LM2759 1A Switched Capacitor Flash LED Driver with I2C Compatible Interface General Description Features LM2759 is an integrated low-noise, high-current switched capacitor DC/DC converter with a regulated current source. The device requires only four small ceramic capacitors making the total solution area less than 22 mm2 and the height less than 1 mm. The LM2759 is capable of driving loads up to 1A from a single-cell 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. The LED current can be programmed up to 1A via an I2Ccompatible interface, along with eight selectable Flash TimeOut durations. One high-current Flash LED can be driven either in a high-power Flash mode or a low-power Torch mode. The Strobe pin allows the flash to be toggled via a Flash enable signal from a camera module. The TX input pin limits the Flash LED current to the Torch current level during a RF PA pulse, to reduce high loads on the battery. Internal soft-start circuitry limits the amount of inrush current during start-up. LM2759 is offered in a small 12-pin thermally enhanced LLP package. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Up to 1A Output Current Solution Area < 22 mm2 No Inductor Required 90% Peak Efficiency Adaptive 1x, 1.5x and 2x Gains for Maximum Efficiency Load Disconnect in Shutdown Accurate Input Current Control During Gain Transitions Flash Time-Out TX Input Pin Ensures Synchronization with RF Power Amplifier Pulse Torch, Flash, and Indicator Modes External Flash Enable via Strobe Input Pin Strobe Input Disable via I2C Programmable Flash Pulse Duration, and Torch and Flash Currents via I2C-Compatible Interface 1MHz Constant Frequency Operation Low Profile 12–Pin LLP (3mm x 3mm x 0.8mm) Applications ■ Camera Flash in Cellular Phones Typical Application Circuit 30069322 30069301 © 2010 National Semiconductor Corporation 300693 www.national.com LM2759 1A Switched Capacitor Flash LED Driver with I2C Compatible Interface June 10, 2010 LM2759 Connection Diagram 12-Pin LLP Package 3mm x 3mm x 0.8mm NS Package Number SDF12A 30069302 Note 1: The actual physical placement of the package marking will vary from part to part. The package marking placeholder "XXXXX" is a code for the die traceability. "YYYYY" identifies the device (part number, voltage option, etc.). See the Order Information table below for the device ID codes. Pin Descriptions Pin Name 10 VIN 3 VOUT 12 C1− 11 C1+ 2 C2+ 1 C2− Description Input voltage connection. Charge pump regulated output. Flying capacitor connections. 4 GND Ground connection. 6 ISINK Regulated current sink input. 8 SDA Serial data I/O pin. 7 Strobe 5 TX 9 SCL Manual flash enable pin. Flash will remain on for the duration that the Strobe pin is held high or when the Flash Timeout occurs, whichever comes first. Transmission pulse Flash interrupt pin. High = RF PA pulse active, LED current reduced to Torch level, Low = RF PA pulse off, LED at full programmed current level. Serial clock pin. Ordering Information Order Number Package Description Package Marking Supplied as Tape and Reel (Units) LM2759SD No-Pullback LLP-12 ZXYTT L2759 1000 units, tape and reel LM2759SDX www.national.com 2 4500 units, tape and reel Input Voltage Range LED Voltage Range Junction Temperature Range (TJ) Ambient Temperature Range (TA) (Note 7) 3) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN pin: Voltage to GND Strobe, TX, SDA, SCL, ISINK pins: Voltage to GND Continuous Power Dissipation (Note 4) Junction Temperature (TJ-MAX) Storage Temperature Range Maximum Lead Temp. (Soldering) ESD Rating(Note 6) Human Body Model (Note 2, Note 3) -0.3V to 6.0V -0.3V to (VIN + 0.3V) w/ 6.0V max 2.7V to 5.5V 2.0V to 4.0V -30°C to +125°C -30°C to +85°C Thermal Information Junction-to-Ambient Thermal Resistance (θJA), Leadless Leadframe Package (Note 8) Internally Limited 150°C -65°C to 150°C (Note 5) 36.7°C/W 2.5KV 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 3, Note 9) Limits in standard typeface are for TJ = 25°C. Limits in boldface type apply over the full operating junction temperature range (-30° C ≤ TJ ≤ +125 °C). Unless otherwise noted, specifications apply to the LM2759 Typical Application Circuit (pg.1) with VIN = 3.6V, VTX = 0V, VSTROBE = 