1− and from C2+ to C2−. Ordering Information Order Number www.national.com Supplied as Tape and Reel (Units) LM2758TL 250 units, tape and reel LM2758TLX 3000 units, tape and reel 2 Input Voltage Range Junction Temperature Range (TJ) Ambient Temperature Range (TA) (Note 6) 2) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN, CPOUT pins: Voltage to GND EN1, EN2 pins: Voltage to GND Continuous Power Dissipation (Note 3) Junction Temperature (TJ-MAX) Storage Temperature Range Maximum Lead Temp. (Soldering) ESD Ratings (Note 4) Human Body Model Machine Model (Note 1, Note 2) -0.3V to 6.0V -0.3V to (VIN + 0.3V) w/ 6.0V max 2.7V to 5.5V -40°C to +125°C -40°C to +85 °C Thermal Information Junction-to-Ambient Thermal Resistance 56°C/W Resistance (θJA), micro SMD package (Note 7) Internally Limited 150°C -65°C to 150°C (Note 5) 2kV 200V ESD Caution Notice National Semiconductor recommends that all integrated circuits be handled with appropriate ESD precautions. Failure to observe proper ESD handling techniques can result in damage to the device. Electrical Characteristics (Note 2, Note 8) Limits in standard typeface are for TJ = 25°C. Limits in boldface type apply over the full operating junction temperature range (-40° C ≤ TJ ≤ +125 °C). Unless otherwise noted, specifications apply to the LM2758 Typical Application Circuit (pg.1) with VIN = 3.6V, VEN1 = VIN, VEN2 = 0V, C1 = C2 = 1 µF, CIN = COUT = 2.2 µF, RSET = 20 kΩ (Note 9). Symbol Parameter Conditions ILED LED Current Accuracy VSET ISET Pin Voltage ID/ISET LED Current to Set Current Flash Mode Ratio Torch Mode ILED-IND Indicator Current Level ILED = 500 mA, Flash Mode Min Typ Max Units 450 500 550 mA 1.3 V 7650 1639 1/32 x ILED- Indicator Mode 32 kHZ PWM Mode mA TORCH VGDX VOUT ROUT 1x to 1.5x Gain Transition Voltage Threshold on IOUT = 500 mA VLEDOutput Voltage 300 mV 1x Mode, IOUT = 0 mA VIN 1.5x Mode, IOUT = 0 mA (Note 10) 4.8 5.3 1x Mode Output Impedance IOUT = 200 mA, VIN = 3.3V 0.33 0.53 1.5x Mode Output Impedance IOUT = 500 mA, VIN = 3.3V (Note 11) 1.5 2.0 1.25 1.5 0.7 0.8 4 5 0.01 1 µA 814 1000 msec FSW Switching Frequency IQ Quiescent Current 0.8 IOUT = 0 mA 1x Mode IOUT = 0 mA 1.5x Mode ISD Shutdown Current Device Disabled (Note 12) TOUT Time-out Duration (Note 13) 640 VIH Input Logic High Pins: EN1, EN2 1.2 VIL Input Logic Low Pins: EN1, EN2 Ω MHz mA V 0.4 3 V V www.national.com LM2758 Operating Ratings Absolute Maximum Ratings (Note 1, Note LM2758 Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the Electrical Characteristics tables. Note 2: All voltages are with respect to the potential to the GND pin. Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ=150°C (typ.) and disengages at TJ = 140°C (typ.). Note 4: The Human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. The machine model is a 200pF capacitor discharged directly into each pin. MIL-STD-883 3015.7 Note 5: For detailed soldering specifications and information, please refer to National Semiconductor Application Note AN-1112. Note 6: In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may have to be derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operation junction temperature (TJ-MAX-OP = 125ºC), the maximum power dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the part/package in the application (θJA), as given by the following equation: TA-MAX = TJ-MAX-OP - (θJA × PD-MAX). Note 7: Junction-to-ambient thermal resistance (θJA) is taken from a thermal modeling result, performed under the conditions and guidelines set forth in the JEDEC standard JESD51-7. The test board is a 4–layer FR-4 board measuring 102 mm x 76 mm x 1.6 mm with a 2x1 array of thermal vias. The ground plane on the board is 50 mm x 50 mm. Thickness of copper layers are 53µm/35µm/35µm/53µm (1.5oz/1oz/1oz/1.5oz). Ambient temperature in simulation is 22°C, still air. Power dissipation is 1W. The value of θJA of this product in this micro SMD could fall in a range as wide as 50ºC/W to 150ºC/W (if not wider), depending on PWB material, layout, and environmental conditions. In applications where high maximum power dissipation exists (high VIN, high IOUT), special care must be paid to thermal dissipation issues. Note 8: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical (Typ) numbers are not guaranteed, but do represent the most likely norm. Unless otherwise specified, conditions for Typ specifications are: VIN = 3.6V and TA = 25°C. Note 9: CIN, COUT, C1, C2: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics. Note 10: Output voltage is internally limited not to exceed maximum specified value. Note 11: These specification table entries are guaranteed by design. These parameters are not guaranteed by production testing. The temperature limits for test are (-40°C ≤ TA ≤ +85°C). Note 12: The temperature limits for ISD (shutdown current) test are -40°C ≤ TA ≤ +85°C, as in Shutdown mode ambient temperature is equal to junction temperature. Note 13: The time-out specifications are calculated values based on the switching frequency spread. www.national.com 4 LM2758 Block Diagram 30045319 5 www.national.com LM2758 Typical Performance Characteristics Unless otherwise specified: TA = 25°C, VIN = 3.6V, CIN = COUT = 2.2 µF, C1 = C2 = 1 µF. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC's). Luxeon PWF1 Flash LED. Efficiency vs VIN Oscillator Frequency vs VIN 30045304 30045305 Quiescent Current vs VIN Shutdown Current vs VIN 30045307 30045306 www.national.com 6 LM2758 ILED vs VLED- LED Current vs RSET 30045308 30045309 Shutdown to Flash Mode VIN = 3.6V, ILED = 500 mA Shutdown to Torch Mode VIN = 3.6V, ILED = 108 mA, Gain = 1x, EN1 = 0V 30045310 Shutdown to Indicator Mode VIN = 3.6V, ILED(Torch) = 108 mA, EN2 = 0V 30045311 Indicator Mode EN1 = VIN = 3.6V, ILED(Torch) = 108 mA, Gain = 1x, EN2 = 0V 30045312 30045313 7 www.national.com LM2758 Torch to Flash Mode Transition EN2 = VIN = 3.6V, ILED(Flash) = 500 mA, Gain = 1.5x Indicator to Flash Mode Transition EN1 = VIN = 3.6V, ILED(Flash) = 500 mA, Gain = 1.5x 30045315 30045314 www.national.com 8 where excessive current is drawn. Output current is limited to 1.2A typically. CIRCUIT DESCRIPTION The LM2758 is an adaptive 1x and 1.5x CMOS charge pump, optimized for driving Flash LEDs in camera phone and other portable applications. It provides a constant current of 500 mA (typ.) for Flash mode and 107 mA (typ.) for Torch mode with RSET = 20 kΩ. These current can change, see SETTING LED CURRENTS. There are four modes of operation for LM2758: the Flash Mode, Torch Mode, Indicator Mode and Shutdown Mode (see EN1 and EN2 truth table). Torch and Flash modes sink a constant DC current while Indicator mode operates in pulsating DC at 1/32 positive duty cycle with same current magnitude as Torch mode. The LED is driven from CPOUT and connected to the current sink. LED drive current mode is programmed by connecting a resistor, RSET, to the current set pin, ISET. LM2758 also controls CPOUT with variable gain (1x or 1.5x) and adjustable impedance (R OUT) to provide an output voltage that would account for LED forward voltage drop and headroom for the current sink to drive desired current through LED. LOGIC CONTROL PINS LM2758 has two logic pins, EN1 and EN2. The operating modes of the part function according to the tables below: EN1 EN2 Mode 0 0 Shutdown 1 0 Indicator 0 1 Torch 1 1 Flash FLASH TIME-OUT FEATURE Flash Time-out Protection Circuitry disables the current sinks when the signal on EN1 and EN2 is held high for more than 814 msec (typ.). This prevents the device from self-heating due to the high power dissipation during Flash conditions. During the time-out condition, voltage will still be present on CPOUT but the current sinks will be shut off, resulting in no current through the Flash LED. When the device goes into a time-out condition, placing a logic Low signal on EN1 and EN2 will reset the time-out; a subsequent logic High signal on EN1 or EN2 will return the device to normal operation. CHARGE PUMP AND GAIN TRANSITIONS The input to the 1x/1.5x charge pump is connected to the VIN pin, and the loosely regulated output of the charge pump is connected to the CPOUT pin. In 1x mode, as long as the input voltage is less than 4.7V, the output voltage is approximately equal to the input voltage. When input voltage is over 4.7V the output voltage gets regulated to 4.7V. In 1.5x mode, the output voltage is always less than or equal to 4.7V over entire input voltage range. The charge pump’s gain is selected depending on the headroom voltage across the current sink of LM2758. When headroom voltage VLED- (at LED pin) drops below 300 mV (typ.) the charge pump gain transition happens from 1x to 1.5x to maintain current regulation across the LED. Once the charge pump transition to a higher gain, it will remain at that gain for as long as the device remains enabled. Shutting down and then re-enabling the device will resets the gain mode to the minimum gain required to maintain the load. SETTING LED CURRENTS The current through the LED can be set by connecting an appropriately sized resistor RSET between the ISET pin of the LM2758 and GND. The LED current in Torch mode is approximately 1639 times greater than the current of ISET, while the LED current in Flash mode is approximately 7650 times of the same ISET current. The feedback