LED Voltage Supply Consideration

Toshiba Constant Current LED Drivers, Part Numbers TB62706BN/BF, TB62705CP/CF/CFN and TB62706CFA.

The LED voltage supply, or more specifically, the resulting Vce (driver output to ground voltage - output on condition) can have an effect on the performance of the Toshiba Constant Current LED Driver devices. Device Vce has an affect on both device power dissipation and current regulation.

Part 1 - Vce impact on power dissipation.

Total power dissipation = ((ILED) (Vce) (# of outputs)) + residual device consumption.

Power dissipation for the Constant Current driver is almost directly proportional to Vce. As Vce rises the power dissipated by the device increases as well. To calculate the anticipated power dissipation of the constant current device multiply the programmed current by the anticipated Vce by the number of outputs on the device. In addition to the power dissipated as a result of LED current there is also a residual power dissipation from the operation of the device. This is about 50 - 100mW and should also be added into the power dissipation formula.

Example 1

1 1 Residual power dissipation = IDD x VDD = 12mA x 5V = 60mW (for this example)

Example 1 represents a typical circuit. In this case we would sum up the total power dissipation as follows:

Total power dissipation = (40mA) (1V) (8 outputs) + 60mW = 380mW

 
Chart 1 (left) and Chart 2 (right)


Chart 3

In this example we are well within the power dissipation limit of the device (1.47W max. for TB62705CP). Figure 1, Charts 1,2 and 3 illustrate the recommended Iout vs Vce relationship limits for part numbers TB62706BN/BF, TB62705CP/CF/CFN and TB62706CFA.

Part 2 - Vce impact on current regulation

It is necessary to design for an adequate Vce level for the Constant Current LED driver to properly regulate the programmed current. In general, for programmed currents below 50mA, a minimum Vce of 0.4V is recommended for proper current regulation. For programmed currents of 50mA or greater a Vce of at least 0.7V should be maintained. Figure 2 illustrates the relationship of Vce to programmed output current. Observation of these constraints ensures operation in the linear region of the current regulated outputs.

Figure 2

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