LM2754SQX

LM2754 800mA Switched Capacitor Flash LED Driver with Time-Out Protection September 2006 LM2754 800mA Switched Capacitor Flash LED Driver with Time-Out Protection General Description The LM2754 is an integrated low noise, high current switched capacitor DC/DC converter with four regulated current sinks. The device is optimized for driving 1 to 4 high power white LEDs in parallel with a maximum current of 800mA. Maximum efficiency is achieved over the input voltage range by actively selecting the proper gain based on the LED forward voltage and current requirements. Two external low power resistors set the desired current for Torch and Flash modes. The TX pin allows the device to be forced into Torch mode during a Flash pulse, allowing for synchronization between the RF power amplifier pulse and Flash/Torch modes. To protect the device and Flash LEDs, internal Time-Out circuitry turns off the LM2754 in case of a faulty prolonged Flash mode. Internal soft-start circuitry limits the amount of inrush current during start-up. The LM2754 is available in a small 24-pin thermally enhanced LLP package. Features n n n n n n n n n n n n n n n Up to 800mA Output Current Wide Operating Input Voltage Range: 2.8V to 5.5V Drives 1, 2, 3 or 4 LEDs in Parallel Ability to Disable One Current Sink Via the SEL Pin to Accommodate 3-LED Flash Modules Time-Out Circuitry Limits Flash Duration to 1 Second TX Input Ensures Synchronization with RF Power Amplifier Pulse Adaptive 1x, 1.5x and 2x Gains for Maximum Efficiency 1MHz Constant Frequency Operation Output Current Limit True Shutdown Output Disconnect < 1µA Shutdown Current Internal Soft-Start Limits Inrush Current No Inductor Required Total Solution Size without LED < 28mm2 Low Profile 24-Pin LLP Package (4mm x 4mm x 0.8mm) Applications n Camera Flash in Mobile Phones n Flash for Digital Cameras n Supplies for DSP’s, Microprocessors, Memory, MP3 Players, Pagers, Other Portable Devices Typical Application Circuit 20202801 © 2006 National Semiconductor Corporation DS202028 www.national.com LM2754 Connection Diagram LM2754 24-pin No-Pullback Leadless Leadframe Package (LLP-24) 4mm x 4mm x 0.8mm NS Package Number SQA24A 20202802 Pin Descriptions Pin 23,24 22 8 12, 13, 14, 15 1, 2, 7, 5 3 9, 16, 17 21 20 10, 11 Name VINSW VIN VOUT D1, D2, D3, D4* GNDSW GND EN T/F ISET1, ISET2 Description Input Voltage Connection for Switch Array. Pins 23 and 24 are connected internally on the die. Connect VIN and VINSW pins together. Input Voltage Connection. Connect VIN and VINSW pins together. Output Voltage. Connect to LED Anodes. Regulated Current Sink Inputs. (* See SEL PIN description) Switch Array Ground Connection. Connect GND and GNDSW pins together. Ground Connection. Connect GND and GNDSW pins together. Enable Control Pin. Logic High = Normal Operation in Torch Mode. Logic Low = Device Shut-Down. (See Note) Torch/Flash Control Pin. Logic High = Flash Mode. Logic Low = Torch Mode. Device must be enabled for Torch or Flash to operate. (See Note) Current Set Resistor Connections. Connect 1% resistors to ground to set the desired current through the LEDs. LED current is approximated by the equation: 800 x (1.25V ÷ R). This equation corresponds to the current through one current sink. Total LED current is equal to the sum of currents through all current sinks connected to the LED. The equation used for Torch (ISET1) and Flash (ISET2) resistors are the same. RF PA synchronization control pin. Logic High = Force Torch Mode. Logic Low = Normal Operation. (See Applications Information section for the full operational description) (See Note) D4 Control Pin. Logic Low = Normal 4-LED Operation. Logic High = Disable D4 LED Input. Connect D4 to VOUT when not used. (See Note) Do not connect to any node. C1+, C1-, C2+, C2- Flying Capacitor Connections. 