LM2756TMEV

Application Report SNVA318A – March 2008 – Revised April 2013 AN-1779 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD ..................................................................................................................................................... ABSTRACT This application note discusses the ways in which the LM2756 can be configured to drive the eight LEDs in numerous configurations. 1 2 3 4 5 6 7 Contents Typical Application .......................................................................................................... 3 Basic Description ............................................................................................................ 3 Bill of Materials .............................................................................................................. 3 LM2756 Evaluation Board Schematic .................................................................................... 4 LM2756 Evaluation Board Layout ........................................................................................ 4 Board Operation ............................................................................................................. 6 6.1 Basic Connections ................................................................................................. 6 6.2 External Control Interface Connection .......................................................................... 6 6.3 Operation Description ............................................................................................. 6 Software Interface Information ........................................................................................... 11 List of Figures 1 Top Layer .................................................................................................................... 4 2 Middle Layer 1 ............................................................................................................... 5 3 Middle Layer 2 ............................................................................................................... 5 4 Bottom Layer (unmirrored) ................................................................................................. 5 5 Data Validity Diagram ...................................................................................................... 6 6 Start and Stop Conditions.................................................................................................. 7 7 Write Cycle 8 9 .................................................................................................................. Chip Address ................................................................................................................ General Purpose Register Description Internal Hex Address: 10h ................................................... 7 7 8 10 Brightness Control Register Description Internal Hex Address: 0xA0 (GroupA), 0xB0 (GroupB), 0xC0 (GroupC) ..................................................................................................................... 8 11 Ramp Step Time Register Description Internal Hex Address: 20h ................................................. 10 12 VF Monitor Delay Register Description Internal Hex Address: 60h ................................................. 10 13 GUI Start-Up ............................................................................................................... 11 14 Generic Read/Write Field................................................................................................. 12 15 Drop Down Menu .......................................................................................................... 12 16 Control and Configuration Buttons ...................................................................................... 12 17 Brightness Control Sliders ................................................................................................ 12 18 BankA Ramp Step Time .................................................................................................. 13 19 Results of Pressing the Set Button...................................................................................... 13 20 Example Configuration .................................................................................................... 13 List of Tables SNVA318A – March 2008 – Revised April 2013 Submit Documentation Feedback AN-1779 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD Copyright © 2008–2013, Texas Instruments Incorporated 1 www.ti.com 1 2 2 ............................................................................... Brightness Ramp-Up/Ramp-Down Times .............................................................................. Brightness Level Control Table (GroupA) AN-1779 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD Copyright © 2008–2013, Texas Instruments Incorporated 9 11 SNVA318A – March 2008 – Revised April 2013 Submit Documentation Feedback Typical Application www.ti.com 1 Typical Application GROUP A GROUP B D1A D2A D3A D4A D53 D62 GROUP C D1B VIN + - 1 µF D1C VOUT C1+ 1 µF C1C2+ LM2756 1 µF GND 1 µF C2- HWEN SDIO SCL ISET 2 I C Control Signals Capacitors: Murata GNM1M2R61C105ME18D 1 µF dual capacitors , or 1 µF single capacitor equivalent 2 Basic Description The LM2756 is a highly integrated, switched-capacitor, multi-display LED driver that can drive up to eight LEDs in parallel. The regulated internal current sources on the evaluation boards are set-up to deliver 20mA to each LED delivering excellent current and brightness matching. Utilizing the I2C compatible interface, the user can configure the LM2756 evaluation board to drive the eight LEDs in any of the numerous LED group configurations (4:3:1. 