TLC5957 Datasheet, PDF (15/25 Pages) Texas Instruments – TLC5957 48-Channel, 16-Bit ES-PWM LED Driver with Pre-Charge FET, LED OPEN Detection and Caterpillar Cancelling

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TLC5957
SLVSCQ4 – OCTOBER 2014
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, R_IREF = 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, R_IREF = 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 (T_J)
exceeds 170°C (typ). It resumes normal operation when T_J falls below 160°C (typ).
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