LM2756

LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in micro SMD December 5, 2007 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in micro SMD General Description The LM2756 is a highly integrated, switched-capacitor, multidisplay LED driver that can drive up to 8 LEDs in parallel with a total output current of 180mA. Regulated internal current sources deliver excellent current and brightness matching in all LEDs. The LED driver current sinks are split into three independently controlled groups. The primary group (Group A) can be configured to drive four, five or six LEDs for use in the main phone display, while the secondary group (Group B) can be configured to drive one, two or three LEDs for driving secondary displays, keypads and/or indicator LEDs. An additional driver, D1C, is provided for additional indicator lighting functions. The device provides excellent efficiency without the use of an inductor by operating the charge pump in a gain of 3/2 or in Pass-Mode. The proper gain for maintaining current regulation is chosen, based on LED forward voltage, so that efficiency is maximized over the input voltage range. The LM2756 is available in National’s tiny 20-bump, 0.4mm pitch, thin micro SMD package. Features ■ Drives up to 8 LEDs with up to 30mA of Diode Current Each ■ 32 Exponential Dimming Steps with 800:1 Dimming Ratio ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ for Group A (Up to 6 LEDs) 8 Linear Dimming States for Groups B (Up to 3 LEDs) and D1C (1 LED) Programmable Auto-Dimming Function 3 Independently Controlled LED Groups Via I2C Compatible Interface Up to 90% Efficiency Total Solution Size < 21mm2 Low Profile 20 Bump micro SMD Package (1.615mm × 2.015mm × 0.6mm) 0.4% Accurate Current Matching Internal Soft-Start Limits Inrush Current True Shutdown Isolation for LED’s Wide Input Voltage Range (2.7V to 5.5V) Active High Hardware Enable Applications ■ Dual Display LCD Backlighting for Portable Applications ■ Large Format LCD Backlighting ■ Display Backlighting with Indicator Light Typical Application Circuit 30009701 © 2007 National Semiconductor Corporation 300097 www.national.com LM2756 30009741 Minimum Layout Connection Diagram 20 Bump micro SMD Package NS Package Number TMD20AAA 30009702 Pin Descriptions Bump #s TMD20AAA A3 A2 A1, C1, B1, B2 D3, E3,E4, D4 C4, B4 B3 C3 D2 Pin Names VIN VOUT C1+, C1-, C2+, C2D1A-D4A D53, D62 D1B D1C ISET Pin Descriptions Input voltage. Input range: 2.7V to 5.5V. Charge Pump Output Voltage Flying Capacitor Connections LED Drivers - GroupA LED Drivers - Configurable Current Sinks. Can be assigned to GroupA or GroupB LED Drivers - GroupB LED Driver - Indicator LED Placing a resistor (RSET) between this pin and GND sets the full-scale LED current for DxA , DxB, D53, D62 and D1C LEDs. Full-Scale LED Current = 189 × (1.25V ÷ RSET) Hardware Enable Pin. High = Normal Operation, Low = RESET Serial Data Input/Output Pin Serial Clock Pin Ground E1 C2 E2 A4, D1 HWEN SDIO SCL GND Ordering Information Order Information LM2756TM LM2756TMX Package TMD20AAA Supplied As 250 Units, Tape & Reel 3000 Units, Tape & Reel www.national.com 2 LM2756 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 pin voltage SCL, SDIO, HWEN pin voltages IDxx Pin Voltages Continuous Power Dissipation (Note 3) Junction Temperature (TJ-MAX) Storage Temperature Range Maximum Lead Temperature (Soldering) ESD Rating(Note 5) Human Body Model -0.3V to 6.0V -0.3V to (VIN+0.3V) w/ 6.0V max -0.3V to (VVOUT+0.3V) w/ 6.0V max Internally Limited 150°C -65°C to +150° C (Note 4) Operating Rating (Notes 1, 2) Input Voltage Range LED Voltage Range Junction Temperature (TJ) Range Ambient Temperature (TA) Range (Note 6) 2.7V to 5.5V 2.0V to 4.0V -30°C to +105°C -30°C to +85°C Thermal Properties Junction-to-Ambient Thermal Resistance (θJA), TMD20 Package (Note 7) 40°C/W ESD Caution Notice 