TLC5916_10 Datasheet, PDF (22/30 Pages) Texas Instruments – 8-BIT CONSTANT-CURRENT LED SINK DRIVERS

TLC5916, TLC5917
8-BIT CONSTANT-CURRENT LED SINK DRIVERS
SLVS695A – JUNE 2007 – REVISED MARCH 2008
8-Bit Configuration Code and Current Gain
Bit definition of the Configuration Code in the Configuration Latch is shown in Table 4.
Meaning
Default
Bit 0
CM
1
Table 4. Bit Definition of 8-Bit Configuration Code
Bit 1
HC
1
Bit 2
CC0
1
Bit 3
CC1
1
Bit 4
CC2
1
Bit 5
CC3
1
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Bit 6
CC4
1
Bit 7
CC5
1
Bit 7 is first sent into TLC5916/TLC5917 via 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).
• VG: the relationship between {HC,CC[0:5]} and the voltage gain is calculated as shown below:
VG = (1 + HC) × (1 + D/64) / 4
D = CC0 × 2⁵ + CC1 × 2⁴ + CC2 × 2³ + CC3 × 2² + CC4 × 2¹ + CC5 × 2⁰
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 sub-bands:
Low voltage sub-band (HC = 0): VG = 1/4 ~ 127/256, linearly divided into 64 steps
High voltage sub-band (HC = 1): VG = 1/2 ~ 127/128, linearly divided into 64 steps
• CM: In addition to determining the ratio I_OUT,target/I_ref, CM limits the output current range.
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
• CG: The total Current Gain is defined as the following.
V_R-EXT = 1.26 V × VG
I_ref = V_R-EXT/R_ext, if the external resistor, R_ext, is connected to ground.
I_OUT,target = I_ref × 15 × 3^{CM – 1} = 1.26 V/R_ext × VG × 15 × 3^{CM – 1} = (1.26 V/R_ext × 15) × CG
CG = VG × 3^{CM – 1}
Therefore, CG = (1/12) to (127/128), and it is divided into 256 steps. If CG = 127/128 = 0.992, the
I_OUT,target-R_ext.
Examples
• Configuration Code {CM, HC, CC[0:5]} = {1,1,111111}
VG = 127/128 = 0.992 and CG = VG × 3⁰ = VG = 0.992
• Configuration Code = {1,1,000000}
VG = (1 + 1) × (1 + 0/64)/4 = 1/2 = 0.5, and CG = 0.5
• Configuration Code = {0,0,000000}
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 19.
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