SLVSCQ4 October 2014 TLC5957
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.
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.
Repeat step 4 through 6.
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.
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.
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.
|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
|BC = 0h
|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
|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|
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.
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)
|BC DATA||GAIN||RATIO OF
GAIN / GAIN_MAX (AT MAX BC)
|000 (recommend)||0 (recommend)||20.0||12.9%|
|100 (default)||4 (default)||100.0||64.7%|
|NOTE: Recommend to use smaller BC code for better performance. For noise immunity purposes, suggest RIREF < 60 kΩ.|
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.