SLVSCQ4 October   2014 TLC5957

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Typical Application Circuit (Multiple Daisy Chained TLC5957s)
  5. 5 Revision History
  6. 6 Pin 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 Timing Requirements
    7. 7.7 Typical Characteristics
  8. 8 Parameter Measurement Information
    1. 8.1 Pin Equivalent Input and Output Schematic Diagrams
    2. 8.2 Test Circuit
  9. 9 Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Device Functional Modes
      1. 9.3.1  Brightness Control (BC) Function
      2. 9.3.2  Color Control (CC) Function
      3. 9.3.3  Select RIREF For a Given BC
      4. 9.3.4  Choosing BC/CC For a Different Application
        1. 9.3.4.1 Example 1: Red LED Current is 20mA, Green LED Needs 12mA, Blue LED needs 8mA
        2. 9.3.4.2 Example 2: Red LED Current is 5mA, Green LED Needs 2mA, Blue LED Needs 1mA.
      5. 9.3.5  LED Open Detection (LOD)
      6. 9.3.6  Poker Mode
      7. 9.3.7  Internal Circuit for Caterpillar Removal
      8. 9.3.8  Internal Pre-charge FET for Ghost Removal
      9. 9.3.9  Thermal Shutdown (TSD)
      10. 9.3.10 IREF Resistor Short Protection (ISP)
      11. 9.3.11 Noise Reduction
  10. 10Application and Implementation
    1. 10.1 Application Information
  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 Trademarks
    3. 13.3 Electrostatic Discharge Caution
    4. 13.4 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

9 Detailed Description

9.1 Overview

The TLC5957 is a 48-channel constant-current sink driver for multiplexing an LED display system. Each channel has an individually-adjustable, 65536-step, pulse width modulation (PWM) grayscale control.

The TLC5957 supports output current range from 1 mA to 25 mA. Channel-to-channel accuracy is 3% max, device-to-device accuracy is 2% max in all current ranges. Also, the TLC5957 implements Low Grayscale Enhancement (LGSE) technology to improve the display quality at low grayscale conditions. These features improve the performance of the TLC5957-multiplexed display system.

The output channels are grouped in three groups, each group has 16 channels for one color. Each group has a 512-step color brightness control (CC) function. The maximum current value of all 48 channels can be set by 8-step global brightness control (BC) function. GS, CC and BC data are accessible via a serial interface port.

The TLC5957 has one error flag: LED open detection (LOD), that can be read via a serial interface port. The TLC5957 also has an enhanced circuit to solve the caterpillar issue caused by open LEDs. Thermal shutdown (TSD) and Iref resistor short protection (ISP) assure TLC5957 of a higher system reliability.

9.2 Functional Block Diagram

fbd_slvscq4.gif

9.3 Device Functional Modes

After power on, all OUTXn of TLC5957 are turned off. All the internal counters and function control registers are initialized. Below is a brief summary of the sequence to operate TLC5957, just give users a general idea how this part works. After that, the function block related to each step will be detailed in following sections.

  1. According to required LED current, choose BC and CC code, select the current programming resistor RIREF.
  2. Send WRTFC command to set FC register value if the default value need be changed.
  3. Write GS data of line 1 into GS data latch. Using LATGS command for the last group of 48bit GS data loading, the GS data written just now will be displayed.
  4. Input GCLK continuously, 2N GCLK (N>=9) as a segment. Between the interval of two segments, supply voltage should be switched from one line to next line accordingly.
  5. During the same period of step4, GS data for next line should be written into GS data latch. Using LATGS command for the last group of 48bit GS data loading.
  6. Repeat step 4-5 until it comes to the last line for a multiplexing panel. Input 2N GCLK (N>=9) as a segment, at the same time, GS data for 1st line should be written into GS data latch. Using LINERESET command for the last group of 48bit GS data loading.

Repeat step 4 through 6.

9.3.1 Brightness Control (BC) Function

The TLC5957 is able to adjust the output current of all constant-current outputs simultaneously. This function is called global brightness control (BC). The global BC for all outputs is programmed with a 3-bit word, thus all output currents can be adjusted in 8 steps from 12.9% to 100% (See Table 2) for a given current programming resistor(RIREF)

BC data can be set via the serial interface. When the BC data change, the output current also changes immediately. When the device is powered on, the BC data in the function control (FC) register is set to 4h as the initial value.