loop of an internal amplifier sets the voltage of the ISET pin to 1.3V (typ.). The statements above are simplified in the equations below: ILED = GAINFLASH/TORCH x (1.3/RSET) The maximum recommended current through LED is 500 mA in Torch mode / 700 mA in Flash mode. Note: If the ISET for Torch Mode setting at 500 mA, the Flash mode would be over 700 mA (max). See the graph LED Current vs RSET. Using the part in conditions where the junction temperature might rise above the rated maximum requires that the operating ranges and/or conditions be de-rated. The printed circuit board also must be carefully laid out to account for high thermal dissipation in the part. SOFT START The LM2758 contains internal soft-start circuitry to limit inrush currents when the part is enabled. Soft start is implemented internally with a controlled turn-on of the internal voltage reference. CURRENT LIMIT PROTECTION The LM2758 charge pump contains current limit protection circuitry that protects the device during VOUT fault conditions 9 www.national.com LM2758 Application Information LM2758 ANALOG BRIGHTNESS CONTROL 30045318 The current though the LED could be varied dynamically by changing the ISET current. The above figure shows the circuit. The current though the LED can be calculated as follows. tance tolerance (as good as ±10%) and hold their value over temperature (X7R: ±15% over -55°C to 125°C; X5R: ±15% over -55°C to 85°C). Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM2758. Capacitors with these temperature characteristics typically have wide capacitance tolerance (+80%, -20%) and vary significantly over temperature (Y5V: +22%, -82% over -30°C to +85°C range; Z5U: +22%, -56% over +10°C to +85° C range). Under some conditions, a nominal 1 μF Y5V or Z5U capacitor could have a capacitance of only 0.1 μF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum capacitance requirements of the LM2758. The voltage rating of the output capacitor should be 6.3V or more. For example, a 6.3V 0603 2.2 μF output capacitor (TDK C1608X5R0J225) is acceptable for use with the LM2758, as long as the capacitance on the output does not fall below a minimum of 1 μF in the intended application. All other capacitors should have a voltage rating at or above the maximum input voltage of the application and should have a minimum capacitance of 1 μF. 30045317 CAPACITOR SELECTION The LM2758 requires 4 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR <20 mΩ typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors are not recommended for use with the LM2758 due to their high ESR, as compared to ceramic capacitors. For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with the LM2758. These capacitors have tight capaci- Suggested Capacitors and Suppliers MFG Part No. Type MFG Voltage Rating Case Size Inch (mm) 2.2 µF for CIN and COUT C1608X5R0J225 Ceramic X5R TDK 6.3V 0603 (1608) JMK107BJ225 Ceramic X5R Taiyo-Yuden 6.3V 0603 (1608) C1608X5R0J105 Ceramic X5R TDK 6.3V 0603 (1608) JMK107BJ105M Ceramic X5R Taiyo-Yuden 6.3V 0603 (1608) 1 µF for C1 and C2 www.national.com 10 MICRO SMD PACKAGE ASSEMBLY AND USE Use of the micro SMD package requires specialized board layout, precision mounting and careful re-flow techniques as detailed in National Semiconductor Application Note 1112. Refer to the section "Surface Mount Technology (SMD) Assembly Considerations". For best results in assembly, alignment ordinals on the PC board should be used to facilitate placement of the device. The pad style used with the micro SMD package must be the NSMD (non-solder mask defined) typ. This means that the solder-mask opening is larger than the pad size. This prevents a lip that otherwise forms if the solder mask and pad overlap, from holding the device off the surface of the board and interfering with mounting. See Application Note 1112 for specific instructions how to do this. The 12-bump package used for LM2758 has 300 micron solder balls and requires 10.82 mils pads for mounting on the circuit board. The trace to each pad should enter the pad with a 90° entry angle to prevent debris from being caught in deep corners. Initially, the trace to each pad should be 7 mil. wide, for a section approximately 7 mil. long or longer, as a thermal relief. Then each trace should neck up or down to its optimal width. The important criteria is symmetry. This ensures the solder bumps on the LM2758 re-flow evenly and that the device solders level to the board. In particular, special attention must be paid to the pads for bumps C1 and D3, because VIN and GND are typically connected to large copper planes, thus inadequate thermal relief can result in late or