LM27964SQ-I

LM27964 White LED Driver System with I2C Compatible Brightness Control August 2007 LM27964 White LED Driver System with I2C Compatible Brightness Control General Description 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. 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). Features ■ 87% Peak LED Drive Efficiency ■ 0.2% Current Matching between Current Sinks ■ Drives 6 LEDs with up to 30mA per LED in two distinct ■ ■ ■ ■ ■ ■ ■ groups, for backlighting two displays (main LCD and sub LCD) Dedicated Keypad LED Driver with up to 80mA of drive current Independent Resistor-Programmable Current Settings I2C Compatible Brightness Control Interface Adaptive 1×- 3/2× Charge Pump Extended Li-Ion Input: 2.7V to 5.5V Small low profile industry standard leadless package, LLP 24 : (4mm x 4mm x 0.8mm) LM27964SQ-I LED PWM frequency = 10kHz, LM27964SQ-C LED PWM frequency = 23kHz Applications ■ ■ ■ ■ Mobile Phone Display Lighting Mobile Phone Keypad Lighting PDAs Backlighting General LED Lighting Typical Application Circuit 20138101 © 2007 National Semiconductor Corporation 201381 www.national.com LM27964 Connection Diagram 24 Pin Quad LLP Package NS Package Number SQA24A 20138102 Pin Descriptions Pin #s 24 23 19, 22 (C1) 20, 21 (C2) 13, 14, 15, 16 4, 5 6 17 3 12 1 2 7 9, 10, 18, DAP 8, 11 Pin Names VIN POUT C1, C2 Charge Pump Output Voltage Flying Capacitor Connections Pin Descriptions Input voltage. Input range: 2.7V to 5.5V. D4A, D3A, D2A, D1A LED Drivers - GroupA D1B, D2B DKEY ISETA ISETB ISETK SCL SDIO VIO GND NC LED Drivers - GroupB LED Driver - KEYPAD Placing a resistor (RSETA) between this pin and GND sets the full-scale LED current for Group A LEDs. LED Current = 200 × (1.25V ÷ RSETA) Placing a resistor (RSETB) between this pin and GND sets the full-scale LED current for Group B LEDs. LED Current = 200 × (1.25V ÷ RSETB) Placing a resistor (RSETK) between this pin and GND sets the total LED current for the KEYPAD LEDs. Keypad LED Current = 800 × (1.25V ÷ RSETK) Serial Clock Pin Serial Data Input/Output Pin Serial Bus Voltage Level Pin Ground No Connect Ordering Information Order Information LM27964SQ-I LM27964SQX-I LM27964SQ-C LM27964SQX-C Current Source PWM Frequency 10kHz. 23kHz. Package Supplied As 1000 Units, Tape & Reel 4500 Units, Tape & Reel 1000 Units, Tape & Reel 4500 Units, Tape & Reel SQA24 LLP SQA24 LLP www.national.com 2 LM27964 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 -0.3V to 6.0V SCL, SDIO, VIO pin voltages -0.3V to (VIN+0.3V) w/ 6.0V max IDxx Pin Voltages -0.3V to (VPOUT+0.3V) w/ 6.0V max Continuous Power Dissipation Internally Limited (Note 3) Junction Temperature (TJ-MAX) 150ºC Storage Temperature Range -65ºC to +150º C Maximum Lead Temperature (Soldering) ESD Rating (Note 5) Human Body Model - IDxx Pins: Human Body Model - All other Pins: (Note 4) 1.0kV 2.0kV 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 +100°C -30°C to +85°C Thermal Properties Juntion-to-Ambient Thermal Resistance (θJA), SQA24A Package (Note 7) 41.3°C/W 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; 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 3.0V ≤ VIN ≤ 5.5V BankA or BankB Half-Scale ENA or ENB = "1", ENK = “0” 2.7V ≤ VIN ≤ 3.0V BankA or BankB Full-Scale ENA or ENB = "1", ENK = “0” Output Current Regulation Keypad Driver Enabled 3.0V ≤ VIN ≤ 5.5V DKEY Full-Scale ENA = ENB = “0”, ENK = “1” 3.2V ≤ VIN ≤ 5.5V Output Current Regulation BankA and DKEY Enabled (Note 10) Open-Loop Charge Pump Output Resistance VDxx 1x to 3/2x Gain Transition Threshold RSETA = 8.3kΩ, RSETK = 16.9kΩ VLED = 3.6V BankA and DKEY Full-Scale ENA = ENK = “1”, ENB = “0” Gain = 3/2 Gain = 1 VDxA and/or VDxB Falling IDxx = 95% ×IDxx (nom.) (IDxx (nom) ≈ 15mA) BankA and/or BankB Full-Scale Gain = 3/2, ENA and/or ENB = "1" IDKEY = 95% ×IDKEY (nom.) (IDKEY (nom) ≈ 60mA) DKEY Full-Scale Gain = 3/2, ENK = "1" 180 52.8 (-12%) Min 13.77 (-10%) Typ 15.3 Max 16.83 (+10%) Units mA (%) 7.5 mA 15 mA IDxx 60 30 DxA 60 DKEY 2.75 1 375 67.2 (+12%) mA (%) mA ROUT VDxTH Ω mV 180 mV VHR Current Source Headroom Voltage Requirement (Note 11) 3 www.national.com LM27964 Symbol IQ ISD VSET IDxA-B / ISETA-B IDKEY / ISETK fSW tSTART fPWM