SLVS677C July   2008  – October 2015 TLC5926 , TLC5927

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison Table
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1  Absolute Maximum Ratings
    2. 7.2  ESD Ratings
    3. 7.3  Recommended Operating Conditions
    4. 7.4  Thermal Information
    5. 7.5  Electrical Characteristics: VDD = 3 V
    6. 7.6  Electrical Characteristics: VDD = 5.5 V
    7. 7.7  Timing Recommendations
    8. 7.8  Switching Characteristics: VDD = 3 V
    9. 7.9  Switching Characteristics: VDD = 5.5 V
    10. 7.10 Typical Characteristics
  8. 8 Parameter Measurement Information
  9. 9 Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 Open-Circuit Detection Principle
      2. 9.3.2 Short-Circuit Detection Principle (TLC5927 Only)
      3. 9.3.3 Overtemperature Detection and Shutdown
    4. 9.4 Device Functional Modes
      1. 9.4.1 Operation Mode Switching
      2. 9.4.2 Normal Mode Phase
      3. 9.4.3 Special Mode Phase
        1. 9.4.3.1 Reading Error Status Code in Special Mode
      4. 9.4.4 Writing Configuration Code in Special Mode
  10. 10Application and Implementation
    1. 10.1 Application Information
      1. 10.1.1 Constant Current
      2. 10.1.2 Adjusting Output Current
      3. 10.1.3 16-Bit Configuration Code and Current Gain
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
      3. 10.2.3 Application Curve
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
  13. 13Device and Documentation Support
    1. 13.1 Related Links
    2. 13.2 Community Resources
    3. 13.3 Trademarks
    4. 13.4 Electrostatic Discharge Caution
    5. 13.5 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

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10 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

10.1 Application Information

10.1.1 Constant Current

In LED display applications, TLC592x provides nearly no current variations from channel to channel and from IC to IC. While IOUT ≤ 50 mA, the maximum current skew between channels is less than ±6% and between ICs is less than ±6%.

10.1.2 Adjusting Output Current

TLC592x scales up the reference current, Iref, set by the external resistor Rext to sink a current, Iout, at each output port. Users can follow Equation 1, Equation 2, and Equation 3 to calculate the target output current IOUT,target in the saturation region:

Equation 1. VR-EXT = 1.26 V × VG
Equation 2. Iref = VR-EXT/Rext, if another end of the external resistor Rext is connected to ground.
Equation 3. IOUT,target = Iref × 15 × 3CM – 1

Where Rext is the resistance of the external resistor connected to the R-EXT terminal, and VR-EXT is the voltage of R-EXT, which is controlled by the programmable voltage gain (VG), which is defined by the Configuration Code. The Current Multiplier (CM) determines that the ratio IOUT,target/Iref is 15 or 5. After power on, the default value of VG is 127/128 = 0.992, and the default value of CM is 1, so that the ratio IOUT,target/Iref = 15. Based on the default VG and CM.

Equation 4. VR-EXT = 1.26 V × 127/128 = 1.25 V
Equation 5. IOUT,target = (1.25 V/Rext) × 15

Therefore, the default current is approximately 52 mA at 360 Ω and 26 mA at 720 Ω. The default relationship after power on between IOUT,target and Rext is shown in Figure 15.

TLC5926 TLC5927 g_iout_rext.gif Figure 15. Default Relationship Curve Between IOUT,target and Rext

10.1.3 16-Bit Configuration Code and Current Gain

Table 5 lists bit definition of the Configuration Code in the Configuration Latch.

Table 5. Bit Definition of 8-Bit Configuration Code

Bit 0 Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 Bit 8–15
Meaning CM HC CC0 CC1 CC2 CC3 CC4 CC5 Don't care
Default 1 1 1 1 1 1 1 1 X

Bit 7 is first sent into TLC592x through SDI. Bits 1 to 7 {HC, CC[0:5]} determine the voltage gain (VG) that affects the voltage at R-EXT and indirectly affects the reference current, Iref, flowing through the external resistor at R-EXT. Bit 0 is the Current Multiplier (CM) that determines the ratio IOUT,target/Iref. Each combination of VG and CM gives a specific Current Gain (CG).

  • VG: the relationship between {HC,CC[0:5]} and the voltage gain is calculated as shown in Equation 6 and Equation 7:
    • Equation 6. VG = (1 + HC) × (1 + D/64) / 4
    Equation 7. D = CC0 × 25 + CC1 × 24 + CC2 × 23 + CC3 × 22 + CC4 × 21 + CC5 × 20

    Where HC is 1 or 0, and D is the binary value of CC[0:5]. So, the VG could be regarded as a floating-point number with 1-bit exponent HC and 6-bit mantissa CC[0:5]. {HC,CC[0:5]} divides the programmable voltage gain VG into 128 steps and two subbands:

    Low voltage subband (HC = 0): VG = 1/4 ~ 127/256, linearly divided into 64 steps

    High voltage subband (HC = 1): VG = 1/2 ~ 127/128, linearly divided into 64 steps

  • CM: In addition to determining the ratio IOUT,target/Iref, CM limits the output current range.