9.3.2 Color Control (CC) Function

The TLC5957 is able to adjust the output current of each of the three color groups OUTR0-OUTR15, OUTG0-OUTG15, and OUTB0-OUTB15 separately. This function is called color brightness control (CC). For each color, it has 9-bit data latch CCR, CCG, or CCB in FC register. Thus, all color group output currents can be adjusted in 512 steps from 0% to 100% of the maximum output current, IOLCMax. (See next section for more details about IOLCMax). The CC data are entered via the serial interface. When the CC data change, the output current also changes immediately.

When the IC is powered on, the CC data are set to ‘100h’.

Equation 1 calculates the actual output current.

Equation 1. Iout(mA) = IOLCMax(mA) × ( CCR/511d or CCB/511d)

Where:

IOLCMax = the maximum channel current for each channel, determined by BC data and RIREF (See Equation 2)

CCR/G/B = the color brightness control value for each color group in the FC1 register (000h to 1FFh)

Table 1 shows the CC data versus the constant-current against IOLCMax.

Table 1. CC Data vs Current Ratio and Set Current Value

CC DATA (CCR or CCG or CCB) RATIO OF OUTPUT CURRENT
TO IolcMax(%, typical) 		OUTPUT CURRENT (mA, RIREF = 7.41 kΩ)
BINARY DECIMAL HEX BC = 7h
(IolcMax =25mA) 		BC = 0h
(IolcMax=3.2mA)
0 0000 0000 0 00 0 0 0
0 0000 0001 1 01 0.2 0.05 0.006
0 0000 0010 2 02 0.4 0.10 0.013
— — — — — —
1 0000 0000
(Default) 		256
(Default)		 100
(Default)		 50.1 12.52 1.621
— — — — — —
1 1111 1101 509 1FD 99.6 24.90 3.222
1 1111 1110 510 1FE 99.8 24.95 3.229
1 1111 1111 511 1FF 100.0 25 3.235

9.3.3 Select RIREF For a Given BC

The maximum output current per channel, IOLCMax is determined by resistor RIREF placed between the IREF and IREFGND pins, and the BC code in FC register. The voltage on IREF is typically 1.209V. RIREF can be calculated by Equation 2.

Equation 2. Riref(kΩ) = Viref(V) / IOLCMax(mA) × Gain

Where:

VIREF = the internal reference voltage on IREF (1.209V, typical)

IOLCMax is the largest current for each output at CCR/G/B=1FFh.

Gain = the current gain at a selected BC code (See Table 2)

Table 2. Current Gain Versus BC Code

BC DATA GAIN RATIO OF
GAIN / GAIN_MAX (AT MAX BC)
BINARY HEX
000 (recommend) 0 (recommend) 20.0 12.9%
001 1 39.5 25.6%
010 2 58.6 37.9%
011 3 80.9 52.4%
100 (default) 4 (default) 100.0 64.7%
101 5 113.3 73.3%
110 6 141.6 91.7%
111 7 154.5 100%
NOTE: Recommend to use smaller BC code for better performance. For noise immunity purposes, suggest RIREF < 60 kΩ.

9.3.4 Choosing BC/CC For a Different Application

BC is mainly used for global brightness adjustment between day and night. Suggested BC is 4h, which is in the middle of the range; thus, one can change brightness up and down flexibly.

CC can be used to fine tune the brightness in 512 steps, this is suitable for white balance adjustment between RGB color groups. To get a pure white color, the general requirement for the luminous intensity ratio of R, G, B LED is 3:6:1. Depending on LED’s characteristics (Electro-Optical conversion efficiency), the current ratio of R, G, B LED will be much different from this ratio. Usually, the Red LED will need the largest current. One can choose 511d(the max value) CC code for the color group which needs the largest current at first, then choose proper CC code for the other two color groups according to the current ratio requirement of the LED used.