2.0kV 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 (Notes 2, 8) Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range. Unless otherwise specified: VIN = 3.6V; VHWEN = VIN; VDxA = VDxB = VDxC = 0.4V; RSET = 11.8kΩ; GroupA = GroupB = GroupC = Fullscale Current; ENA, ENB, ENC Bits = “1”; SD53, SD62, 53A, 62A Bits = "0"; C1 = C2 = CIN= COUT= 1.0µF; Specifications related to output current(s) and current setting pins (IDxx and ISET) apply to GroupA and GroupB. (Note 9) Symbol Parameter Condition 2.7V ≤ VIN ≤ 5.5V ENA = '1', 53A = 62A = '0'', ENB = ENC = '0' 4 LEDs in GroupA Min 18.65 (-8%) Typ 20.28 Max 21.90 (+8%) 22.10 (+8.5%) 21.60 (+8%) 21.20 (+7.5%) Units mA (%) mA (%) mA (%) mA (%) mA Output Current Regulation GroupA 2.7V ≤ VIN ≤ 5.5V 18.70 ENA = '1', 53A = 62A = '1', ENB = ENC = '0' (-8.5%) 6 LEDs in GroupA 2.7V ≤ VIN ≤ 5.5V ENB = '1', 53A = 62A = '0', ENA = ENC = '0' 3 LEDs in GroupB 2.7V ≤ VIN ≤ 5.5V ENC = '1', ENA = ENB = '0' RSET = 8.33kΩ 18.40 (-8%) 18.20 (-7.5%) 20.40 Output Current Regulation GroupB IDxx Output Current Regulation IDC Maximum Diode Current per Dxx Output(Note 10) Output Current Regulation GroupA, GroupB, and GroupC Enabled (Note 10) 20.00 19.70 30 22.5 DxA 22.5 DxB 22.5 DxC 3.2V ≤ VIN ≤ 5.5V VLED = 3.6V RSET = 10.5kΩ GroupA (4 LEDs) mA IDxxMATCH 0.4 1.0 0.7 150 1.8 2.7 2.5 mV % LED Current Matching(Note 11) VDxx 1x to 3/2x Gain Transition Threshold Current sink Headroom Voltage Requirement (Note 12) Open-Loop Charge Pump Output Resistance Quiescent Supply Current 2.7V ≤ VIN ≤ 5.5V GroupA (6 LEDs) GroupB (3 LEDs) VDxTH VHR VDxA and/or VDxB Falling IDxx = 95% ×IDxx (nom.) (IDxx (nom) ≈ 20mA) Gain = 3/2 Gain = 1 Gain = 1.5x, No Load 65 2.4 0.9 2.1 2.5 mV ROUT IQ Ω mA 3 www.national.com LM2756 Symbol ISD VSET IDxA-B-C / ISET fSW tSTART VHWEN Parameter Shutdown Supply Current ISET Pin Voltage Output Current to Current Set Ratio GroupA, GroupB, GroupC Switching Frequency Start-up Time HWEN Voltage Thresholds Condition All ENx bits = "0" 2.7V ≤ VIN ≤ 5.5V Min Typ 3.7 1.25 189 Max 5.5 Units µA V 1.0 VOUT = 90% steady state 2.7V ≤ VIN ≤ 5.5V Reset Normal Operation 0 1.075 1.3 250 1.6 0.580 VIN MHz µs V I2C Compatible Interface Voltage Specifications (SCL, SDIO) VIL VIH VOL t1 t2 t3 t4 t5 Input Logic Low "0" Input Logic High "1" Output Logic Low "0" SCL (Clock Period) Data In Setup Time to SCL High Data Out stable After SCL Low SDIO Low Setup Time to SCL Low (Start) SDIO High Hold Time After SCL High (Stop) 2.7V ≤ VIN ≤ 5.5V 2.7V ≤ VIN ≤ 5.5V ILOAD = 3.5mA (Note 14) 294 100 0 100 100 0 1.225 0.710 VIN 400 V V mV ns ns ns ns ns I2C Compatible Interface Timing Specifications (SCL, SDIO)(Note 13) 30009713 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 pins. Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ = 160°C (typ.) and disengages at TJ = 155°C (typ.). Note 4: For detailed soldering specifications and information, please refer to National Semiconductor Application Note 1112: Micro SMD Wafer Level Chip Scale Package (AN-1112). Note 5: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. (MIL-STD-883 3015.7) 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 operating junction temperature (TJ-MAX-OP = 105°C), 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 is highly dependent on application and board layout. In applications where high maximum power dissipation exists, special care must be paid to thermal dissipation issues in board design. For more information, please refer to National Semiconductor Application Note 1112: Micro SMD Wafer Level Chip Scale Package (AN-1112). Note 8: 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 9: CIN, CVOUT, C1, and C2 : Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics www.national.com 4 LM2756 Note 10: The maximum total output current for the LM2756 should be limited to 180mA. The total output current can be