Parameter (Note 12) Quiescent Supply Current Shutdown Supply Current ISET Pin Voltage Output Current to Current Set Ratio BankA and BankB Output Current to Current Set Ratio DKEY Switching Frequency Start-up Time Internal Diode Current PWM Frequency Condition Gain = 1.5x, No Load All ENx bits = "0" 2.7V ≤ VIN ≤ 5.5V Min Typ 0.2 1.3 3.0 1.25 200 800 Max 2 1.7 5 Units % mA µA V IDxx-MATCH LED Current Matching 500 POUT = 90% steady state LM27964SQ-I LM27964SQ-C 700 250 10 23 1/16 900 kHz µs kHz Fullscal e D.C. Step Diode Current Duty Cycle Step I2C Compatible Interface Voltage Specifications (SCL, SDIO, VIO) VIO VIL VIH VOL t1 t2 t3 t4 t5 Serial Bus Voltage Level 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 = 2mA 2.5 100 0 100 100 1.8 0 0.73 × VIO VIN 0.27 × VIO VIO 400 V V V mV µs ns ns ns ns I2C Compatible Interface Timing Specifications (SCL, SDIO, VIO)(Note 13) 20138113 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 × PD-MAX). www.national.com 4 LM27964 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 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 20138120 www.national.com 6 LM27964 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 Circuit Description OVERVIEW The LM27964 is a white LED driver system based upon an adaptive 1.5×/1× 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. 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. 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. It is not recommended that any of the BankA or BankB drivers be left disconnected if either bank will be used in the applica- tion. 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". 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. 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. 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. 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. 8 www.national.com LM27964 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. 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. 20138107 FIGURE 6. General Purpose Register Example 20138109 FIGURE 4. Chip Address INTERNAL REGISTERS OF LM27964 Register General Purpose Register Bank A and Bank B Birghtness Control Register Internal Hex Address 10h A0h Power On Value 20138105 0000 0000 0000 0000 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 × 1.25V / RSETx Brightness Level Segments = 1/16th of Fullscale KEYPAD B0h Brightness Control 0000 0000 20138108 FIGURE 5. General Purpose Register Description Internal Hex Address: 10h Note: ENA: Enables DxA LED drivers (Main Display) ENB: Enables DxB LED drivers (Sub Display) ENK: Enables Keypad Driver 9 www.national.com LM27964 BankB is a fixed 10kHz (LM27964SQ-I) or 23kHz (LM27964SQ-C) 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. 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 × 1.25V / RSETx Brightness Level are= 100% (Fullscale), 70%, 40%, 20% 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 × ROUT)] / [(Nx x ROUT) + kHRx] (eq. 1) ILED_MAX = [(1.5 x VIN ) - VLED - (IADDITIONAL × 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: VPOUT = (1.5 × VIN) – [(NA× ILEDA + NB × ILEDB + NK × ILEDK) × ROUT] (eq. 2) 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: (VPOUT – VLEDx) > kHRx × ILEDx (eq. 3) 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 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 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 × (VISET / RSETA/B) RSETA/B = 200 × (1.25V / IDxA/B) IDKEY = 800 × (VISET / RSETK) RSETK = 800 × (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 www.national.com 10 LM27964 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 LM27964 is 180mA. Each driver bank has a maximum allotted current per Dxx sink that must not be exceeded. DRIVER TYPE DxA DxB DKEY MAXIMUM Dxx CURRENT 30mA per DxA Pin 30mA per DxB Pin 80mA 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. 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 × VIN × ILEDA + LEDB + LEDK) - (VLEDA × NA × ILEDA) (VLEDB × NB × ILEDB) - (VLEDK × NK × 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. 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. 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