TLC59108 Datasheet, PDF (17/31 Pages) Texas Instruments – 8-BIT Fm+ I2C-BUS CONSTANT-CURRENT LED SINK DRIVER

TLC59108
www.ti.com .................................................................................................................................................................................................. SLDS156 – MARCH 2009
ADDRESS
11h
Table 11. IREF – Output Gain Control Register (Address 12h) Bit Description
REGISTER
BIT
SYMBOL ACCESS⁽¹⁾
VALUE DESCRIPTION
7
CM
R/W
1 ⁽²⁾
High/low current multiplier
ALLCALLADR
6
HC
R/W
1 ⁽²⁾
Subcurrent
5:0
CC[5:0]
R/W
11 1111⁽²⁾ Current multiplier
(1) R = read, W = write
(2) Default value
I_REF determines the voltage gain (VG), which affects the voltage at the R_EXT terminal and indirectly the reference
current (I_REF) flowing through the external resistor at terminal R_EXT. Bit 0 is the Current Multiplier (CM) bit, which
determines the ratio I_OUT,target/I_ref. Each combination of VG and CM sets a 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 to 127/256, linearly divided into 64 steps
High voltage sub-band (HC = 1): VG = 1/2 to 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_REXT = 1.26 V × VG
I_ref = V_REXT/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
• I_REF Code {CM, HC, CC[0:5]} = {1,1,111111}
VG = 127/128 = 0.992 and CG = VG × 3⁰ = VG = 0.992
• I_REF Code {CM, HC, CC[0:5]} = {1,1,000000}
VG = (1 + 1) × (1 + 0/64)/4 = 1/2 = 0.5, and CG = 0.5
• I_REF Code {CM, HC, CC[0:5]} = {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 9.
Copyright © 2009, Texas Instruments Incorporated
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