split among any of the three Groups (IDxA = IDxB = IDxC = 30mA Max.). Under maximum output current conditions, special attention must be given to input voltage and LED forward voltage to ensure proper current regulation. See the Maximum Output Current section of the datasheet for more information. Note 11: For the two groups of current sinks on a part (GroupA and GroupB), the following are determined: the maximum sink current in the group (MAX), the minimum sink current in the group (MIN), and the average sink current of the group (AVG). For each group, two matching numbers are calculated: (MAX-AVG)/ AVG and (AVG-MIN)/AVG. The largest number of the two (worst case) is considered the matching figure for the Group. The matching figure for a given part is considered to be the highest matching figure of the two Groups. The typical specification provided is the most likely norm of the matching figure for all parts. Note 12: For each Dxx pin, headroom voltage is the voltage across the internal current sink connected to that pin. For Group A, B, and C current sinks, VHRx = VOUT -VLED. If headroom voltage requirement is not met, LED current regulation will be compromised. Note 13: SCL and SDIO should be glitch-free in order for proper brightness control to be realized. Note 14: SCL is tested with a 50% duty-cycle clock. Block Diagram 30009703 5 www.national.com LM2756 Typical Performance Characteristics Unless otherwise specified: TA = 25°C; VIN = 3.6V; VHWEN = VIN; VLEDxA = VLEDxB = VLED1C = 3.6V; RSET = 11.8kΩ; C1=C2= CIN = CVOUT = 1µF; ENA = ENB = ENC = '1'. LED Drive Efficiency vs Input Voltage LED Drive Efficiency vs Input Voltage 30009719 30009721 Input Current vs Input Voltage GroupA Diode Current vs Input Voltage 30009720 30009726 GroupB Diode Current vs Input Voltage GroupC Diode Current vs Input Voltage 30009727 30009728 www.national.com 6 LM2756 GroupA Current Matching vs Input Voltage 6 LEDs GroupA Current Matching vs Input Voltage 4 LEDs 30009716 30009717 GroupB Current Matching vs Input Voltage 3 LEDs GroupA Diode Current vs GroupA Brightness Code 30009722 30009718 GroupB Diode Current vs GroupB Brightness Code GroupC Diode Current vs GroupC Brightness Code 30009723 30009724 7 www.national.com LM2756 Quiescent Current in Gain 1.5× vs Input Voltage Shutdown Current vs Input Voltage 30009714 30009715 www.national.com 8 LM2756 Circuit Description OVERVIEW The LM2756 is a white LED driver system based upon an adaptive 3/2× - 1× CMOS charge pump capable of supplying up to 180mA of total output current. With three separately controlled Groups of constant current sinks, the LM2756 is an ideal solution for platforms requiring a single white LED driver for main display, sub display, and indicator lighting. The tightly matched current sinks ensure uniform brightness from the LEDs across the entire small-format display. Each LED is configured in a common anode configuration, with the peak drive current being programmed through the use of an external RSET resistor. An I2C compatible interface is used to enable the device and vary the brightness within the individual current sink Groups. For GroupA , 32 exponentially-spaced analog brightness control levels are available. GroupB and GroupC have 8 linearly-spaced analog brightness levels. CIRCUIT COMPONENTS Charge Pump The input to the 3/2× - 1× charge pump is connected to the VIN pin, and the regulated output of the charge pump is connected to the VOUT pin. The recommended input voltage range of the LM2756 is 2.7V to 5.5V. The device’s regulated charge pump has both open loop and closed loop modes of operation. When the device is in open loop, the voltage at VOUT is equal to the gain times the voltage at the input. When the device is in closed loop, the voltage at VOUT is regulated to 4.6V (typ.). The charge pump gain transitions are actively selected to maintain