19 TX 18 4, 6 SEL No Connect Note: EN, T/F, TX, and SEL pins each have a 500kΩ resistor connected internally to GND Ordering Information Order Number LM2754SQ LM2754SQX Package Description No-Pullback LLP-24 Package Marking UZXYTT LM2754 Supplied as Tape and Reel (Units) 1000 4500 www.national.com 2 LM2754 Absolute Maximum Ratings (Notes 1, 2) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN, VOUT pins EN, T/F, TX, SEL pins Continuous Power Dissipation (Note 3) Junction Temperature (TJ-MAX-ABS) Storage Temperature Range Lead Temp. (Soldering, 5 sec.) ESD Rating (Note 4) Human Body Model -0.3V to 6.0V -0.3V to (VIN + 0.3V) w/ 6.0V max Internally Limited 150˚C -65˚C to 150˚C 260˚C 2kV (Notes 2, 7) Operating Ratings (Notes 1, 2) Input Voltage (VIN) Junction Temperature Range (TJ) Ambient Temperature Range (TA) (Note 5) 2.8V to 5.5V -40˚C to +125˚C -40˚C to +85 ˚C Thermal Information Junction-to-Ambient Thermal Resistance, LLP-24 Package (θJA) (Note 6) 42˚C/W Electrical Characteristics Limits in standard typeface are for TJ = 25˚C, and limits in boldface type apply over the full operating junction temperature range (-40˚C to +125 ˚C). Unless otherwise noted, specifications apply to the LM2754 Typical Application Circuit (pg.1) with V(IN, INSW) = 3.6V, VEN = 1.8V, VT/F = 0V, VTX = 0V, VSEL = 0V, CIN = C1 = C2 = 2.2µF, COUT = 4.7µF. (Note 8) Symbol VSETx IDx/ISETx VHR VOUT Parameter ISETx Pin Voltage LED Current to Set Current Ratio (Note 11) Current Sink Headroom Voltage (Note 10) Output Voltage RSETx = 20kΩ IDx = 50mA to 100mA IDx = 200mA IDx = 200mA IDx = 50mA 1x Mode, IDx = 0mA 1.5x Mode, IDx = 0mA 2x Mode, IDx = 0mA ROUT Output Impedance 1x Mode 1.5x Mode 2x Mode IQ Quiescent Supply Current 1x Mode, IDx = 0mA 1.5x Mode, IDx = 0mA 2x Mode, IDx = 0mA ISD fSW VIH VIL IIH IIL Shutdown Supply Current Switching Frequency Logic Input High Logic Input Low Logic Input High Current (Note 9) Logic Input Low Current (Note 9) Input Pins: EN, T/F, TX, SEL Input Pins: EN, T/F, TX, SEL V(EN, T/F, TX, SEL) = 1.8V V(EN, T/F, TX, SEL) = 0V 4 0.5 VEN = 0V 0.7 1.2 0.4 µA µA Conditions Min −3.5% −7% −11.5% Typ 1.244 795 820 550 150 4.7 4.7 5.1 0.25 1.3 1.5 0.7 3.4 6.3 0.1 1 8 1 1.3 µA MHz V mA Ω V Max +3.5% +7% +11.5% mV Units V mA/mA 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 at 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 = 120˚C (typ.). Note 4: The Human-body model is a 100 pF capacitor discharged through a 1.5kΩ resistor into each pin. Note 5: 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 x PD-MAX). Note 6: 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 102mm x 76mm x 1.6mm. The 2 imbedded copper layers cover roughly the same area as the board. Thickness of copper layers are 70µm/35µm/35µm/70µm (2oz/1oz/1oz/2oz). Thermal vias are placed between the die attach pad in the 1st copper layer 3 www.national.com LM2754 Electrical Characteristics (Notes 2, 7) (Continued) and the 2nd copper layer. Ambient temperature in simulation is 22˚C, still air. Power dissipation is 1W. The value of θJA of the LM2754 in LLP-24 could fall in a range as wide as 35oC/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 Gain, 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 7: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm. Note 8: CIN, COUT, C1, C2: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics Note 9: There is a 500kΩ resistor connected internally between each logic pin (EN, T/F, TX, SEL) and GND. Note 10: Headroom Voltage (VHR) is the voltage across the current sinks (VDx) at which the current falls to 95% of the nominal programmed current. VHR is measured from VDx to GND. If the headroom voltage requirement is not met, LED current regulation will be compromised. Note 11: IDx/ISETx Ratio was tested with the Charge Pump in a gain of 1x. Block Diagram 20202803 www.national.com 4 LM2754 Typical Performance Characteristics Unless otherwise specified: TA = 25˚C, VDx = 1V, V(IN, INSW) = 3.6V, VEN = VIN, VT/F = VTX = VSEL = 0V, CIN = C1 = C2 = 2.2µF, COUT = 4.7µF. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC’s). Dx Current vs. RSET Dx Current vs. Headroom Voltage 20202804 20202805 Quiescent Current vs. Input Voltage Shutdown Current vs. Input Voltage 20202806 20202807 Efficiency vs. Input Voltage Oscillator Frequency vs. Input Voltage 20202808 20202809 5 www.national.com LM2754 Typical Performance Characteristics Unless otherwise specified: TA = 25˚C, VDx = 1V, V(IN, INSW) = 3.6V, VEN = VIN, VT/F = VTX = VSEL = 0V, CIN = C1 = C2 = 2.2µF, COUT = 4.7µF. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC’s). (Continued) Flash Mode with TX Pulse Torch to Flash Mode 20202810 20202811 VIN = 3.6V, Load = 700mA (Flash), LED = PWF1 CH1 (TOP): VTX; Scale: 1V/Div, DC Coupled CH2 (BOTTOM): ILED; Scale: 200mA/Div Time scale: 1ms/Div VIN = 3.6V, Load = 200mA/800mA (Torch/Flash), LED = PWF1 CH1 (TOP): VIN; Scale: 1V/Div, DC Coupled CH2 (MIDDLE): VOUT; Scale: 1V/Div, DC Coupled CH3 (BOTTOM): IIN; Scale: 200mA/Div Time scale: 100ms/Div Torch to Flash Mode Rising Edge Flash to Torch Mode Falling Edge 20202812 VIN = 3.6V, Load = 200mA/800mA (Torch/Flash), LED = PWF1 CH1 (TOP): VIN; Scale: 1V/Div, DC Coupled CH2 (MIDDLE): VOUT; Scale: 1V/Div, DC Coupled CH3 (BOTTOM): IIN; Scale: 200mA/Div Time scale: 400µs/Div CH1 (TOP): VIN; Scale: 1V/Div, DC Coupled 20202813 VIN = 3.6V, Load = 800mA/200mA (Flash/Torch), LED = PWF1 CH2 (MIDDLE): VOUT; Scale: 1V/Div, DC Coupled CH3 (BOTTOM): IIN; Scale: 200mA/Div Time scale: 100µs/Div www.national.com 6 LM2754 Typical Performance Characteristics Unless otherwise specified: TA = 25˚C, VDx = 1V, V(IN, INSW) = 3.6V, VEN = VIN, VT/F = VTX = VSEL = 0V, CIN = C1 = C2 = 2.2µF, COUT = 4.7µF. Capacitors are low-ESR multi-layer ceramic capacitors (MLCC’s). (Continued) Battery Ripple, Gain = 1.5x Battery Ripple, Gain = 2x 20202814 20202815 VIN = Li-Ion Battery at 3.7V, Load = 400mA, LED = PWF1 CH1 (MID-TOP): VOUT; Scale: 20mV/Div, AC Coupled CH2 (MID-BOTTOM): VIN; Scale: 50mV/Div, AC Coupled CH3 (TOP): VIN; Scale: 1V/Div, DC Coupled CH4 (BOTTOM): IIN; Scale: 500mA/Div Time scale: 1µs/Div VIN = Li-Ion Battery at 3.7V, Load = 400mA, LED = PWF1 CH1 (MID-TOP): VOUT; Scale: 20mV/Div, AC Coupled CH2 (MID-BOTTOM): VIN; Scale: 50mV/Div, AC Coupled CH3 (TOP): VIN; Scale: 1V/Div, DC Coupled CH4 (BOTTOM): IIN; Scale: 500mA/Div Time scale: 1µs/Div Startup and Shutdown Response Torch Mode (400mA) 20202816 VIN = 3.6V, Load = 400mA, LED = PWF1 CH1 (TOP): VOUT; Scale: 1V/Div, DC Coupled CH4 (MIDDLE): IIN; Scale: 200mA/Div CH3 (BOTTOM): IOUT; Scale: 200mA/Div Time scale: 100ms/Div 7 www.national.com LM2754 Application Information CIRCUIT DESCRIPTION The LM2754 is an adaptive 1x/1.5x/2x CMOS charge pump, optimized for driving Flash LEDs in camera phone and other portable applications. It provides four constant current inputs, each capable of sinking up to 200mA for Flash mode, and 100mA for Torch mode. Each LED is driven from VOUT and connected to one of the four current sinks. LED drive current for Torch mode is programmed by connecting a resistor, RSET1, to the current set pin, ISET1. LED drive current for Flash mode is set by connecting a resistor, RSET2, to the current set pin, ISET2. Torch mode is enabled by the EN pin, and the transition from Torch to Flash mode is controlled by the T/F pin. This device also has an option to disable the D4 current sink via the SEL pin, for Flash LED modules with only 3 LEDs. To prevent high battery load during a simultaneous RF PA transmission pulse and Flash condition, this device has a Flash interrupt pin (TX) to reduce the LED current to the Torch mode level for the duration of the RF PA transmission pulse. CHARGE PUMP 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. The device’s loosely-regulated charge pump has both open loop and closed loop modes of operation. Under no-load conditions, open loop operation occurs when VOUT is equal to the product of the input voltage and the charge pump gain, and is less than the nominal output regulation voltage. Over the recommended input voltage range of 3.0V to 5.5V, unloaded open loop operation will only occur in 1x and 1.5x gains. When the LM2754 is in closed loop operation with no-load, the voltage at VOUT is loosely regulated to 4.7V (typ.) for the 1x and 1.5x gains, and 5.1V (typ.) for the 2x gain. 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 LM2754. The charge pump gain transitions are actively selected to maintain regulation based on LED forward voltage and load requirements. The charge pump only transitions to higher gains, from 1x to 1.5x and 1.5x to 2x. Each transition from one gain to the next takes 125ms (typ.) for Torch mode and 2ms (typ.) for Flash 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. SOFT START The LM2754 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 LM2754 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 1.2A (typ.). LOGIC CONTROL PINS There are 4 logic control pins for the LM2754. All pins are active-High logic (High = Function ON). There is an internal pull-down resistor (500kΩ typ.) connected between