SLVS814A January 2008 – May 2015 TLC5916-Q1 , TLC5917-Q1
PRODUCTION DATA.
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.
In LED display applications, the TLC591x-Q1 provides nearly no current variations from channel to channel and from IC to IC. While IOUT ≤ 100 mA, the maximum current skew between channels is less than ±3% and between ICs is less than ±6%.
The TLC591x-Q1 scales up the reference current, Iref, set by the external resistor Rext to sink a current, Iout, at each output port. Use the following formulas to calculate the target output current
where
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.
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 19.
The TLC5917/TLC5917-Q1 Constant-Current LED Sink Drivers is designed to work alone or cascaded. Figure 20 shows implementation of a single TLC591x-Q1 device.
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.
DESIGN PARAMETER | EXAMPLE VALUE |
---|---|
Number of LED strings | 8 |
Number of LEDs per string | 3 |
LED Current (mA) | 20 |
Forward voltage of each LED (V) | 3.5 |
Junction-to-ambient thermal resistance (°C/W) | 86.9 |
Ambient temperature of application (°C) | 115 |
VDD (V) | 5 |
IDD (mA) | 10 |
Max operating junction temperature (°C) | 150 |
Use the following equations to determine the design parameters.
where
where
where
where
VO must not be too high as this causes excess power dissipation inside the current sink. However, VO also must not be too low as this does not allow the full LED current Figure 21.
With VLED = 12 V:
Using PD_CS, calculate:
Using PD_TOT, calculate:
This design example demonstrates 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, add each result together to calculate the total power dissipation in the current sinks.
The TLC5917/TLC5917-Q1 Constant-Current LED Sink Drivers is designed to work alone or cascaded. Figure 22 shows a cascaded driver implementation.