regulation based on LED forward voltage and load requirements. LED Forward Voltage Monitoring The LM2756 has the ability to switch gains (1x or 3/2x) based on the forward voltage of the LED load. This ability to switch gains maximizes efficiency for a given load. Forward voltage monitoring occurs on all diode pins. At higher input voltages, the LM2756 will operate in pass mode, allowing the VOUT voltage to track the input voltage. As the input voltage drops, the voltage on the Dxx pins will also drop (VDXX = VVOUT – VLEDx). Once any of the active Dxx pins reaches a voltage approximately equal to 150mV, the charge pump will switch to the gain of 3/2. This switch-over ensures that the current through the LEDs never becomes pinched off due to a lack of headroom across the current sinks. Once a gain transition occurs, the LM2756 will remain in the gain of 3/2 until an I2C write to the part occurs. At that time, the LM2756 will re-evaluate the LED conditions and select the appropriate gain. Only active Dxx pins will be monitored. For example, if only GroupA is enabled, the LEDs in GroupB or GroupC will not affect the gain transition point. If all 3 Groups are enabled, all diodes will be monitored, and the gain transition will be based upon the diode with the highest forward voltage. Configurable Gain Transition Delay To optimize efficiency, the LM2756 has a user selectable gain transition delay that allows the part to ignore short duration input voltage drops. By default, the LM2756 will not change gains if the input voltage dip is shorter than 3 to 6 milliseconds. There are four selectable gain transition delay ranges available on the LM2756. All delay ranges are set within the VF Monitor Delay Register . Please refer to the INTERNAL REG- ISTERS section of this datasheet for more information regarding the delay ranges. HWEN Pin The LM2756 has a hardware enable/reset pin (HWEN) that allows the device to be disabled by an external controller without requiring an I2C write command. Under normal operation, the HWEN pin should be held high (logic '1') to prevent an unwanted reset. When the HWEN is driven low (logic '0'), all internal control registers reset to the default states and the part becomes disabled. Please see the Electrical Characteristics section of the datasheet for required voltage thresholds. I2C Compatible Interface DATA VALIDITY The data on SDIO 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. 30009725 FIGURE 1. Data Validity Diagram A pull-up resistor between the controller's VIO line and SDIO must be greater than [ (VIO-VOL) / 3.5mA] to meet the VOL requirement on SDIO. 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. START AND STOP CONDITIONS START and STOP conditions classify the beginning and the end of the I2C session. A START condition is defined as SDIO signal transitioning from HIGH to LOW while SCL line is HIGH. A STOP condition is defined as the SDIO 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. 30009711 FIGURE 2. Start and Stop Conditions TRANSFERING DATA Every byte put on the SDIO line must be eight bits long, with the most significant bit (MSB) transferred first. Each byte of data has to be followed by an acknowledge bit. The acknowledge related clock pulse is generated by the master. The 9 www.national.com LM2756 master releases the SDIO line (HIGH) during the acknowledge clock pulse. The LM2756 pulls down the SDIO line during the 9th clock pulse, signifying an acknowledge. The LM2756 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 LM2756 address is 36h. 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. 