each logic pin and GND. The operating modes for the part function according to the Table below: EN 0 1 1 1 1 T/F X 0 0 1 1 TX SEL X X X 0 0 X 0 1 0 1 Mode Part in Shutdown Part Enabled, Current set by RSET1, D1-4 Active Part Enabled, Current set by RSET1, D1-3 Active, D4 Disabled Part Enabled, Current set by RSET2, D1-4 Active Part Enabled, Current set by RSET2, D1-3 Active, D4 Disabled Part Enabled, Current set by RSET1, TX signal from RF PA, D1-4 Active Part Enabled, Current set by RSET1, TX signal from RF PA, D1-3 Active, D4 Disabled 1 1 1 0 1 1 1 1 EN PIN (TORCH) The EN pin is the master enable pin for the part. When the voltage on this pin is Low ( < 0.4V), the part is in shutdown mode. In this mode, all internal circuitry is OFF, VOUT is disconnected from the VIN, and the part consumes very little supply current ( < 1µA typ.). When the voltage on the EN pin is High ( > 1.2V), the part will activate the charge pump and regulate the output voltage to its nominal value. When the output voltage reaches its regulation level, the current sinks will turn on and sink the current programmed by RSET1 (assuming the logic on T/F is Low). Enabling the device is also referred to as Torch Mode. For correct start-up sequencing, power must be applied to VIN before a High logic signal is applied to the EN pin. T/F PIN (FLASH) AND FLASH TIMEOUT A logic Low ( < 0.4V) signal on the T/F pin disables the Flash mode, defaulting the current through the LEDs to the Torch level programmed by RSET1. Applying a logic High ( > 1.2V) signal to T/F places the device in Flash mode, with the LED current set by RSET2. Flash Timeout Protection Circuitry disables the current sinks when the signal on T/F is held high for more than 1 second (typ). This prevents the device from self-heating due to the high power dissipation during Flash conditions. During the timeout condition, voltage will still be present on VOUT but the current sinks will be shut off, resulting in no current through the Flash LEDs. When the device goes into a timeout condition, placing a logic Low signal on EN will reset the timeout and a subsequent logic High signal on EN will return the device to normal operation. Flash timeout is not active during TX mode. TX PIN The TX pin on the LM2754 disables the Flash operation during a RF PA transmission pulse, and sets the LED current to the Torch level programmed by RSET1 for the duration of that pulse. At the end of each transmission interrupt pulse www.national.com 8 LM2754 Application Information (Continued) signal on the TX pin, the LED current level returns to the Flash current level set by RSET2. The TX pin responds to the typical logic High ( > 1.2V) and logic Low ( < 0.4V) signal levels. Flash Timeout is not active during the TX mode operation. SEL PIN Connecting the SEL pin to a logic Low ( < 0.4V) signal places the device in normal operation, with all 4 current sinks active. To accommodate Flash LED modules with only 3 LEDs, place a logic High ( > 1.2V) signal on the SEL pin to disable the current sink D4. If only 3 current sinks are used, the 200mA per current sink recommendation still applies, and the maximum Flash current will be 600mA. Connect D4 to VOUT when the logic in the SEL pin is High. Optional use of the SEL pin is to reduce the LED current used for Torch or Flash by 25% for high battery load conditions. SETTING LED CURRENTS The current through the LEDs connected to D1-4 can be set simply by connecting an appropriately sized resistor (RSETx) between the ISET1 pin of the LM2754 and GND for Torch mode and the ISET2 pin and GND for Flash Mode. The LED currents are proportional to the current that flows out of the ISETx pin and are a factor of approximately 800 times greater than the ISETx current. The feedback loop of an internal amplifier sets the voltage of the ISET pin to 1.25V (typ.). The statements above are simplified in the equations below: IDx = 800 x(VSET / RSET) RSET = 800 x (1.25V / IDx) The maximum recommended current through each current sink is 100mA during Torch mode and 200mA during Flash mode. Maximum recommended total Flash current with all 4 current sinks used is 800mA (max 200mA per current sink). 