5:2:1, 6:1:1, etc.). 3 Bill of Materials Component Symbol Value Manufacturer Part # LM2756 -- Texas Instruments LM2756SDX LM2756 Evaluation Board -- Texas Instruments 551013004-002 RevA D1A-D4A, D53B,D62,D1B,D1C White LED Nichia NSSW020BT COUT/CIN 1µF, 16V Dual Capacitor Murata GNM1M2R61C105 ME18D C2/C1 1µF, 16V Dual Capacitor Murata GNM1M2R61C105 ME18D RSET 11.8kΩ Vishay Dale CRCW04021182F RSCL, RSDA,RHWEN 10kΩ Vishay Dale CRCW08051002F RESET Momentary Switch Panasonic EVQ-P2K02Q X4 USB Dock Connector 3M 8516-4500JL SNVA318A – March 2008 – Revised April 2013 Submit Documentation Feedback AN-1779 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD Copyright © 2008–2013, Texas Instruments Incorporated 3 LM2756 Evaluation Board Schematic www.ti.com 4 LM2756 Evaluation Board Schematic 5 LM2756 Evaluation Board Layout Figure 1. Top Layer 4 AN-1779 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD Copyright © 2008–2013, Texas Instruments Incorporated SNVA318A – March 2008 – Revised April 2013 Submit Documentation Feedback LM2756 Evaluation Board Layout www.ti.com Figure 2. Middle Layer 1 Figure 3. Middle Layer 2 Figure 4. Bottom Layer (unmirrored) SNVA318A – March 2008 – Revised April 2013 Submit Documentation Feedback AN-1779 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD Copyright © 2008–2013, Texas Instruments Incorporated 5 Board Operation 6 Board Operation 6.1 Basic Connections www.ti.com To operate the LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD, connect a supply voltage (2.7V-5.5V) between board connectors VIN and GND and attach an I2C interface using one of the methods described in . There is a RESET button provided on the board to exercise the RESET pin on the LM2756. By default, this pins is pulled high through a resistor to allow normal operation. Depressing this button during board operation will shutdown the LM2756 and will clear all of the internal registers resetting them to their default values. Default Jumper Connections: • EN_LEDS: This connects VOUT to the anodes of the LEDs. Removing the jumper disconnects the onboard LED power and allows external diodes / measurement equipment to be connected between VOUT and the Dx Pins • VIN_CON: Connects the adjustable voltage supply of the USB Docking board to the VIN of the LM2756. If the USB board is not used, this jumper does not need to be placed. If the USB Docking board is going to be used for the I2C interface, but not for VIN, make sure the VIN_CON jumper is removed. With the default jumper connections made, the board will be ready to operate once an input voltage and an I2C interface generator (external or USB docking board) are connected. 6.2 External Control Interface Connection The LM2756 evaluation board provides two ways to connect an I2C compatible interface to the LM2756 IC. The first method to connect the interface is through a set of connectors on the bottom of the evaluation board that allow the board to plug into TI's USB interface board directly. The second method of interface connection is through a header strip located on the left hand side of the evaluation board. There are pins available to connect VIO (controller reference voltage), SCL (Interface Clock Line), and SDA (Interface Data Line) each separated by a ground pin. The evaluation board has two external pull-ups that connect both SCL and SDA to VIO to compliment the open drain inputs found on the LM2756. Section 6.3 describes the internal registers and I2C compatible interface in greater detail. 6.3 Operation Description I2C Compatible Interface 6.3.1 6.3.1.1 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. Figure 5. 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. 6 AN-1779 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD Copyright © 2008–2013, Texas Instruments Incorporated SNVA318A – March 2008 – Revised April 2013 Submit Documentation Feedback Board Operation www.ti.com 6.3.1.2 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. Figure 6. Start and Stop Conditions 6.3.1.3 Transferring 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 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. w = write (SDIO = "0") r = read (SDIO = "1") ack = acknowledge (SDIO pulled down by either master or slave) id = chip address, 36h for LM2756 Figure 7. Write Cycle 6.3.1.4 I2C Compatible Chip Address The chip address for LM2756 is 0110110, or 36h. Figure 8. Chip Address SNVA318A – March 2008 – Revised April 2013 Submit Documentation Feedback AN-1779 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD Copyright © 2008–2013, Texas Instruments Incorporated 7 Board Operation 6.3.1.5 www.ti.com Internal Registers of LM2756 Register Internal Hex Address Power On Value General Purpose Register 10h 0000 0000 Group A Brightness Control Register A0h 1110 0000 Group B Brightness Control Register B0h 1111 1000 Group C Brightness Control Register C0h 1111 1000 Ramp Step Time Register 20h 1111 0000 VF Monitor Delay Ragister 60h 1111 1100 Figure 9. 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 Figure 10. Brightness Control Register Description Internal Hex Address: 0xA0 (GroupA), 0xB0 (GroupB), 0xC0 (GroupC) 8 AN-1779 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD Copyright © 2008–2013, Texas Instruments Incorporated SNVA318A – March 2008 – Revised April 2013 Submit Documentation Feedback Board Operation www.ti.com 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 Table 1. Brightness Level Control Table (GroupA) Brightness Code (hex) Perceived Brightness Level (%) 00 0.125 01 0.313 02 0.625 03 1 04 1.125 05 1.313 06 1.688 07 2.063 08 2.438 09 2.813 0A 3.125 0B 3.75 0C 4.375 0D 5.25 0E 6.25 0F 7.5 10 8.75 11 10 12 12.5 13 15 14 16.875 15 18.75 16 22.5 17 26.25 18 31.25 19 37.5 1A 43.75 1B 52.5 1C 61.25 1D 70 1E 87.5 1F 100 SNVA318A – March 2008 – Revised April 2013 Submit Documentation Feedback AN-1779 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD Copyright © 2008–2013, Texas Instruments Incorporated 9 Board Operation www.ti.com GroupB and GroupC Brightness Levels (% of Full-Scale) = 10%, 20%, 30%, 40%, 50%, 60%, 70%, 100% Ramp Step Time Register Register Address: 0x20 MSB 1 bit7 1 bit6 1 bit5 1 bit4 0 bit3 LSB 0 bit2 RS1 bit1 RS0 bit0 Figure 11. 