30009712 FIGURE 3. Write Cycle w = write (SDIO = "0") r = read (SDIO = "1") ack = acknowledge (SDIO pulled down by either master or slave) id = chip address, 36h for LM2756 I2C COMPATIBLE CHIP ADDRESS The chip address for LM2756 is 0110110, or 36h. 30009708 FIGURE 5. General Purpose Register Description Internal Hex Address: 10h Note: ENA: Enables DxA LED drivers (Main Display) ENB: Enables DxB LED drivers (Aux Lighting) ENC: Enables D1C LED driver (Indicator Lighting) SD53: Shuts down driver D53 SD62: Shuts down driver D62 53A: Configures D53 to GroupA 62A: Configures D62 to GroupA 30009709 FIGURE 4. Chip Address INTERNAL REGISTERS OF LM2756 Register General Purpose Register Internal Hex Address 10h Power On Value 0000 0000 1110 0000 Group A A0h Brightness Control Register Group B B0h Brightness Control Register Group C C0h Brightness Control Register Ramp Step Time Register VF Monitor Delay Ragister 20h 60h 30009705 1111 1000 30009706 1111 1000 1111 0000 30009707 1111 1100 FIGURE 6. Brightness Control Register Description www.national.com 10 LM2756 Internal Hex Address: 0xA0 (GroupA), 0xB0 (GroupB), 0xC0 (GroupC) Note: DxA4-DxA0, D53, D62: Sets Brightness for DxA pins (GroupA). 11111=Fullscale DxB2-DxB0: Sets Brightness for DxB pins (GroupB). 111=Fullscale DxC2-DxC0: Sets Brightness for D1C pin. 111 = Fullscale Full-Scale Current set externally by the following equation: IDxx = 189 × 1.25V / RSET 30009735 FIGURE 7. Ramp Step Time Register Description Internal Hex Address: 20h Note: RS1-RS0: Sets Brightness Ramp Step Time. The Brightness ramp settings only affect GroupA current sinks. ('00' = 100µs, '01' = 25ms, '10' = 50ms, '11' = 100ms). Brightness Level Control Table (GroupA) Brightness Code (hex) 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F Perceived Brightness Level (%) 0.125 0.313 0.625 1 1.125 1.313 1.688 2.063 2.438 2.813 3.125 3.75 4.375 5.25 6.25 7.5 8.75 10 12.5 15 16.875 18.75 22.5 26.25 31.25 37.5 43.75 52.5 61.25 70 87.5 100 30009739 FIGURE 8. VF Monitor Delay Register Description Internal Hex Address: 60h Note: VF1-VF0: Sets the Gain Transition Delay Time. The VF Monitor Delay can be set to four different delay times. ('00' (Default) = 3-6msec., '01' = 1.5-3msec., '10' = 0.4-0.8msec., '11' = 60-90µsec.). Application Information LED CONFIGURATIONS The LM2756 has a total of 8 current sinks capable of sinking 180mA of total diode current. These 8 current sinks are configured to operate in three independently controlled lighting regions. GroupA has four dedicated current sinks, while GroupB and GroupC each have one. To add greater lighting flexibility, the LM2756 has two additional drivers (D53 and D62) that can be assigned to either GroupA or GroupB through a setting in the general purpose register. At start-up, the default condition is four LEDs in GroupA, three LEDs in GroupB and a single LED in GroupC (NOTE: GroupC only consists of a single current sink (D1C) under any configuration). Bits 53A and 62A in the general purpose register control where current sinks D53 and D62 are assigned. By writing a '1' to the 53A or 62A bits, D53 and D62 become assigned to the GroupA lighting region. Writing a '0' to these bits assigns D53 and D62 to the GroupB lighting region. With this added flexibility, the LM2756 is capable of supporting applications requiring 4, 5, or 6 LEDs for main display lighting, while still providing additional current sinks that can be used for a wide variety of lighting functions. SETTING LED CURRENT The current through the LEDs connected to DxA and DxB can be set to a desired level simply by connecting an appropriately sized resistor (RSET) between the ISET pin of the LM2756 and GND. The DxA, DxB and D1C LED currents are proportional to the current that flows out of the ISET pin and are a factor of 189 times greater than the ISET current. The feedback loops of the internal amplifiers set the voltage of the ISET pin to 1.25V (typ.). The statements above are simplified in the equations below: IDxA/B/C (A)= 189 × (VISET / RSET) RSET (Ω)= 189 × (1.25V / IDxA/B/C) Once the desired RSET value has been chosen, the