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. PARALLEL DX OUTPUTS FOR INCREASED CURRENT DRIVE Outputs D1-4 may be connected together to drive a one or two LEDs at higher currents. In applications using a single LED, all four parallel current sinks of equal value drive the single LED. For this type of configuration, the LED current should be programmed so that the current through each of the outputs is 25% of the total desired LED current. For example, if 200mA is the desired drive current for the single LED, RSET should be selected such that the current through each of the current sink inputs is 50mA. Similarly, if two LEDs are to be driven by pairing up the D1-4 inputs (i.e D1-2, D3-4), RSET should be selected such that the current through each current sink input is 50% of the desired LED current. Connecting the outputs in parallel does not affect internal operation of the LM2754 and has no impact on the Electrical Characteristics and limits previously presented. The available diode output current, maximum diode voltage, and all other specifications provided in the Electrical Characteristics table apply to this parallel output configuration, just as they do to the standard 4-LED application circuit. Maximum recommended LED current for any configuration is 200mA per current sink, and 800mA total. For situations where only 3 current sinks will be used for the application, see the SEL PIN operation section. CAPACITOR SELECTION The LM2754 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 < 20mΩ typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors are not recommended for use with the LM2754 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 LM2754. 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 LM2754. 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 1µF Y5V or Z5U capacitor could have a capacitance of only 0.1µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum capacitance requirements of the LM2754. The voltage rating of the output capacitor should be 10V or more. For example, a 10V 0603 4.7µF output capacitor (TDK C1608X5R1A475) is acceptable for use with the LM2754, 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 1µF. 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/2x charge pump, the input current is equal to the charge pump gain times the output current (total LED current). The efficiency of the LM2754 can be predicted as follows: PLED = N x VLED x ILED PIN = VIN x IIN PIN = VIN x (Gain x N x 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 x 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 (100-800mA). 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. 9 www.national.com LM2754 Application Information THERMAL PROTECTION (Continued) TJ = TA + (PDISSIPATION x θJA) The junction temperature rating takes precedence over the ambient temperature rating. The LM2754 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 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. Internal thermal protection circuitry disables the LM2754 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 LEDs, TA is the ambient temperature, and θJA is the junction-to-ambient thermal resistance for the LLP-24 package. VIN is the input voltage to the LM2754, VLED is the nominal LED forward voltage, and ILED is the programmed LED current. PDISSIPATION = PIN - PLED = [Gain x VIN x (4 x ILED)] − (VLED x 4 x ILED) PCB Layout Considerations The LLP is a leadframe based Chip Scale Package (CSP) with very good thermal properties. This package has an exposed DAP (die attach pad) at the center of the package measuring 2.6mm x 2.6mm. The main advantage of this exposed DAP is to offer lower thermal resistance when it is soldered to the thermal land on the PCB. For PCB layout, National highly recommends a 1:1 ratio between the package and the PCB thermal land. To further enhance thermal conductivity, the PCB thermal land may include vias to a ground plane. For more detailed instructions on mounting LLP packages, please refer to National Semiconductor Application Note AN-1187. www.national.com 10 LM2754 800mA Switched Capacitor Flash LED Driver with Time-Out Protection Physical Dimensions inches (millimeters) unless otherwise noted 24-Pin LLP NS Package Number SQA24A National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. For the most current product information visit us at www.national.com. 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