0V, CIN = C1 = C2 = 2.2 µF, COUT = 4.7 µF. (Note 10) Symbol Parameter Conditions ILED LED Current Sink Accuracy IFLASH Max Flash Output Current Flash Mode ADDR xB0 = 0x0F VGDX Gain Transition Voltage Threshold on ISINK VOUT Output Voltage ROUT Flash Mode ADDR xB0 = 0x02 Min Typ Max Units 198 −10% 220 242 +10% mA 1 A ILED = 500mA (VISINK falling) 350 mV 1x Mode, IOUT = 0 mA (VIN >VOUT)(Note 12) 4.7 4.9 1.5x Mode, IOUT = 0 mA 4.7 4.9 2x Mode, IOUT = 0 mA 5.1 5.4 x1 Mode Output Impedance IOUT = 200mA, VIN = 3.3V 0.33 1.5x Mode Output Impedance IOUT = 500mA, VIN = 3.3V 1.9 x2 Mode Output Impedance V Ω 2.25 FSW Switching Frequency 2.7V ≤ VIN ≤ 5.5V 0.7 VIH Input Logic High Pins: TX, Strobe 1.26 VIL Input Logic Low Pins: TX, Strobe IQ Quiescent Current ISD Shutdown Current 1 1.3 MHz V 0.7 IOUT = 0 mA, 1x Mode 0.6 0.9 IOUT = 0 mA, 1.5x Mode 3.4 4.0 IOUT = 0 mA, 2x Mode 5.9 7.0 Device Disabled 5.8 9.7 V mA µA 2.7V ≤ VIN ≤ 5.5V 3 www.national.com LM2759 Operating Ratings Absolute Maximum Ratings (Note 2, Note LM2759 Limits in standard typeface are for TJ = 25°C. Limits in boldface type apply over the full operating junction temperature range (-30° C ≤ TJ ≤ +125 °C). Unless otherwise noted, specifications apply to the LM2759 Typical Application Circuit (pg.1) with VIN = 3.6V, VTX = 0V, VSTROBE = 0V, CIN = C1 = C2 = 2.2 µF, COUT = 4.7 µF. (Note 10) Symbol Parameter Conditions Min Typ Max Units 0.72 V I2C Compatible Interface Voltage Specifications (SCL, SDA). VIL Input Logic Low “0” 2.7V ≤ VIN ≤ 5.5V VIH Input Logic High “1' 2.7V ≤ VIN ≤ 5.5V VOL Output Logic Low “0” ILOAD = 3 mA I2C V 1.25 300 mV Compatible Interface Timing Voltage Specifications (SCL, SDA) (Note 11) t1 SCL (Clock Period) 2.5 µs t2 Data in Setup Time to SCL High 100 ns t3 Data Out Stable After SCL Low 0 ns t4 SDA Low Setup Time to SCL Low (Start) 100 ns t5 SDA High Hold Time After SCL High (Stop) 100 ns Note 2: 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 3: All voltages are with respect to the potential to the GND pin. Note 4: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ=150°C (typ.) and disengages at TJ = 120°C (typ.). Note 5: For detailed soldering specifications and information, please refer to National Semiconductor Application Note AN-1187. Note 6: The Human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. (MIL-STD-883 3015.7) Note 7: 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 8: 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 the LLP package could fall in a range as wide as 30ºC/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. For more information on these topics, please refer to Application Note 1187: Leadless Leadframe Package (LLP) and the Power Efficiency and Power Dissipation section of this datasheet. Note 9: 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 10: CIN, COUT, C1, C2: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics. Note 11: SCL and SDA should be glitch-free in order for proper brightness control to be realized. Note 12: For input voltage below the regulation target during the gain of 1x, the output voltage will typically be equal to the input voltage. www.national.com 4 LM2759 Block Diagram 30069319 5 www.national.com LM2759 Typical Performance Characteristics Unless otherwise specified: TA = 25°C, VIN = 3.6V, CIN = C1 = C2 = 2.2µF, COUT = 4.7µF. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC's). Luxeon PWF3 Flash LED. Efficiency vs VIN Input Current vs VIN 30069326 30069327 Quiescent Current vs VIN, Gain = 1X Quiescent Current vs VIN, Gain = 2X 30069328 30069329 ILED vs VISINK Shutdown Current vs VIN 30069330 www.national.com 30069320 6 LM2759 Oscillator Frequency vs VIN Torch Code Levels 30069321 30069331 Flash Code Levels Shutdown to Torch Mode, 100mA 30069333 CH1: SDA; Scale: 2V/Div, DC Coupled CH2: VOUT; Scale: 2V/Div, DC Coupled CH3: IIN; Scale: 100mA/Div, DC Coupled CH4: ILED; Scale: 100mA/Div, DC Coupled Time scale: 400µs/Div 30069332 Shutdown to Flash Mode, 1A Torch to Flash Mode, 100mA to 1A 30069334 30069335 CH1: SDA; Scale: 2V/Div, DC Coupled CH2: VOUT; Scale: 2V/Div, DC Coupled CH3: IIN; Scale: 1A/Div, DC Coupled CH4: ILED; Scale: 1A/Div, DC Coupled Time scale: 1ms/Div CH1: SDA; Scale: 2V/Div, DC