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). VF Monitor Delay Register Register Address: 0x60 MSB 1 bit7 1 bit6 1 bit5 1 bit4 1 bit3 LSB 1 bit2 VF1 bit1 VF0 bit0 Figure 12. 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' = 0.4msec., '11' = 6090µsec.). 6.3.2 LED Configurations The LM2756 has a total of eight 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. 6.3.3 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) (1) 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 to 32 different levels. GroupB and GroupC(D1C) have 8 analog current levels. Please refer to Section 6.3.1 for detailed instructions on how to adjust the brightness control registers. 10 AN-1779 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD Copyright © 2008–2013, Texas Instruments Incorporated SNVA318A – March 2008 – Revised April 2013 Submit Documentation Feedback Software Interface Information www.ti.com 6.3.4 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 ramp-up/ramp-down time is determined 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 ramp-down (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. Table 2 the approximate ramp-up/ramp-down times if the GroupA brightness register is set to full-scale, or brightness code 31. Table 2. Brightness Ramp-Up/Ramp-Down Times 7 Ramp Code RS1-RS0 Ramp Step Time Total Ramp Time 00 100µs 3.2ms 01 25ms 0.8s 10 50ms 1.6s 11 100ms 3.2s Software Interface Information In order to fully evaluate the LM2756 part, an I2C Compatible interface must be used for any functionality to occur. A detailed description of the interface control is described in the LM2756 data sheet. Texas Instruments has created an I2C compatible interface generation program and USB docking board that can help exercise the part in a simple way. Contained in this document is a description of how to use the USB docking board and interface software. The LM2756 evaluation board has the means to “plug into” the USB docking board. The USB docking board can provide all of the control signals and power required to operate the evaluation board. A standard USB cable must be connected to the board from a PC. The I2C compatible interface program provides all of the control that the LM2756 part requires. For proper operation, the USB docking board should be plugged into the PC before the interface program is opened. Once connected, and the program is executed, a basic interface window will open. Figure 13. GUI Start-Up SNVA318A – March 2008 – Revised April 2013 Submit Documentation Feedback AN-1779 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD Copyright © 2008–2013, Texas Instruments Incorporated 11 Software Interface Information www.ti.com At the top of the interface, the user can read or write to any of the data registers on the LM2756 part using the two pull down menus (for the slave i.d. and the desired data address), the data field, and the read and write buttons. Figure 14. Generic Read/Write Field Figure 15. Drop Down Menu Just below the pull down menus are convenient toggle buttons to set/reset the control bits in the General Purpose Register. Figure 16. Control and Configuration Buttons • • • 62toA and 53toA: Assigns D62 and D63 current sinks to BankA when depressed. By default, D62 and D53 are assigned to BankB SD62 and SD53: Disabled drivers D62 and D53 when depressed ENC, ENB and ENA: These bits, when depressed, enable BankA, BankB and BankC. Figure 17. Brightness Control Sliders • • • 12 BankA Slider: Sets the BankA brightness to any allowable brightness code (0 to 31) BankB Slider: Sets the BankB brightness to any allowable brightness code (0 to 7) BankCSlider: Sets the BankC brightnessn to any allowable brightness code (0 to 7) AN-1779 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD Copyright © 2008–2013, Texas Instruments Incorporated SNVA318A – March 2008 – Revised April 2013 Submit Documentation Feedback Software Interface Information www.ti.com Figure 18. BankA Ramp Step Time • Ramp Step Time: This field sets the BankA brightness control ramp-up/ramp-down times. The time shown in the field corresponds to the time the LM2756 remains at each brightness code. Figure 19. Results of Pressing the Set Button Pressing the Set button places the LM2756 into the 4:3:1 configuration and sets the brightness levels in each bank to full-scale. Figure 20. Example Configuration In this configuration, the LM2756 will have 6 LEDs in BankA set to the full-scale brightness with a ramp step time equal to 13ms. BankB and BankC are each set to brightness code3 and are both active. SNVA318A – March 2008 – Revised April 2013 Submit Documentation Feedback AN-1779 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD Copyright © 2008–2013, Texas Instruments Incorporated 13 Software Interface Information www.ti.com NOTE: If the part is enabled to any level of brightness or state and the program is closed (by either hitting the OK or cancel buttons) , the LM2756 part will remain in the last controlled state. 14 AN-1779 LM2756 Multi-Display Inductorless LED Driver with 32 Exponential Dimming Steps in µSMD Copyright © 2008–2013, Texas Instruments Incorporated SNVA318A – March 2008 – Revised April 2013 Submit Documentation Feedback IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. 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