LM2756 has the ability to internally dim the LEDs using analog current scaling. The analog current level is set through the I2C compatible interface. LEDs connected to GroupA can be dimmed 11 www.national.com GroupB and GroupC Brightness Levels (% of Full-Scale) = 10%, 20%, 30%, 40%, 50%, 60%, 70%, 100% LM2756 to 32 different levels. GroupB and GroupC(D1C) have 8 analog current levels. Please refer to the I2C Compatible Interface section of this datasheet for detailed instructions on how to adjust the brightness control registers. LED CURRENT RAMPING The LM2756 provides an internal LED current ramping function that allows the GroupA LEDs to turn on and turn off gradually over time. The target current level is set in the GroupA Brightness Control Register (0xA0). The total rampup/ramp-down time is determind by the GroupA brightness level (0-31) and the user configurable ramp step time. Bits RS1 and RS2 in the Ramp Step Time Register (0x20) set the ramp step time to the following four times: '00' = 100µsec., '01' = 25msec., '10' = 50msec., '11' = 100msec. The LM2756 will always ramp-up (upon enable) and rampdown (upon disable) through the brightness levels until the target level is reached. At the default setting of '00', the LM2756's current ramping feature looks more like a current step rather than a current ramp. The following table gives the approximate ramp-up/ramp-down times if the GroupA brightness register is set to full-scale, or brightness code 31. Brightness Ramp-Up/Ramp-Down Times Ramp Code RS1-RS0 00 01 10 11 Ramp Step Time 100µs 25ms 50ms 100ms Total Ramp Time 3.2ms 0.8s 1.6s 3.2s kHR – Headroom constant. This parameter models the minimum voltage required to be present across the current sinks for them to regulate properly. This minimum voltage is proportional to the programmed LED current, so the constant has units of mV/mA. The typical kHR of the LM2756 is 3.25mV/mA. In equation form: (VVOUT – VLEDx) > kHRx × ILEDx (eq. 3) Typical Headroom Constant Values kHRA = kHRB = kHRC = 3.25 mV/mA The "ILED-MAX" equation (eq. 1) is obtained from combining the ROUT equation (eq. 2) with the kHRx equation (eq. 3) and solving for ILEDx. Maximum LED current is highly dependent on minimum input voltage and LED forward voltage. Output current capability can be increased by raising the minimum input voltage of the application, or by selecting an LED with a lower forward voltage. Excessive power dissipation may also limit output current capability of an application. Total Output Current Capability The maximum output current that can be drawn from the LM2756 is 180mA. Each driver Group has a maximum allotted current per Dxx sink that must not be exceeded. DRIVER TYPE DxA DxB D1C MAXIMUM Dxx CURRENT 30mA per DxA Pin 30mA per DxB Pin 30mA The 180mA load can be distributed in many different configurations. Special care must be taken when running the LM2756 at the maximum output current to ensure proper functionality. PARALLEL CONNECTED AND UNUSED OUTPUTS Connecting the outputs in parallel does not affect internal operation of the LM2756 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 LED application circuit. All Dx current sinks utilize LED forward voltage sensing circuitry to optimize the charge-pump gain for maximum efficiency. Due to the nature of the sensing circuitry, it is not recommended to leave any of the DxA (D1A-D4A, D53, D62) pins open if diode GroupA is going to be used during normal operation. Leaving DxA pins unconnected will force the charge-pump into 3/2× mode over the entire VIN range negating any efficiency gain that could have been achieved by switching to 1× mode at higher input voltages. If the D1B or D1C drivers are not going to be used, make sure that the ENB and ENC bits in the general purpose register are set to '0' to ensure optimal efficiency. The D53 and D62 pins can be completely shutdown through the general purpose register by writing a '1' to the SD53 or SD62 bits. Care must be taken when selecting the proper RSET value. The current on any DxX pin must not exceed the maximum current rating for any given current sink pin. 