TLC5940-EP Datasheet, PDF (16/33 Pages) Texas Instruments – 16-CHANNEL LED DRIVER WITH DOT CORRECTION AND GRAYSCALE PWM CONTROL

TLC5940-EP
SLVSA51D – MARCH 2010 – REVISED MAY 2010
www.ti.com
LOD: LED OPEN DETECTION
The TLC5940 has an LED-open detector that detects broken or disconnected LEDs. The LED open detector
pulls the XERR pin to GND when an open LED is detected. XERR and the corresponding error bit in the Status
Information Data is only active under the following open-LED conditions.
1. OUTn is on and the time tpd2 (1 ms typical) has passed.
2. The voltage of OUTn is < 0.3V (typical)
The LOD status of each output can be also read out from the SOUT pin. See STATUS INFORMATION OUTPUT
section for details. The LOD error bits are latched into the Status Information Data when XLAT returns to a low
after a high. Therefore, the XLAT pin must be pulsed high then low while XERR is active in order to latch the
LOD error into the Status Information Data for subsequent reading via the serial shift register.
DELAY BETWEEN OUTPUTS
The TLC5940 has graduated delay circuits between outputs. These circuits can be found in the constant current
driver block of the device (see the functional block diagram). The fixed-delay time is 20ns (typical), OUT0 has no
delay, OUT1 has 20ns delay, and OUT2 has 40ns delay, etc. The maximum delay is 300ns from OUT0 to
OUT15. The delay works during switch on and switch off of each output channel. These delays prevent large
inrush currents which reduces the bypass capacitors when the outputs turn on.
OUTPUT ENABLE
All OUTn channels of the TLC5940 can be switched off with one signal. When BLANK is set high, all OUTn
channels are disabled, regardless of logic operations of the device. The grayscale counter is also reset. When
BLANK is set low, all OUTn channels work under normal conditions. If BLANK goes low and then back high
again in less than 300ns, all outputs programmed to turn on still turn on for either the programmed number of
grayscale clocks, or the length of time that the BLANK signal was low, which ever is lower. For example, if all
outputs are programmed to turn on for 1ms, but the BLANK signal is only low for 200ns, all outputs still turn on
for 200ns, even though some outputs are turning on after the BLANK signal has already gone high.
Table 3. BLANK Signal Truth Table
BLANK
LOW
HIGH
OUT0 - OUT15
Normal condition
Disabled
SETTING MAXIMUM CHANNEL CURRENT
The maximum output current per channel is programmed by a single resistor, R_(IREF), which is placed between
IREF pin and GND pin. The voltage on IREF is set by an internal band gap V_(IREF) with a typical value of
1.24V. The maximum channel current is equivalent to the current flowing through R_(IREF) multiplied by a factor of
31.5. The maximum output current per channel can be calculated by Equation 6:
Imax
+
V(IREF)
R(IREF)
31.5
(6)
where:
V_(IREF) = 1.24 V
R_(IREF) = User-selected external resistor.
I_max must be set between 5 mA and 120 mA. The output current may be unstable if I_max is set lower than 5 mA.
Output currents lower than 5 mA can be achieved by setting Imax to 5 mA or higher and then using dot
correction.
Figure 4 shows the maximum output current I_O versus R_(IREF). R_(IREF) is the value of the resistor between IREF
terminal to GND, and I_O is the constant output current of OUT0 to OUT15. A variable power supply may be
connected to the IREF pin through a resistor to change the maximum output current per channel. The maximum
output current per channel is 31.5 times the current flowing out of the IREF pin.
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