LM2758TL

LM2758 Switched Capacitor Flash LED Driver in micro SMD October 9, 2009 LM2758 Switched Capacitor Flash LED Driver in micro SMD General Description 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. Features ■ 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 Connection Diagram 12-Bump Thin Micro SMD 1.514mm x 1.996mm x 0.6mm NS Package Number TLA12 30045302 Pin Descriptions Pin C1 B2 B1 D2 C2 D1 A2 C3 A1 B3 A3 D3 Name VIN CPOUT C1+ C1− C2+ C2− SGND LED− EN1 EN2 ISET PGND Analog and control ground for charge pump. This pin should be connected directly to a low impedance ground plane. Regulated current source output. Connect flash LED cathode to this pin. The EN1 and EN2 pins are used to select the modes (Torch, Indictor, Flash), as well as to put the part into Shutdown mode. LED current programming resistor pin. A resistor connected between this pin and GND are used to set Torch, Flash and Indicator currents. 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−. 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. Description Ordering Information Order Number LM2758TL LM2758TLX Supplied as Tape and Reel (Units) 250 units, tape and reel 3000 units, tape and reel www.national.com 2 LM2758 Absolute Maximum Ratings (Note 1, Note 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 -0.3V to 6.0V -0.3V to (VIN + 0.3V) w/ 6.0V max Internally Limited 150°C -65°C to 150°C (Note 5) 2kV 200V Operating Ratings (Note 1, Note 2) 2.7V to 5.5V -40°C to +125°C -40°C to +85 °C Input Voltage Range Junction Temperature Range (TJ) Ambient Temperature Range (TA) (Note 6) Thermal Information Junction-to-Ambient Thermal Resistance Resistance (θJA), micro SMD package (Note 7) 56°C/W 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 ILED VSET ID/ISET ILED-IND Indicator Current Level VGDX Parameter LED Current Accuracy ISET Pin Voltage LED Current to Set Current Flash Mode Ratio Torch Mode Indicator Mode 32 kHZ PWM Mode Conditions ILED = 500 mA, Flash Mode Min 450 Typ 500 1.3 7650 1639 1/32 x ILEDTORCH Max 550 Units mA V mA 1x to 1.5x Gain Transition Voltage Threshold on IOUT = 500 mA VLEDOutput Voltage 1x Mode Output Impedance 1.5x Mode Output Impedance 1x Mode, IOUT = 0 mA 1.5x Mode, IOUT = 0 mA (Note 10) IOUT = 200 mA, VIN = 3.3V IOUT = 500 mA, VIN = 3.3V (Note 11) 0.8 IOUT = 0 mA 1x Mode IOUT = 0 mA 1.5x Mode Device Disabled (Note 12) (Note 13) Pins: EN1, EN2 Pins: EN1, EN2 640 1.2 300 VIN 4.8 0.33 1.5 1.25 0.7 4 0.01 814 5.3 0.53 2.0 1.5 0.8 5 1 1000 0.4 mV VOUT ROUT V Ω FSW IQ ISD TOUT VIH VIL Switching Frequency Quiescent Current Shutdown Current Time-out Duration Input Logic High Input Logic Low MHz mA µA msec V V 3 www.national.com 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 30045306 30045307 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 30045311 Shutdown to Indicator Mode VIN = 3.6V, ILED(Torch) = 108 mA, EN2 = 0V 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 30045314 30045315 www.national.com 8 LM2758 Application Information 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. 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. 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 where excessive current is drawn. Output current is limited to 1.2A typically. LOGIC CONTROL PINS LM2758 has two logic pins, EN1 and EN2. The operating modes of the part function according to the tables below: EN1 0 1 0 1 EN2 0 0 1 1 Mode Shutdown Indicator Torch 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. 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. 9 www.national.com 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. 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