10.1.1 Constant Current

In LED display applications, TLC591x provides nearly no current variations from channel to channel and from IC to IC. While 5 mA ≤ I_OUT ≤ 100 mA, the maximum current skew between channels is less than ±3% and between ICs is less than ±6%.

10.1.2 Adjusting Output Current

TLC591x 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 the below formulas to calculate the target output current I_OUT,target in the saturation region. In the equations,

R_ext is the resistance of the external resistor connected between the R-EXT terminal and ground and V_R-EXT is the voltage of R-EXT, which is controlled by the programmable voltage gain (VG). VG 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 19.

Figure 19. Default Relationship Curve Between I_OUT,target and R_ext After Power Up

10.1.3 Cascading Implementation of TLC591x Device

Figure 20. Cascading Implementation of TLC591x Device

10.1.4 8-Bit Configuration Code and Current Gain

Bit definition of the Configuration Code in the Configuration Latch is shown in Table 5.

Bit 7 is first sent into TLC591x 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).

• VG: the relationship between {HC,CC[0:5]} and the voltage gain is calculated as shown in Equation 6 and Equation 7:
Equation 6. VG = (1 + HC) × (1 + D/64) / 4
Equation 7. 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 = 3 mA to 40 mA

• CG: The total Current Gain is defined as the following.
Equation 8. V_R-EXT = 1.26 V × VG
Equation 9. I_ref = V_R-EXT/R_ext, if the external resistor, R_ext, is connected to ground.
Equation 10. 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
Equation 11. 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 21.

Figure 21. Current Gain vs Configuration Code

10.2.1 Design Requirements

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.

10.2.2 Detailed Design Procedure

V_O must not be too high as this causes excess power dissipation inside the current sink. However, V_O also must not be too low as this does not allow the full LED current (Figure 4). With V_LED = 12 V:

Using P_{D_CS}, calculate:

Using P_{D_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.

10.2.3 Application Curve

Figure 23. Output Current vs Output Voltage