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 3/2× - 1× charge pump, the input current is equal to the charge pump gain times the output MAXIMUM OUTPUT CURRENT, MAXIMUM LED VOLTAGE, MINIMUM INPUT VOLTAGE The LM2756 can drive 8 LEDs at 22.5mA each (GroupA , GroupB, GroupC) from an input voltage as low as 3.2V, so long as the LEDs have a forward voltage of 3.6V or less (room temperature). The statement above is a simple example of the LED drive capability of the LM2756. The statement contains the key application parameters that are required to validate an LEDdrive design using the LM2756: LED current (ILEDx), number of active LEDs (Nx), LED forward voltage (VLED), and minimum input voltage (VIN-MIN). The equation below can be used to estimate the maximum output current capability of the LM2756: ILED_MAX = [(1.5 x VIN) - VLED - (IADDITIONAL × ROUT)] / [(Nx x ROUT) + kHRx] (eq. 1) ILED_MAX = [(1.5 x VIN ) - VLED - (IADDITIONAL × 2.4Ω)] / [(Nx x 2.4Ω) + kHRx] IADDITIONAL is the additional current that could be delivered to the other LED Groups. ROUT – Output resistance. This parameter models the internal losses of the charge pump that result in voltage droop at the pump output VOUT. Since the magnitude of the voltage droop is proportional to the total output current of the charge pump, the loss parameter is modeled as a resistance. The output resistance of the LM2756 is typically 2.4Ω (VIN = 3.6V, TA = 25°C). In equation form: VVOUT = (1.5 × VIN) – [(NA× ILEDA + NB × ILEDB + NC × ILEDC) × ROUT] (eq. 2) www.national.com 12 LM2756 current (total LED current). The efficiency of the LM2756 can be predicted as follow: PLEDTOTAL = (VLEDA × NA × ILEDA) + (VLEDB × NB × ILEDB) + (VLEDC × ILEDC) PIN = VIN × IIN PIN = VIN × (GAIN × ILEDTOTAL + IQ) E = (PLEDTOTAL ÷ PIN) The LED voltage is the main contributor to the charge-pump gain selection process. Use of low forward-voltage LEDs (3.0V- to 3.5V) will allow the LM2756 to stay in the gain of 1× for a higher percentage of the lithium-ion battery voltage range when compared to the use of higher forward voltage LEDs (3.5V to 4.0V). See the LED Forward Voltage Monitoring section of this datasheet for a more detailed description of the gain selection and transition process. For an advanced analysis, it is recommended that power consumed by the circuit (VIN x IIN) for a given load be evaluated rather than power efficiency. POWER DISSIPATION The power dissipation (PDISS) and junction temperature (TJ) can be approximated with the equations below. PIN is the power generated by the 3/2× - 1× 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 micro SMD 20-bump package. V IN is the input voltage to the LM2756, VLED is the nominal LED forward voltage, N is the number of LEDs and ILED is the programmed LED current. PDISS = PIN - PLEDA - PLEDB - PLEDC PDISS= (GAIN × VIN × IGroupA + GroupB + GroupC ) - (VLEDA × NA × ILEDA) - (VLEDB × NB × ILEDB) - (VLEDC × ILEDC) TJ = TA + (PDISS x θJA) The junction temperature rating takes precedence over the ambient temperature rating. The LM2756 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. THERMAL PROTECTION Internal thermal protection circuitry disables the LM2756 when the junction temperature exceeds 160°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 155°C (typ.). It is important that the board layout provide good thermal conduction to keep the junction temperature within the specified operating ratings. CAPACITOR SELECTION The LM2756 requires 4 external capacitors for proper operation (C1 = C2 = CIN = COUT = 1µF). Surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR