LM27964 White LED Driver System with I2C Compatible Brightness Control General Description Features The LM27964 is a charge-pump-based white-LED driver that is ideal for mobile phone display backlighting. The LM27964 can drive up to 6 LEDs in parallel along with multiple keypad LEDs, with a total output current up to 180mA. Regulated internal current sources deliver excellent current matching in all LEDs. n 87% Peak LED Drive Efficiency n 0.2% Current Matching between Current Sinks n Drives 6 LEDs with up to 30mA per LED in two distinct groups, for backlighting two displays (main LCD and sub LCD) n Dedicated Keypad LED Driver with up to 80mA of drive current n Independent Resistor-Programmable Current Settings n I2C Compatible Brightness Control Interface n Adaptive 1x- 3/2x Charge Pump n Extended Li-Ion Input: 2.7V to 5.5V n Small low profile industry standard leadless package, LLP 24 : (4mm x 4mm x 0.8mm) n LM27964SQ-I LED PWM frequency = 10kHz, LM27964SQ-A LED PWM frequency = 23kHz The LED driver current sources are split into two independently controlled groups. The primary group (4 LEDs) can be used to backlight the main phone display and the second group (2 LEDs) can be used to backlight a secondary display. A single Keypad LED driver can power up to 16 keypad LEDs with a current of 5mA each. The LM27964 has an I2C compatible interface that allows the user to independently control the brightness on each bank of LEDs. The LM27964 works off an extended Li-Ion input voltage range (2.7V to 5.5V). 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 LM27964 is available in National’s small 24-pin Leadless Leadframe Package (LLP-24). Applications n n n n Mobile Phone Display Lighting Mobile Phone Keypad Lighting PDAs Backlighting General LED Lighting Typical Application Circuit 20138101 © 2005 National Semiconductor Corporation DS201381 www.national.com LM27964 White LED Driver System with I2C Compatible Brightness Control October 2005 LM27964 Connection Diagram 24 Pin Quad LLP Package NS Package Number SQA24A 20138102 Pin Descriptions Pin #s Pin Names 24 VIN Pin Descriptions Input voltage. Input range: 2.7V to 5.5V. 23 POUT Charge Pump Output Voltage 19, 22 (C1) 20, 21 (C2) C1, C2 Flying Capacitor Connections 13, 14, 15, 16 D4A, D3A, D2A, D1A LED Drivers - GroupA 4, 5 D1B, D2B LED Drivers - GroupB 6 DKEY LED Driver - KEYPAD 17 ISETA Placing a resistor (RSETA) between this pin and GND sets the full-scale LED current for Group A LEDs. LED Current = 200 x (1.25V ÷ RSETA) 3 ISETB Placing a resistor (RSETB) between this pin and GND sets the full-scale LED current for Group B LEDs. LED Current = 200 x (1.25V ÷ RSETB) 12 ISETK Placing a resistor (RSETK) between this pin and GND sets the total LED current for the KEYPAD LEDs. Keypad LED Current = 800 x (1.25V ÷ RSETK) 1 SCL Serial Clock Pin 2 SDIO Serial Data Input/Output Pin 7 VIO Serial Bus Voltage Level Pin 9, 10, 18, DAP GND Ground 8, 11 NC No Connect Ordering Information Order Information LM27964SQ-I LM27964SQX-I LM27964SQ-A LM27964SQX-A www.national.com Current Source PWM Frequency Package 10kHz. SQA24 LLP 23kHz. SQA24 LLP 2 Supplied As 1000 Units, Tape & Reel 4500 Units, Tape & Reel 1000 Units, Tape & Reel 4500 Units, Tape & Reel Operating Rating If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN pin voltage -0.3V to (VPOUT+0.3V) w/ 6.0V max Continuous Power Dissipation (Note 3) Internally Limited Junction Temperature (TJ-MAX) 150oC Storage Temperature Range -65oC to +150o C Maximum Lead Temperature (Soldering) (Note 4) ESD Rating (Note 5) Human Body Model - IDxx Pins: Human Body Model - All other Pins: Electrical Characteristics 2.7V to 5.5V LED Voltage Range 2.0V to 4.0V -30˚C to +100˚C Ambient Temperature (TA) Range(Note 6) -0.3V to (VIN+0.3V) w/ 6.0V max IDxx Pin Voltages Input Voltage Range Junction Temperature (TJ) Range -0.3V to 6.0V SCL, SDIO, VIO pin voltages (Notes 1, 2) -30˚C to +85˚C Thermal Properties 41.3˚C/W Juntion-to-Ambient Thermal Resistance (θJA), SQA24A Package (Note 7) 1.0kV 2.0kV (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; VDxA = 0.4V; VDxB = 0.4V; VDKEY = 0.4V; RSETA = RSETB = RSETK = 16.9kΩ; BankA, BankB, and DKEY = Fullscale Current; ENA, ENB, ENK Bits = “1”; C1=C2=1.0µF, CIN=COUT=2.2µF; Specifications related to output current(s) and current setting pins (IDxx and ISETx) apply to BankA, BankB and DKEY. (Note 9) Symbol Parameter Condition 3.0V ≤ VIN ≤ 5.5V BankA or BankB Full-Scale ENA or ENB = "1", ENK = “0” Output Current Regulation BankA or BankB Enabled IDxx ROUT VDxTH Min Typ Max Units 13.77 (-10%) 15.3 16.83 (+10%) mA (%) 3.0V ≤ VIN ≤ 5.5V BankA or BankB Half-Scale ENA or ENB = "1", ENK = “0” 7.5 mA 2.7V ≤ VIN ≤ 3.0V BankA or BankB Full-Scale ENA or ENB = "1", ENK = “0” 15 mA Output Current Regulation Keypad Driver Enabled 3.0V ≤ VIN ≤ 5.5V DKEY Full-Scale ENA = ENB = “0”, ENK = “1” Output Current Regulation BankA and DKEY Enabled (Note 10) 3.2V ≤ VIN ≤ 5.5V RSETA = 8.3kΩ, RSETK = 16.9kΩ VLED = 3.6V BankA and DKEY Full-Scale ENA = ENK = “1”, ENB = “0” Open-Loop Charge Pump Output Resistance Gain = 3/2 VDxx 1x to 3/2x Gain Transition Threshold VDxA and/or VDxB Falling 52.8 (-12%) 60 mA 60 DKEY 1 3 mA (%) 30 DxA 2.75 Gain = 1 67.2 (+12%) 375 Ω mV www.national.com LM27964 Absolute Maximum Ratings (Notes 1, 2) LM27964 Electrical Characteristics (Notes 2, 8) (Continued) 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; VDxA = 0.4V; VDxB = 0.4V; VDKEY = 0.4V; RSETA = RSETB = RSETK = 16.9kΩ; BankA, BankB, and DKEY = Fullscale Current; ENA, ENB, ENK Bits = “1”; C1=C2=1.0µF, CIN=COUT=2.2µF; Specifications related to output current(s) and current setting pins (IDxx and ISETx) apply to BankA, BankB and DKEY. (Note 9) Symbol VHR Parameter Condition Min Current Source Headroom Voltage IDxx = 95% xIDxx (nom.) Requirement (IDxx (nom) ≈ 15mA) BankA and/or BankB Full-Scale (Note 11) Gain = 3/2, ENA and/or ENB = "1" IDxx-MATCH LED Current Matching Typ Max Units 180 mV IDKEY = 95% xIDKEY (nom.) (IDKEY (nom) ≈ 60mA) DKEY Full-Scale Gain = 3/2, ENK = "1" 180 (Note 12) 0.2 2 % IQ Quiescent Supply Current Gain = 1.5x, No Load 1.3 1.7 mA ISD Shutdown Supply Current All ENx bits = "0" 3.0 5 µA VSET ISET Pin Voltage 2.7V ≤ VIN ≤ 5.5V 1.25 IDxA-B / ISETA-B Output Current to Current Set Ratio BankA and BankB 200 IDKEY / ISETK Output Current to Current Set Ratio DKEY 800 fSW Switching Frequency tSTART Start-up Time POUT = 90% steady state 250 fPWM Internal Diode Current PWM Frequency LM27964SQ-I 10 LM27964SQ-A 23 500 D.C. Step Diode Current Duty Cycle Step 700 V 900 kHz µs kHz 1/16 Fullscale 2 I C Compatible Interface Voltage Specifications (SCL, SDIO, VIO) VIO Serial Bus Voltage Level 1.8 VIN V VIL Input Logic Low "0" 2.7V ≤ VIN ≤ 5.5V 0 0.27 x VIO V VIH Input Logic High "1" 2.7V ≤ VIN ≤ 5.5V 0.73 x VIO VIO VOL Output Logic Low "0" ILOAD = 2mA 400 V mV I2C Compatible Interface Timing Specifications (SCL, SDIO, VIO)(Note 13) 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 SDIO Low Setup Time to SCL Low (Start) 100 ns t5 SDIO High Hold Time After SCL High (Stop) 100 ns 20138113 www.national.com 4 (Continued) 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 = 170˚C (typ.) and disengages at TJ = 165˚C (typ.). Note 4: For detailed soldering specifications and information, please refer to National Semiconductor Application Note 1187: Leadless Leadframe Package (AN-1187). 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 = 100˚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 x 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 1187: Leadless Leadframe Package (AN-1187). 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, CPOUT, C1, and C2 : Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics Note 10: The maximum total output current for the LM27964 should be limited to 180mA. The total output current can be split among any of the three banks (IDxA = IDxB = 30mA Max., IDKEY = 80mA 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 each IDxx output pin, headroom voltage is the voltage across the internal current sink connected to that pin. For Group A and B outputs, VHR = VOUT -VDxx. If headroom voltage requirement is not met, LED current regulation will be compromised. Note 12: For the two groups of outputs on a part (BankA and BankB), the following are determined: the maximum output current in the group (MAX), the minimum output current in the group (MIN), and the average output 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 bank. The matching figure for a given part is considered to be the highest matching figure of the two banks. The typical specification provided is the most likely norm of the matching figure for all parts. Note 13: SCL and SDIO should be glitch-free in order for proper brightness control to be realized. Block Diagram 20138103 5 www.national.com LM27964 Electrical Characteristics (Notes 2, 8) LM27964 Typical Performance Characteristics Unless otherwise specified: VIN = 3.6V; VLEDxA = 3.6V, VLEDxB = 3.6V; RSETA = RSETB = RSETK = 16.9kΩ; C1=C2=1µF , and CIN = CPOUT = 2.2µF. LED Drive Efficiency vs Input Voltage Charge Pump Output Voltage vs Input Voltage 20138117 20138123 Shutdown Current vs Input Voltage Diode Current vs Input Voltage 20138119 20138124 BankA/BankB Diode Current vs Brightness Register Code BankA Diode Current vs BankA Headroom Voltage 20138118 www.national.com 20138120 6 = 3.6V; RSETA = RSETB = RSETK = 16.9kΩ; C1=C2=1µF , and CIN = CPOUT = 2.2µF. (Continued) BankB Diode Current vs BankB Headroom Voltage Keypad Driver Current vs Input Voltage 20138121 20138115 Keypad Driver Current vs. Brightness Register Code Keypad Diode Current vs Keypad Headroom Voltage 20138114 20138122 Keypad Driver Current vs Keypad RSET Resistance 20138116 7 www.national.com LM27964 Typical Performance Characteristics Unless otherwise specified: VIN = 3.6V; VLEDxA = 3.6V, VLEDxB LM27964 It is not recommended that any of the BankA or BankB drivers be left disconnected if either bank will be used in the application. If Dxx pin/s are left unconnected, the LM27964 will default to the gain of 3/2. If the BankA or BankB drivers are not going to be used in the application, leaving the Dxx pins is acceptable as long as the ENx bit in the general purpose register is set to "0". Circuit Description OVERVIEW The LM27964 is a white LED driver system based upon an adaptive 1.5x/1x CMOS charge pump capable of supplying up to 180mA of total output current. With three separately controlled banks of constant current sinks, the LM27964 is an ideal solution for platforms requiring a single white LED driver for main and sub displays, as well as other general purpose lighting needs. The tightly matched current sinks ensure uniform brightness from the LEDs across the entire small-format display. 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 CLK is LOW. Each LED is configured in a common anode configuration, with the peak drive current being programmed through the use of external RSETx resistors. An I2C compatible interface is used to enable and vary the brightness within the individual current sink banks. For BankA and BankB, 16 levels of PWM brightness control are available, while 4 analog levels are present for the DKEY driver. CIRCUIT COMPONENTS Charge Pump The input to the 1.5x/1x 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 LM27964 is 3.0V 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. This allows the charge pump to stay in the most efficient gain (1x) over as much of the input voltage range as possible, reducing the power consumed from the battery. 20138106 FIGURE 1. Data Validity Diagram A pull-up resistor between VIO and SDIO must be greater than [ (VIO-VOL) / 2mA] 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. The data on SDIO line must be stable during the HIGH period of the clock signal (SCL). In other words, the state of the data line can only be changed when CLK is LOW. LED Forward Voltage Monitoring The LM27964 has the ability to switch converter 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 within BankA and BankB (DKEY is not monitored). At higher input voltages, the LM27964 will operate in pass mode, allowing the POUT voltage to track the input voltage. As the input voltage drops, the voltage on the DXX pins will also drop (VDXX = VPOUT – VLEDx). Once any of the active Dxx pins reaches a voltage approximately equal to 375mV, the charge pump will then switch to the gain of 3/2. This switchover ensures that the current through the LEDs never becomes pinched off due to a lack of headroom on the current sources. Only active Dxx pins will be monitored. For example, if only BankA is enabled, the LEDs in BankB will not affect the gain transition point. If both banks are enabled, all diodes will be monitored, and the gain transition will be based upon the diode with the highest forward voltage. The DKEY pin is not monitored as it is intended to be for keypad LEDs. Keypad LEDs generally require lower current, resulting in lower forward voltage compared to the BankA and BankB LEDs that have higher currents. In the event that only the DKEY driver is enabled without either BankA or BankB, the charge pump will default to 3/2 mode to ensure the DKEY driver has enough headroom. www.national.com 20138111 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) being transferred first. Each byte of data has to be followed by an acknowledge bit. The acknowledge related clock pulse is generated by the master. 8 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 LM27964 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. (Continued) The master releases the SDIO line (HIGH) during the acknowledge clock pulse. The LM27964 pulls down the SDIO line during the 9th clock pulse, signifying an acknowledge. The LM27964 generates an acknowledge after each byte has been received. 20138112 FIGURE 3. Write Cycle w = write (SDIO = "0") r = read (SDIO = "1") ack = acknowledge (SDIO pulled down by either master or slave) rs = repeated start id = chip address, 36h for LM27964 I2C COMPATIBLE CHIP ADDRESS The chip address for LM27964 is 0110110, or 36h. 20138108 FIGURE 5. General Purpose Register Description Internal Hex Address: 10h 20138109 Note: ENA: Enables DxA LED drivers (Main Display) ENB: Enables DxB LED drivers (Sub Display) FIGURE 4. Chip Address ENK: Enables Keypad Driver INTERNAL REGISTERS OF LM27964 Internal Hex Address Power On Value General Purpose Register 10h 0000 0000 Bank A and Bank B Birghtness Control Register A0h 0000 0000 Register 20138107 KEYPAD B0h Brightness Control FIGURE 6. General Purpose Register Example 0000 0000 9 www.national.com LM27964 Circuit Description LM27964 Circuit Description the LM27964 and GND. The DxA and DxB LED currents are proportional to the current that flows out of the ISETA and ISETB pins and are a factor of 200 times greater than the ISETA/B currents. The DKEY current is proportional to the current that flows out of the ISETK pin and is a factor of 800 times greater than the ISETK current. The feedback loops of the internal amplifiers set the voltage of the ISETx pins to 1.25V (typ.). Separate RSETx resistor should be used on each ISETx pin. The statements above are simplified in the equations below: IDxA/B = 200 x (VISET / RSETA/B) RSETA/B = 200 x (1.25V / IDxA/B) IDKEY = 800 x (VISET / RSETK) RSETK = 800 x (1.25V / IDKEY) Once the desired RSETx values have been chosen, the LM27964 has the ability to internally dim the LEDs by Pulse Width Modulating (PWM) the current. The PWM duty cycle is set through the I2C compatible interface. LEDs connected to BankA and BankB current sinks (DxA and DxB) can be dimmed to 16 different