9.3.4.1 Example 1: Red LED Current is 20mA, Green LED Needs 12mA, Blue LED needs 8mA

  1. Red LED needs the largest current, so choose 511d for CCR
  2. 511 x 12mA / 20mA = 306.6, thus choose 307d for CCG. With same method, choose 204d for CCB.
  3. According to the required red LED current, choose 7h for BC.
  4. According to Equation 2, RIREF = 1.2V/20mA x 154.5 = 9.27 kΩ

In this example, we choose 7h for BC, instead of using the default 4h. This is because the Red LED current is 20mA, approaching the upper limit of current range. To prevent the constant output current from exceeding the upper limit in case a larger BC code is input accidently, we choose the maximum BC code here.

9.3.4.2 Example 2: Red LED Current is 5mA, Green LED Needs 2mA, Blue LED Needs 1mA.

  1. Red LED needs the largest current, so choose 511d for CCR.
  2. 511 x 2mA / 5mA = 204.4, thus choose 204d for CCG. With same method, choose 102d for CCB.
  3. According to the required blue LED current, choose 0h for BC.
  4. According to Equation 2, RIREF = 1.2V / 5mA x 20 = 4.8 kΩ

In this example, we choose 0h for BC, instead of using the default 4h. This is because the Blue LED current is 1mA, which is approaching the lower limit of current range. To prevent the constant output current from exceeding the lower limit in case a lower BC code is input accidently, we choose the min BC code here.

In general, if LED current is in the middle of range(i.e, 10mA), one can just use the default 4h as BC code.

9.3.5 LED Open Detection (LOD)

LOD function detects a fault caused by an open circuit in any LED string, or a short from OUTXn to ground with low impedance, by comparing the OUTXn voltage to the LOD detection threshold voltage level set by LODVLT in the FC register. If the OUTXn voltage is lower than the programmed voltage, the corresponding output LOD bit will be set to '1' to indicate a opened LED. Otherwise, the output of that LOD bit is '0'. LOD data output by the detect circuit are valid only during the ‘on’ period of that OUTXn output channel. LOD data are always ‘0’ for outputs that are turned off.

9.3.6 Poker Mode

Poker Mode provides the TLC5957 with a flexible PWM bit, from 9 bit to 16 bit. Therefore, data length can be reduced. In high multiplexing applications, Poker Mode can significantly increase visual refresh rate.

9.3.7 Internal Circuit for Caterpillar Removal

Caterpillar effect is a very common issue on LED panels. It is usually caused by an LED lamp open, LED lamp leakage or LED lamp short. The TLC5957 implements an internal circuit that can eliminate the caterpillar issue caused by LED open. This function can be enabled and disabled by LINERESET command. If the function is enabled, the IC automatically detects the broken LED lamp, and the lamp will not light until IC reset.

9.3.8 Internal Pre-charge FET for Ghost Removal

The internal pre-charge FET can prevent ghosting of multiplexed LED modules. One cause of this phenomenon is the charging current for parasitic capacitance of the OUTXn through the LED when the supply voltage switches from one common line to the next common line.

To prevent this unwanted charging current, the TLC5957 uses an internal FET to pull OUTXn up to VCC-1.4V during the common line switching period. Thus, no charging current flows through LED and the ghosting is eliminated.

9.3.9 Thermal Shutdown (TSD)

The thermal shutdown (TSD) function turns off all IC constant-current outputs when the junction temperature (TJ) exceeds 170°C (typ). It resumes normal operation when TJ falls below 160°C (typ).

9.3.10 IREF Resistor Short Protection (ISP)

The Iref resistor short protection (ISP) function prevents unwanted large currents from flowing though the constant-current output when the Iref resistor is shorted accidently. The TLC5957 turns off all output channels when the Iref pin voltage is lower than 0.19V (typ). When the Iref pin voltage goes higher than 0.33V (typ), the TLC5957 resumes normal operation.

9.3.11 Noise Reduction

Large surge currents may flow through the IC and the board on which the device is mounted if all 48 LED channels turn on simultaneously at the 1st GCLK rising edge. This large surge current could induce detrimental noise and electromagnetic interference (EMI) into other circuits.

The TLC5957 separate the LED channels into 12 groups. Each group turns on sequentially with some delay between one group and the next group. By this means, a soft-start feature is provided and the inrush current is minimized.