SLVS677C July 2008 – October 2015 TLC5926 , TLC5927
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, TLC592x provides nearly no current variations from channel to channel and from IC to IC. While I_{OUT} ≤ 50 mA, the maximum current skew between channels is less than ±6% and between ICs is less than ±6%.
TLC592x scales up the reference current, I_{ref}, set by the external resistor R_{ext} to sink a current, I_{out}, at each output port. Users can follow Equation 1, Equation 2, and Equation 3 to calculate the target output current I_{OUT,target} in the saturation region:
Where R_{ext} is the resistance of the external resistor connected to the R-EXT terminal, and V_{R-EXT} is the voltage of R-EXT, which is controlled by the programmable voltage gain (VG), which is defined by the Configuration Code. The Current Multiplier (CM) determines that the ratio I_{OUT,target}/I_{ref} 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 I_{OUT,target}/I_{ref} = 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 I_{OUT,target} and R_{ext} is shown in Figure 15.
Table 5 lists bit definition of the Configuration Code in the Configuration Latch.
Bit 0 | Bit 1 | Bit 2 | Bit 3 | Bit 4 | Bit 5 | Bit 6 | Bit 7 | Bit 8–15 | |
---|---|---|---|---|---|---|---|---|---|
Meaning | CM | HC | CC0 | CC1 | CC2 | CC3 | CC4 | CC5 | Don't care |
Default | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | X |
Bit 7 is first sent into TLC592x through SDI. Bits 1 to 7 \{HC, CC[0:5]\} determine the voltage gain (VG) that affects the voltage at R-EXT and indirectly affects the reference current, I_{ref}, flowing through the external resistor at R-EXT. Bit 0 is the Current Multiplier (CM) that determines the ratio I_{OUT,target}/I_{ref}. Each combination of VG and CM gives a specific Current Gain (CG).
Where HC is 1 or 0, and D is the binary value of CC[0:5]. So, the VG could be regarded as a floating-point number with 1-bit exponent HC and 6-bit mantissa CC[0:5]. \{HC,CC[0:5]\} divides the programmable voltage gain VG into 128 steps and two subbands:
Low voltage subband (HC = 0): VG = 1/4 ~ 127/256, linearly divided into 64 steps
High voltage subband (HC = 1): VG = 1/2 ~ 127/128, linearly divided into 64 steps
High Current Multiplier (CM = 1): I_{OUT,target}/I_{ref} = 15, suitable for output current range I_{OUT} = 10 mA to 120 mA.
Low Current Multiplier (CM = 0): I_{OUT,target}/I_{ref} = 5, suitable for output current range I_{OUT} = 5 mA to 40 mA
Therefore, CG = (1/12) to (127/128) divided into 256 steps.
Examples
VG = 127/128 = 0.992 and CG = VG × 3^{0} = VG = 0.992
VG = (1 + 1) × (1 + 0/64)/4 = 1/2 = 0.5, and CG = 0.5
VG = (1 + 0) × (1 + 0/64)/4 = 1/4, and CG = (1/4) × 3^{–1} = 1/12
After power on, the default value of the Configuration Code \{CM, HC, CC[0:5]\} is \{1,1,111111\}. Therefore, VG = CG = 0.992. The relationship between the Configuration Code and the Current Gain is shown in Figure 16.
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 PARAMETERS | EXAMPLE VALUES |
---|---|
No. of LED strings | 16 |
No. of LEDs per string | 3 |
LED current (mA) | 20 |
Forward voltage of each LED (V) | 3.5 |
Junction-to-ambient thermal resistance (°C/W) | 40 |
Ambient temperature of application (°C) | 115 |
V_{DD} (V) | 5 |
I_{DD} (mA) | 17 |
Max operating junction temperature (°C) | 150 |
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V_{O} should not be too high as this will cause excess power dissipation inside the current sink. However, V_{O} should also not be loo low as this will not allow the full LED current (refer to the output voltage vs. output current graph). With V_{LED} = 12 V:
Using P_{D_CS}, calculate:
Using P_{D_TOT}, calculate:
This design example has demonstrated 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 (n_{LED}, I_{O}, V_{F}, V_{LED}). If the parameters are unique for each channel, then the power dissipation must be calculated for each current sink separately. Then, each result must be added together to calculate the total power dissipation in the current sinks.