levels/duty-cycles (1/16th of full-scale to full-scale). The internal PWM frequency for BankA and BankB is a fixed 10kHz (LM27964SQ-I) or 23kHz (LM27964SQ-A) depending on the option. The DKEY current sink uses an analog current scaling method to control LED brightness. The brightness levels are 100% (Fullscale), 70%, 40%, and 20%. When connecting multiple LEDs in parallel to the DKEY current sink, it is recommended that ballast resistors be placed in series with the LEDs. The ballast resistors help reduce the affect of LED forward voltage mismatch, and help equalize the diode currents. Ballast resistor values must be carefully chosen to ensure that the current source headroom voltage is sufficient to supply the desired current. Please refer to the I2C Compatible Interface section of this datasheet for detailed instructions on how to adjust the brightness control registers. (Continued) 20138105 FIGURE 7. Brightness Control Register Description Internal Hex Address: A0h Note: DxA3-DxA0: Register Sets Current Level Supplied to DxA LED drivers DxB3-DxB0: Register Sets Current Level Supplied to DxB LED drivers Full-Scale Current set externally by the following equation: IDxx = 200 x 1.25V / RSETx Brightness Level Segments = 1/16th of Fullscale MAXIMUM OUTPUT CURRENT, MAXIMUM LED VOLTAGE, MINIMUM INPUT VOLTAGE The LM27964 can drive 4 LEDs at 30mA each (BankA) and 12 keypad LEDs at 5mA each (60mA total at DKEY) 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 capabilities of the LM27964. The statement contains the key application parameters that are required to validate an LEDdrive design using the LM27964: 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 LM27964: ILED_MAX = [(1.5 x VIN) - VLED - (IADDITIONAL x ROUT)] / [(Nx x ROUT) + kHRx] (eq. 1) ILED_MAX = [(1.5 x VIN ) - VLED - (IADDITIONAL x 2.75Ω)] / [(Nx x 2.75Ω) + kHRx] IADDITIONAL is the additional current that could be delivered to the other LED banks. ROUT – Output resistance. This parameter models the internal losses of the charge pump that result in voltage droop at the pump output POUT. 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 LM27964 is typically 2.75Ω (VIN = 3.6V, TA = 25˚C). In equation form: 20138104 FIGURE 8. Brightness Control Register Example 20138110 FIGURE 9. Internal Hex Address: B0h Note: DKEY1-DKEY0: Sets Brightness for DKEY pin (KEYPAD Driver). 11=Fullscale Bit7 to Bit 2: Not Used Full-Scale Current set externally by the following equation: IDKEY = 800 x 1.25V / RSETx Brightness Level are= 100% (Fullscale), 70%, 40%, 20% Application Information SETTING LED CURRENT The current through the LEDs connected to DxA, DxB and DKEY can be set to a desired level simply by connecting an appropriately sized resistor (RSETx) between the ISETx pin of www.national.com 10 DxB pins unused if either diode bank is going to be used during normal operation. Leaving DxA and/or DxB pins unconnected will force the charge-pump into 3/2x mode over the entire VIN range negating any efficiency gain that could be achieve by switching to 1x mode at higher input voltages. Care must be taken when selecting the proper RSETx value. The current on any Dxx pin must not exceed the maximum current rating for any given current sink pin. (Continued) VPOUT = (1.5 x VIN) – [(NAx ILEDA + NB x ILEDB + NK x (eq. 2) ILEDK) x ROUT] kHR – Headroom constant. This parameter models the minimum voltage required to be present across the current sources 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 LM27964 is 12mV/mA. In equation form: (eq. 3) (VPOUT – VLEDx) > kHRx x ILEDx 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 1.5x/1x charge pump, the input current is equal to the charge pump gain times the output current (total LED current). The efficiency of the LM27964 can be predicted as follows: PLEDTOTAL = (VLEDA x NA x ILEDA) + (VLEDB x NB x ILEDB) + (VLEDK x NK x ILEDK) PIN = VIN x IIN Typical