SNVS857 February   2014 LP8555

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Simplified Schematic
  5. 5 Revision History
  6. 6 Terminal Configuration and Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 Handling Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 I2C Serial Bus Timing Parameters (FSET/SDA, ISET/SCL)
    7. 7.7 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Features Description
      1. 8.3.1 Boost Converter Overview
        1. 8.3.1.1 Operation
        2. 8.3.1.2 Protection
        3. 8.3.1.3 Setting Boost Switching Frequency
        4. 8.3.1.4 Adaptive Boost Output Voltage Control
        5. 8.3.1.5 EMI Reduction
      2. 8.3.2 Brightness Control
        1. 8.3.2.1 PWM Input Duty Measurement
        2. 8.3.2.2 BRTMODE = 00
        3. 8.3.2.3 BRTMODE = 01
        4. 8.3.2.4 BRTMODE = 10
        5. 8.3.2.5 BRTMODE = 11
        6. 8.3.2.6 Hybrid PWM and Current Dimming Control
        7. 8.3.2.7 Setting PWM Dimming Frequency
        8. 8.3.2.8 Setting Full-Scale LED Current
        9. 8.3.2.9 Phase-Shift PWM Scheme
      3. 8.3.3 LED Brightness Slopes, Normal and Advanced
      4. 8.3.4 Start-up and Shutdown Sequences
        1. 8.3.4.1 Start-up With PWM Input Brightness Control Mode (BRTMODE = 00b)
        2. 8.3.4.2 Shutdown With PWM Input Brightness Control Mode (BRTMODE = 00b)
        3. 8.3.4.3 Start-up With I2C Brightness Control Mode (BRTMODE = 01b)
        4. 8.3.4.4 Shutdown With I2C Brightness Control Mode (BRTMODE = 01b)
        5. 8.3.4.5 Start-up with I2C + PWM Input Brightness Control Mode (BRTMODE = 10 or 11b)
        6. 8.3.4.6 Shutdown with I2C + PWM Input Brightness Control Mode (BRTMODE = 10 or 11b)
      5. 8.3.5 LED String Count Auto Detection
      6. 8.3.6 Fault Detection
        1. 8.3.6.1 LED Short Detection
        2. 8.3.6.2 LED Open Detection
        3. 8.3.6.3 Undervoltage Detection
        4. 8.3.6.4 Thermal Shutdown
        5. 8.3.6.5 Boost Overcurrent Protection
        6. 8.3.6.6 Boost Overvoltage Protection
        7. 8.3.6.7 Boost Undervoltage Protection
      7. 8.3.7 I2C-Compatible Serial Bus Interface
        1. 8.3.7.1 Interface Bus Overview
        2. 8.3.7.2 Data Transactions
        3. 8.3.7.3 Acknowledge Cycle
        4. 8.3.7.4 “Acknowledge After Every Byte” Rule
        5. 8.3.7.5 Addressing Transfer Formats
        6. 8.3.7.6 Control Register Write Cycle
        7. 8.3.7.7 Control Register Read Cycle
    4. 8.4 Device Functional Modes
      1. 8.4.1 Operation Without I2C Control
      2. 8.4.2 Operation With I2C Control
      3. 8.4.3 Shutdown Mode
      4. 8.4.4 Standby Mode
      5. 8.4.5 Active Mode
    5. 8.5 Register Maps
      1. 8.5.1  COMMAND
      2. 8.5.2  STATUS/MASK
      3. 8.5.3  BRTLO
      4. 8.5.4  BTHI
      5. 8.5.5  CONFIG
      6. 8.5.6  CURRENT
      7. 8.5.7  PGEN
      8. 8.5.8  BOOST
      9. 8.5.9  LEDEN
      10. 8.5.10 STEP
      11. 8.5.11 Brightness Transitions, Typical Times
      12. 8.5.12 VOLTAGE_0
      13. 8.5.13 LEDEN1
      14. 8.5.14 VOLTAGE1
      15. 8.5.15 OPTION
      16. 8.5.16 EXTRA
      17. 8.5.17 ID
      18. 8.5.18 REVISION
      19. 8.5.19 CONF0
      20. 8.5.20 CONF1
      21. 8.5.21 VHR0
      22. 8.5.22 VHR1
      23. 8.5.23 JUMP
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 Application for Default LP8555YFQR EPROM Configuration
        1. 9.2.1.1 Schematic
        2. 9.2.1.2 LP8555YFQR EPROM Configuration
        3. 9.2.1.3 Design Requirements
        4. 9.2.1.4 Detailed Design Procedure
          1. 9.2.1.4.1 Inductor
          2. 9.2.1.4.2 Output Capacitor
          3. 9.2.1.4.3 LDO Capacitor
          4. 9.2.1.4.4 VDD Capacitor
          5. 9.2.1.4.5 Boost Input Capacitor
          6. 9.2.1.4.6 Diode
        5. 9.2.1.5 Application Performance Plots
      2. 9.2.2 Application Example With Different LED Configuration for Each Bank
        1. 9.2.2.1 Schematic
      3. 9.2.3 Application Example With 12 LED Strings and PWM Input Brightness Control
        1. 9.2.3.1 Schematic
      4. 9.2.4 Application With 12 LED Strings, I2C Brightness Control
        1. 9.2.4.1 Schematic
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12 Device and Documentation Support
    1. 12.1 Trademarks
    2. 12.2 Electrostatic Discharge Caution
    3. 12.3 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

9 Application and Implementation

9.1 Application Information

The LP8555 designed for LCD backlighting, especially for high-resolution tablet panels where more backlight power is needed due to smaller aperture ratio of the LCD. With single-boost configuration the inductor selection is difficult for height restricted applications; to overcome this LP8555 uses dual-boost configuration. This shares the total load to two boost inductors and allows using two smaller inductors instead of one large inductor while maintaining good efficiency. 12 LED current sinks allow driving up to 96 LEDs with high efficiency. Better efficiency is achieved with using lower conversion ratio for boost and driving more LEDs in parallel, compared to using fewer LED strings and higher boost conversion ratio. Main limiting factor for output power is inductor current limit, which is calculated in the Detailed Design Procedure. PCB thermal performance must be considered in high power applications where thermal dissipation of LP8555 can become limiting factor.

Due to a flexible input voltage configuration, the LP8555 can be used also in various other applications, such as laptop backlighting, as well as other LED lighting where high number of LEDs are needed and must be driven with highest possible efficiency. The following design procedure can be used to select component values for the LP8555. An example for default EPROM configuration is given with corresponding design parameters. LP8555EVM User Guide has reference bill of materials and example layout pictures.

9.2 Typical Applications

9.2.1 Application for Default LP8555YFQR EPROM Configuration

With the default EPROM configuration PWM input is used for brightness control. The backlight is enabled/disabled and also configuration can be changed before backlight turning on with I2C writes. Up to 12 LED strings can be used with max 28-V boost output voltage. LED current is set to 25 mA by default. See detailed EPROM setup in LP8555YFQR EPROM Configuration.

9.2.1.1 Schematic

eprom_vs_application.gifFigure 43. Application Diagram for Default EPROM Setup

9.2.1.2 LP8555YFQR EPROM Configuration

ADDRESS (HEX) BIT BIT NAME BIT DESCRIPTION VALUE MEANING REG VALUE (HEX)
00 2 SREN Boost slew rate limit enable 0b 0 = Boost slew rate not limited 00
1 SSEN Spread spectrum enable 0b 0 = Spread spectrum disabled
0 ON Backlight enable 0b 0 = Backlight enabled only by writing this bit to 1
10 7 PWMSB Enable automatic PWM input shutdown 0b 0 = Shutdown function disabled 64
6 PWMFILT Enable PWM input filtering 1b 1 = PWM input analog 50 ns filter enabled
5 EN_BPHASE180 Enable boost 180° phase difference 1b 1 = Boosts clocks are 180° shifted
4 - 0b
3 RELOAD Enable EPROM read at every BL enable sequence 0b 0 = EPROM is read only at first start-up
2 AUTO Enable auto detect for number of LEDs during start-up 1b 1 = LED string auto detection enabled
1:0 BRTMODE Brightness control mode 00b 00 = PWM input duty control only
11 7 ISET Enable external resistor setting of LED string current 0b 0 = LED current set with registers 05
6:3 - 0000b
2:0 MAXCURR Set maximum DC current per string 101b 101 = 25 mA
12 7 PFSET Enable external resistor PWM frequency setting 0b 0 = PWM frequency selected with registers 2B
6 - 0b
5:3 THRESHOLD Hybrid PMW and Current Control switch point control 101b 101 = 25% switch point
2:0 PFSET PWM frequency selection 011b 011 = 19.5 kHz
13 7 - 0b 01
6 - 0b
5:2 - 0000b
1 BIND Boost inductor selection 0b 0 = 4.7 µH ... 6.8 µH inductor
0 BFREQ Boost SW frequency 1b 1 = 1 MHz
14 7:6 OV Set LED high comparator detection level 10b 10 = 3 V BF
5:0 ENABLE_0 LED bank A string enable 111111b 1 = Enabled (all six strings)
15 7:6 SMOOTH Advanced Sloping smoothing factor 00b 00 = No smoothing 20
5:3 PWM_IN_HYST PWM input hysteresis 100b 100 = >8 LSB steps
2:0 STEP Linear Slope time 000b 000 = 0ms
16 7:6 VMAX_0 Bank A boost maximum voltage 11b 11 = 28 V F8
5 ADAPT_0 Enable boost adaptive control for bank A 1b 1 = Adaptive headroom enabled
4:0 VINIT_0 Initial voltage for bank A boost 11000b 11000 = 23.26 V
19 5:0 ENABLE_1 LED bank B string enable 111111b 1 = Enabled (all six strings) 3F
1A 7 VMAX_1 Bank B boost maximum voltage 11b 11 = 28 V F8
6 ADAPT_1 Enable boost adaptive control for bank B 1b 1 = Adaptive headroom enabled
5 VINIT_1 Initial voltage for bank B boost 11000b 11000 = 23.26 V
1E 7:4 ID_CUST ID register, Customer ID 0000b 0000 00
3:0 ID_CFG ID register, EPROM config 0000b 0000
76 7 BOOST_IS_DIV2 Option divide Imax peak current by 2 0b 0 = Normal current limit 0B
6:5 - 00b
4 ALTID I2C slave ID selector 0b 0 = 2Ch (7-bit)
3:2 SRON Slowed boost slew rate 10b When COMMAND.SREN is set to 1 this value is used.
10 = Second slowest
1:0 CURR_LIMIT Inductor peak current limit 11b 11 = 3.1 A
77 7:6 FMOD_DIV Spread spectrum modulation frequency divisor 00b When COMMAND.SSEN is set to 1 this value is used.
00 = 0.42%		 17
5:0 - 010111b
78 6:4 VHR_SLOPE LED driver maximum headroom voltage at maximum current (50mA) 110b 110 = 285 mV 60
3 - 0b
2:0 VHR_VERT LED driver maximum headroom voltage at minimum current 000b 000 = 50 mV
79 5:4 VHR_HYST LED driver hysteresis for mid comparator level 01b 01 = 233 mV 11
3:2 - 00b
1:0 VHR_HORZ LED driver headroom control knee percentage of full LED current 01b 01 = 25%
7A 7 JEN Enable boost voltage jumping on brightness change 1b 1 = Jump enabled 88
6:4 - 000b
3:2 JTHR Jump brightness threshold 10b 10 = 25%
1:0 JVOLT Jump voltage 00b 00 = 2 V

9.2.1.3 Design Requirements

Example requirements based on default EPROM setup.

DESIGN PARAMETER EXAMPLE VALUE
Input voltage range 2.7 V to 4.5 V (Single Li-Ion cell battery)
Brightness Control PWM input duty cycle
Backlight enabled Writing ON bit 1 with I2C
PWM output frequency 19.2 kHz with PSPWM enabled
LED Current 25 mA / channel
Number of Channels Up to 12 with string auto detection enabled
Brightness slopes Disabled
External set resistors Disabled
Inductor 4.7 µH to 6.8 µH, at least 3.1-A saturation current
Boost SW frequency 1 MHz
Maximum output voltage 28 V
SW current limit 3.1 A
CABC Jump enabled for >25% brightness changes

9.2.1.4 Detailed Design Procedure

9.2.1.4.1 Inductor

There are two main considerations when choosing an inductor; the inductor should not saturate, and the inductor current ripple should be small enough to achieve the desired output voltage ripple. Different saturation current rating specifications are followed by different manufacturers so attention must be given to details. Saturation current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of application should be requested from the manufacturer. Shielded inductors radiate less noise and should be preferred.

The saturation current should be greater than the sum of the maximum load current and the worst case average to peak inductor current.

Figure 44 shows the worst case conditions.

inductor_equation.gifFigure 44. Calculating Inductor Maximum Current

  • IRIPPLE: peak inductor current
  • IOUTMAX: maximum load current
  • VIN: minimum input voltage in application
  • L : min inductor value including worst case tolerances
  • f : minimum switching frequency
  • VOUT: output voltage
  • D: Duty Cycle for CCM Operation
  • VOUT : Output Voltage

As a result the inductor should be selected according to the ISAT. A more conservative and recommended approach is to choose an inductor that has a saturation current rating greater than the maximum current limit of 3.1 A. A 4.7-µH to 6.8-µH inductor with a saturation current rating of at least 3.1 A is recommended for most applications. The inductor’s resistance should be less than 300 mΩ for good efficiency.

9.2.1.4.2 Output Capacitor

A ceramic capacitor with 50-V voltage rating is recommended for the output capacitor. The DC-bias effect can reduce the effective capacitance by up to 80% especially with small package size capacitors, which needs to be considered in capacitance value and package selection. Typically one 10-µF or two 4.7-µF capacitors is sufficient. Effectively the capacitance should be at least 2 µF at boost maximum output voltage.

9.2.1.4.3 LDO Capacitor

A ceramic capacitor with at least 10 V voltage rating is recommended for the output capacitor of the LDO. The DC-bias effect can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Typically 10 µF capacitor is sufficient.

9.2.1.4.4 VDD Capacitor

A ceramic capacitor with at least 10-V voltage rating is recommended for the VDD input capacitor. If input voltage is higher, then the rating should be selected accordingly. The DC-bias effect can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Typically, a 1-μF capacitor is sufficient. X5R/X7R are recommended types.

9.2.1.4.5 Boost Input Capacitor

A ceramic capacitor with at least 10-V voltage rating is recommended for the boost input capacitor. If input voltage is higher, then the rating should be selected accordingly. The DC-bias effect can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Typically, a 10-μF capacitor per boost is sufficient. X5R/X7R are recommended types.

9.2.1.4.6 Diode

A Schottky diode should be used for the output diode. Peak repetitive current should be greater than inductor peak current (3.1 A) to ensure reliable operation. Average current rating should be greater than the maximum output current. Schottky diodes with a low forward drop and fast switching speeds are ideal for increasing efficiency in portable applications. Choose a reverse breakdown voltage of the Schottky diode significantly larger (~40 V) than the output voltage. Do not use ordinary rectifier diodes, since slow switching speeds and long recovery times cause the efficiency and the load regulation to suffer.

9.2.1.5 Application Performance Plots

Typical performance plots with default EPROM configuration. The LP8555EVM was used for taking the oscilloscope plots.

Startup_waveform.png
VDD = 3.7V ILED = 25 mA 12 x 7 LEDs
Figure 45. Start-up Waveform with I2C Write
Full Brightness Slope Function Disabled.
Boost_180deg_waveform.png
VDD = 3.7V fSW = 1 MHz
Figure 47. 180° Phase Difference Between Boost A and B
Boost_SW_Waveforms.png
VDD = 3.7V VBOOST = 28V Load = 150 mA
Figure 46. Typical Boost Waveform, Boost A
LED_waveforms.png
ƒPWM = 19.5 kHz 6 Strings
Figure 48. Typical LED Current and Voltage Waveforms.

9.2.2 Application Example With Different LED Configuration for Each Bank

In the following example schematic it is shown how the LED banks can have different LED configuration. Bank A has 4 active LED outputs and Bank B has 5 active LED outputs. LP8555 will automatically detect open outputs and adjust phase shifting for both banks optimally. Control in this example is from I2C bus and PWM/INT terminal is used for interrupt signal, notifying processor on possible fault conditions. Component selection and performance plots follow the examples shown in first application example with default EPROM configuration, but the PSPWM is adjusted based on the number of connected strings. Details on I2C registers and EPROM settings are seen in the Register Map section. If custom EPROM is required, please contact TI Sales representative for availability.

9.2.2.1 Schematic

typ_app_different_LED_snvs857.gifFigure 49. Application Example With Different LED Configurations on Each Bank

9.2.3 Application Example With 12 LED Strings and PWM Input Brightness Control

In the following example schematic it is shown how the LP8555 can be configured for operating without I2C control. Only controls needed are PWM input for brightness control and EN for enabling and disabling device. PWM frequency is set with RFSET resistor and LED current is set with ISET resistor. Full 12 channels are used in this example, but other configurations can be used as well. Component selection and performance plots follow the examples shown in first application example with default EPROM configuration. Details on I2C registers and EPROM settings are seen in the Register Map section. If custom EPROM is required, please contact TI Sales representative for availability. Since this configuration relies on pre-programmed EPROM for basic setup (although LED current and PWM frequency are set with resistors), special EPROM configuration is needed for this application.

9.2.3.1 Schematic

typ_app_PWM_snvs857.gifFigure 50. Application Example with 12 LED Strings and PWM Input Brightness Control

9.2.4 Application With 12 LED Strings, I2C Brightness Control

In the following example full 12 channels are used with I2C brightness control. PWM/INT terminal is used for interrupt signal, notifying processor on possible fault conditions. LED current and PWM frequency are set with I2C writes, or default EPROM values can be used as well. Component selection and performance plots follow the examples shown in first application example with default EPROM configuration. Configuration registers can be set before backlight is enabled, so special pre-set EPROM is not necessarily needed. Details on I2C registers and EPROM settings are seen in the Register Map section. If custom EPROM is required, please contact TI Sales representative for availability.

9.2.4.1 Schematic

typ_app_I2C_snvs857.gifFigure 51. Application Example With 12 LED Strings, I2C Brightness Control