inadequate re-flow of these bumps. The micro SMD package is optimized for the smallest possible size in applications with red or infrared opaque cases. Because the micro SMD package lacks the plastic encapsulation characteristic of larger devices, it is vulnerable to light. Backside metallization and/or epoxy coating, along with front side shading by the printed circuit board, reduce this sensitivity. However, the package has exposed die edges. In particular, micro SMD devices are sensitive to light, in the red and infrared range, shining on the package’s exposed die edges. PLED = VLED × ILED PIN = VIN × IIN PIN = VIN × (Gain × ILED + IQ) E = (PLED ÷ PIN) For a simple approximation, the current consumed by internal circuitry (IQ) can be neglected, and the resulting efficiency will become: E = VLED ÷ (VIN × Gain) Neglecting IQ will result in a slightly higher efficiency prediction, but this impact will be negligible due to the value of IQ being very low compared to the typical Torch and Flash current levels (100-500 mA). It is also worth noting that efficiency as defined here is in part dependent on LED voltage. Variation in LED voltage does not affect power consumed by the circuit and typically does not relate to the brightness of the LED. For an advanced analysis, it is recommended that power consumed by the circuit (VIN x IIN) be evaluated rather than power efficiency. THERMAL PROTECTION Internal thermal protection circuitry disables the LM2758 when the junction temperature exceeds 150°C (typ.). This feature protects the device from being damaged by high die temperatures that might otherwise result from excessive power dissipation. The device will recover and operate normally when the junction temperature falls below 140°C (typ.). It is important that the board layout provide good thermal conduction to keep the junction temperature within the specified operating ratings. POWER DISSIPATION The power dissipation (PDISSIPATION) and junction temperature (TJ) can be approximated with the equations below. PIN is the power generated by the 1x/1.5x charge pump, PLED is the power consumed by the LEDs, TA is the ambient temperature, and θJA is the junction-to-ambient thermal resistance for the 12–bump micro SMD package. VIN is the input voltage to the LM2758, VLED is the nominal LED forward voltage, and ILED is the programmed LED current. BOARD LAYOUT CONSIDERATIONS PC board layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance of a DCDC converter and surrounding circuitry by contributing to EMI, ground bounce, and resistive voltage loss in the traces. These can send erroneous signals to the DC-DC converter IC, resulting in poor regulation or instability. Poor layout can also result in re-flow problems leading to poor solder joints between the micro SMD package and board pads. Poor solder joints can result in erratic or degraded performance. PDISSIPATION = PIN - PLED = (Gain × VIN × ILED) − (VLED × ILED) TJ = TA + (PDISSIPATION × θJA) The junction temperature rating takes precedence over the ambient temperature rating. The LM2758 may be operated outside the ambient temperature rating, so long as the junc- 11 www.national.com LM2758 tion temperature of the device does not exceed the maximum operating rating of 125°C. The maximum ambient temperature rating must be derated in applications where high power dissipation and/or poor thermal resistance causes the junction temperature to exceed 125°C. POWER EFFICIENCY Efficiency of LED drivers is commonly taken to be the ratio of power consumed by the LEDs (PLED) to the power drawn at the input of the part (PIN). With a 1x/1.5x charge pump, the input current is equal to the charge pump gain times the output current (total LED current). The efficiency of the LM2758 can be predicted as follows: LM2758 Physical Dimensions inches (millimeters) unless otherwise noted 12–bump micro SMD NS Package Number TLA12 X1 = 1.514 mm ± 30 µm X2 = 1.996 mm ± 30 µm X3 = 0.600 mm ± 75 µm www.national.com 12 LM2758 Notes 13 www.national.com LM2758 Switched Capacitor Flash LED Driver in micro SMD Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Design Support Amplifiers www.national.com/amplifiers WEBENCH® Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage Reference www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Solutions www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise® Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagic™ www.national.com/solarmagic Wireless (PLL/VCO) www.national.com/wireless www.national.com/training PowerWise® Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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