Headroom Constant Values kHRA = 12mV/mA kHRB = 12 mV/mA kHRK = 3 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. PIN = VIN x (GAIN x ILEDTOTAL + IQ) E = (PLEDTOTAL ÷ PIN) 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. Total Output Current Capability The maximum output current that can be drawn from the LM27964 is 180mA. Each driver bank has a maximum allotted current per Dxx sink that must not be exceeded. DRIVER TYPE MAXIMUM Dxx CURRENT DxA 30mA per DxA Pin DxB 30mA per DxB Pin DKEY 80mA POWER DISSIPATION The power dissipation (PDISS) and junction temperature (TJ) can be approximated with the equations below. PIN is the power generated by the 1.5x/1x 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 LM27964, VLED is the nominal LED forward voltage, N is the number of LEDs and ILED is the programmed LED current. PDISS = PIN - PLEDA - PLEDB - PLEDK PDISS= (GAIN x VIN x ILEDA + LEDB + LEDK) - (VLEDA x NA x ILEDA) (VLEDB x NB x ILEDB) - (VLEDK x NK x ILEDK) TJ = TA + (PDISS x θJA) The junction temperature rating takes precedence over the ambient temperature rating. The LM27964 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 100˚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 100˚C. The 180mA load can be distributed in many different configurations. Special care must be taken when running the LM27964 at the maximum output current to ensure proper functionality. PARALLEL CONNECTED OUTPUTS Outputs D1A-4A or D1B-D2B may be connected together to drive one or two LEDs at higher currents. In such a configuration, all four parallel current sinks (BankA) of equal value can drive a single LED. The LED current programmed for BankA should be chosen so that the current through each of the outputs is programmed to 25% of the total desired LED current. For example, if 60mA is the desired drive current for a single LED, RSETA should be selected such that the current through each of the current sink inputs is 15mA. Similarly, if two LEDs are to be driven by pairing up the D1A-4A inputs (i.e D1A-2A, D3A-4A), RSETA should be selected such that the current through each current sink input is 50% of the desired LED current. The same RSETx selection guidelines apply to BankB diodes. Connecting the outputs in parallel does not affect internal operation of the LM27964 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. Both BankA and BankB utilize LED forward voltage sensing circuitry on each Dxx pin 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 or THERMAL PROTECTION Internal thermal protection circuitry disables the LM27964 when the junction temperature exceeds 170˚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 165˚C (typ.). It is important that the board layout provide good thermal conduction to keep the junction temperature within the specified operating ratings. 11 www.national.com LM27964 Application Information LM27964 Application Information 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 LM27964. The minimum voltage rating acceptable for all capacitors is 6.3V. The recommended voltage rating of the output capacitor is 10V to account for DC bias capacitance losses. (Continued) CAPACITOR SELECTION The LM27964 requires 4 external capacitors for proper operation (C1 = C2 = 1µF, CIN = COUT = 2.2µF). 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 LM27964 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 LM27964. 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). 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 2.5mm. 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. Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM27964. 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 www.national.com 12 inches (millimeters) unless otherwise noted SQA24: 24 Lead LLP X1 = 4.0mm X2 = 4.0mm X3 = 0.8mm 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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