Coupled CH2: VOUT; Scale: 2V/Div, DC Coupled CH3: IIN; Scale: 1A/Div, DC Coupled CH4: ILED; Scale: 1A/Div, DC Coupled Time scale: 1ms/Div 7 www.national.com LM2759 Flash Timeout, Timeout Code (x03) = 325ms 30069336 Torch Level (x0F) = 180mA, Flash Level (x05) = 410mA CH1(bottom): IIN; Scale: 200mA/Div, DC Coupled CH2(middle): SDA; Scale: 2V/Div, DC Coupled CH3(top): VOUT; Scale: 2V/Div, DC Coupled Time scale: 100ms/Div www.national.com 8 TX STROBE CIRCUIT DESCRIPTION The LM2759 is an adaptive CMOS charge pump with gains of 1x, 1.5x, and 2x, optimized for driving Flash LEDs in camera phones and other portable applications. It provides a constant current of up to 1A (typ.) for Flash mode and 180 mA (typ.) for Torch mode. The LM2759 has selectable modes including Flash, Torch, Indicator and Shutdown. Flash mode for the LM2759 can also be enabled via the Strobe input pin. The LED is driven from VOUT and connected to the current sink. The LED drive current and operating modes are programmed via an I2C compatible interface. The LM2759 adaptively selects the next highest gain mode when needed to maintain the programmed LED current level. To prevent a high battery load condition during a simultaneous RF PA transmission and Flash event, LM2759 has a Flash interrupt pin (TX) to reduce the LED current to the programmed Torch current level for the duration of the RF PA transmission pulse. FUNCTION Current I2 C 0 0 programmed state (Off, Torch, Flash, Indicator) 1 0 Current I2C programmed state (Off, Torch, Flash, Indicator). If Flash is enabled via I2C and TX is logic High, the LED current will be at the programmed Torch level. 0 1 Flash Mode (Total LED "ON" Duration limited by Flash Timeout) 1 1 Torch Mode (Total LED "ON" Duration limited by Flash Timeout) I2C COMPATIBLE INTERFACE START AND STOP CONDITIONS START and STOP conditions classify the beginning and the end of the I2C session. A START condition is defined as SDA signal transitioning from HIGH to LOW while SCL line is HIGH. A STOP condition is defined as the SDA transitioning from LOW to HIGH while SCL is HIGH. The I2C master always generates START and STOP conditions. The I2C bus is considered to be busy after a START condition and free after a STOP condition. During data transmission, the I2C master can generate repeated START conditions. First START and repeated START conditions are equivalent, function-wise. CHARGE PUMP AND GAIN TRANSITIONS The input to the 1x, 1.5x, 2x charge pump is connected to the VIN pin, and the loosely regulated output of the charge pump is connected to the VOUT pin. In 1x mode, as long as the input voltage is less than 4.7V (typ.), the output voltage is approximately equal to the input voltage. When the input voltage is over 4.7V (typ.) the output voltage is regulated to 4.7V (typ.). In 1.5x mode, the output voltage is regulated to 4.7V (typ.) over entire input voltage range. For the gain of 2x, the output voltage is regulated to 5.1V (typ.). When under load, the voltage at VOUT can be less than the target regulation voltage while the charge pump is still in closed loop operation. This is due to the load regulation topology of the LM2759. The charge pump’s gain is selected according to the headroom voltage across the current sink of LM2759. When the headroom voltage VGDX (at the LED cathode) drops below 350 mV (typ.) the charge pump gain transitions to the next available higher gain mode. Once the charge pump transitions 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 resets the gain mode to the minimum gain required to maintain the load. 30069311 FIGURE 1. Start and Stop Conditions DATA VALIDITY The data on SDA line must be stable during the HIGH period of the clock signal (SCL). In other words, state of the data line can only be changed when SCL is LOW. SOFT START The LM2759 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 LM2759 charge pump contains current limit protection circuitry that protects the device during VOUT fault conditions where excessive current is drawn. Output current is limited to 1.4A typically. 30069325 FIGURE 2. Data Validity Diagram A pull-up resistor between the controller's VIO line and SDA must be greater than [ (VIO-VOL) / 3.5mA] to meet the VOL requirement on SDA. Using a larger pull-up resistor results in lower switching current with slower edges, while using a smaller pull-up results in higher switching currents with faster edges. LOGIC CONTROL PINS LM2759 has two asynchronous logic pins, Strobe and TX. These logic inputs function according to the table below: TRANSFERING DATA Every byte put on the SDA line must be eight bits long, with the most significant bit (MSB) transferred first. Each byte of 9 www.national.com LM2759 Application Information LM2759 data has to be followed by an acknowledge bit. The acknowledge related clock pulse is generated by the master. The master releases the SDA line (HIGH) during the acknowledge clock pulse. The LM2759 pulls down the SDA line during the 9th clock pulse, signifying an acknowledge. The LM2759 generates an acknowledge after each byte is received. After the START condition, the I2C master sends a chip address. This address is seven bits long followed by an eighth bit which is a data direction bit (R/W). The LM2759 address is 53h. For the eighth bit, a “0” indicates a WRITE and a “1” indicates a READ. The second byte selects the register to which the data will be written. The third byte contains data to write to the selected register. 30069312 FIGURE 3. Write Cycle w = write (SDA = "0") r = read (SDA = "1") ack = acknowledge (SDA pulled down by either master or slave) id = chip address, 53h for LM2759 I2C COMPATIBLE CHIP ADDRESS The chip address for LM2759 is 1010011, or 53h. Internal Hex Address Power On Value (lowest 4 bits) General Purpose Register 10h 0000 Flash Current Register B0h 1010 Torch Current Register A0h 0111 Flash Timeout Duration Register C0h 1011 Register 30069309 INTERNAL REGISTERS GENERAL PURPOSE REGISTER AND STROBE INHIBIT FUNCTION The general purpose register (x10) is used set the mode of operation for the LM2759. The selectable operating modes using the lower 4 bits in the general purpose register are listed in the table below. The Strobe Input Pin can be disabled via I2C to ignore external signals into this pin when desired. This function is implemented through bit 3 of the General Purpose Register (See table below). In the default state, input signals on the Strobe Input are enabled. (Bit3 = “0”, inputs into the Strobe Pin are not inhibited). 30069308 30069305 General Purpose Register (Reg x10) 30069306 30069307 www.national.com 10 Bit3 Bit2 Bit1 Bit0 Mode X X X 0 Shutdown X 0 0 1 Torch X X 1 1 Flash X 1 0 1 Indicator (Lowest Torch Level) 1 X X X Inhibit Inputs into the Strobe Pin FLASH TIME-OUT FEATURE Time-out Protection Circuitry disables the current sink when either the Strobe pin is held at logic high or the Flash mode is enabled via the I2C compatible interface longer than the programmed timeout duration. 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 VOUT but the current sink will be shut off, resulting in no current through the Flash LED. When the device goes into a time-out condition, disabling and then re-enabling the device will reset the time-out. Use the table below to set the desired Flash timeout duration. Flash Current Table (Reg xB0) Flash Timeout Duration (Reg xC0) FLASH CURRENT (mA) CODE (Hex) TIME (ms) 00 80 00 60 01 150 01 125 02 220 02 250 280 03 375 04 350 04 500 05 410 05 625 470 06 750 07 530 07 1100 08 590 09 650 0A 710 0B 770 0C 830 0D 890 CODE (Hex) 03 06 0E 950 0F 1010 CAPACITOR SELECTION The LM2759 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 LM2759 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 LM2759. These capacitors have tight capacitance 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 LM2759. 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 LM2759. The voltage rating of the output capacitor should be 6.3V or more. For example, a 6.3V 0603 4.7 μF output capacitor (TDK C1608X5R0J475) is acceptable for use with the LM2759, as long as the capacitance on the output does not fall below a minimum of 3μF 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. Torch Current Table (Reg xA0) CODE (Hex) TORCH CURRENT (mA) 00 15 01 30 02 40 03 50 04 65 05 80 06 90 07 100 08 110 09 120 0A 130 0B 140 0C 150 0D 160 0E 170 0F 180 11 www.national.com LM2759 SETTING LED CURRENT The current through the LED is set by programming the appropriate register with the desired current level code for Flash and Torch. The time that Flash mode is active is dependent on the lesser of the duration that it is set to "ON" (via I2C or the Strobe pin), or the duration of the Flash Timeout. Use the tables below to select the desired current level. 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. LM2759 Suggested Capacitors and Suppliers MFG Part No. Type MFG Voltage Rating Case Size Inch (mm) 4.7 µF for COUT C1608X5R0J475 Ceramic X5R TDK 6.3V 0603 (1608) JMK107BJ475 Ceramic X5R Taiyo-Yuden 6.3V 0603 (1608) C1608X5R0J225 Ceramic X5R TDK 6.3V 0603 (1608) JMK107BJ225 Ceramic X5R Taiyo-Yuden 6.3V 0603 (1608) 2.2 µF for C1, C2, CIN POWER EFFICIENCY Efficiency of LED drivers is commonly taken to be the ratio of power consumed by the LED (PLED) to the power drawn at the input of the part (PIN). With a 1x, 1.5x, 2x charge pump, the input current is equal to the charge pump gain times the output current (total LED current). The efficiency of the LM2759 can be predicted as follows: The junction temperature rating takes precedence over the ambient temperature rating. The LM2759 may be operated outside the ambient temperature rating, so long as the junction temperature of the device does not exceed the maximum operating rating of 105°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 105°C. PLED = VLED × ILED MAXIMUM OUTPUT CURRENT The maximum LED current that can be used for a particular application depends on the rated forward voltage of the LED used, the input voltage range of the application, and the Gain mode of the LM2759’s charge pump. The following equation can be used to approximate the relationship between the maximum LED current, the LED forward voltage, the minimum input voltage, and the charge pump gain: (VIN_MIN x Gain) > (VF + VHR) + (ILED x ROUT_GAIN) VHR or the voltage required across the current sink to remain in regulation can be approximated by (ILED x KHR), where KHR is 0.8 mV/mA (typ). ROUT_GAIN is the output impedance of the charge pump according to its gain mode. When using the equation above, keep in mind that the (VF + VHR) portion of the equation can not be greater than the nominal output regulation voltage for a particular gain. In other words, when making calculations for an application where the term (VF + VHR) is higher than a particular gain’s regulation voltage, the next higher gain level must be used for the calculation. Example: VF = 4V @ 1A, Charge Pump in the Gain of 2x with a ROUT of 2.25Ω (typ.) VIN_MIN > [(4V + 0.8V) + (1A x 2.25Ω) ] ÷ 2 VIN_MIN > 3.53V (typ.) The maximum power dissipation in the LM2759 must also be taken into account when selecting the conditions for an application, such that the junction temperature of the device never exceeds its rated maximum. The input voltage range, operating temperature range, and/or current level of the application may have to be adjusted to keep the LM2759 within normal operating ratings. 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 (100mA - 1A). 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 LM2759 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 120°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, 2x charge pump, PLED is the power consumed by the LED, TA is the ambient temperature, and θJA is the junction-to-ambient thermal resistance for the 12 pin LLP package. VIN is the input voltage to the LM2759, 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 LLP 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) www.national.com 12 LM2759 Physical Dimensions inches (millimeters) unless otherwise noted 12–Pin LLP Package NS Package Number SDF12A X1 = X2 = 3.0mm X3 = 0.8mm 13 www.national.com LM2759 1A Switched Capacitor Flash LED Driver with I2C Compatible Interface Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com 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 References www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Applications & Markets 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 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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