    • High Current Multiplier (CM = 1): IOUT,target/Iref = 15, suitable for output current range IOUT = 10 mA to 120 mA.

    Low Current Multiplier (CM = 0): IOUT,target/Iref = 5, suitable for output current range IOUT = 5 mA to 40 mA

  • CG: The total Current Gain is defined as Equation 8, Equation 9, Equation 10, and Equation 11.
    • Equation 8. VR-EXT = 1.26 V × VG
    Equation 9. Iref = VR-EXT/Rext, if the external resistor, Rext, is connected to ground.
    Equation 10. IOUT,target = Iref × 15 × 3CM – 1 = 1.26 V/Rext × VG × 15 × 3CM – 1 = (1.26 V/Rext × 15) × CG
    Equation 11. CG = VG × 3CM – 1

    Therefore, CG = (1/12) to (127/128) divided into 256 steps.

Examples

  • Configuration Code {CM, HC, CC[0:5]} = {1,1,111111}

    • VG = 127/128 = 0.992 and CG = VG × 30 = VG = 0.992

  • Configuration Code = {1,1,000000}

    • VG = (1 + 1) × (1 + 0/64)/4 = 1/2 = 0.5, and CG = 0.5

  • Configuration Code = {0,0,000000}

    • VG = (1 + 0) × (1 + 0/64)/4 = 1/4, and CG = (1/4) × 3–1 = 1/12

After power on, the default value of the Configuration Code {CM, HC, CC[0:5]} is {1,1,111111}. Therefore, VG = CG = 0.992. The relationship between the Configuration Code and the Current Gain is shown in Figure 16.

TLC5926 TLC5927 currgain_configcode_lvs677.gif Figure 16. Current Gain vs Configuration Code

10.2 Typical Application

TLC5926 TLC5927 typapp_singledev_lvs677.gif Figure 17. Single Implementation of TLC592x Device

10.2.1 Design Requirements

For this design example, use the parameters listed in Table 6. The purpose of this design procedure is to calculate the power dissipation in the device and the operating junction temperature.

Table 6. Design Parameters

DESIGN PARAMETERS EXAMPLE VALUES
No. of LED strings 16
No. of LEDs per string 3
LED current (mA) 20
Forward voltage of each LED (V) 3.5
Junction-to-ambient thermal resistance (°C/W) 40
Ambient temperature of application (°C) 115
VDD (V) 5
IDD (mA) 17
Max operating junction temperature (°C) 150

10.2.2 Detailed Design Procedure

Equation 12. TJ = TA + θJA × PD_TOT

where

  • TJ is the junction temperature
  • TA is the ambient temperature
  • θJA is the junction-to-ambient thermal resistance
  • PD_TOT is the total power dissipation in the IC

space

Equation 13. PD_TOT = PD_CS + IDD × VDD

where

  • PD_CS is the power dissipation in the LED current sinks
  • IDD is the IC supply current
  • VDD is the IC supply voltage

space

Equation 14. PD_CS = IO × VO × nCH

where

  • IO is the LED current
  • VO is the voltage at the output pin
  • nCH is the number of LED strings

space

Equation 15. VO = VLED – (nLED × VF)

where

  • VLED is the voltage applied to the LED string
  • nLED is the number of LEDs in the string
  • VF is the forward voltage of each LED

space

VO should not be too high as this will cause excess power dissipation inside the current sink. However, VO should also not be loo low as this will not allow the full LED current (refer to the output voltage vs. output current graph). With VLED = 12 V:

Equation 16. VO = 12 V – (3 × 3.5 V) = 1.5 V
Equation 17. PD_CS = 20 mA × 1.5 V × 16 = 0.48 W

Using PD_CS, calculate:

Equation 18. PD_TOT = PD_CS + IDD × VDD = 0.48 W + 0.017 A × 5 V = 0.565 W

Using PD_TOT, calculate:

Equation 19. TJ = TA + θJA × PD_TOT = 115°C + 40°C/W × 0.565 W = 137.6°C

This design example has demonstrated how to calculate power dissipation in the IC and ensure that the junction temperature is kept below 150°C.

NOTE

This design example assumes that all channels have the same electrical parameters (nLED, IO, VF, VLED). If the parameters are unique for each channel, then the power dissipation must be calculated for each current sink separately. Then, each result must be added together to calculate the total power dissipation in the current sinks.

10.2.3 Application Curve

TLC5926 TLC5927 g_io_vo_lvs677.gif Figure 18. Output Current vs Output Voltage