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Light-emitting diodes (LEDs) are being exploited by many automotive lighting applications. Interior lighting is one area where LEDs are well suited because of their small size and high efficiency. Consequently, with recent advancements in efficiency and manufacturing to lower cost, these light sources have become the technology of choice for automotive interior lighting systems. But, driving these devices for optimal performance, is an art.

Since LEDs require a specific operating current, the method and precision of setting the LED forward current, in the midst of a typical wide ranging, automotive battery, and charging system, requires circuitry beyond the usual series-limiting resistor.

An innovative use of a standard N-channel depletion mode JFET will be shown to have better advantages when compared to a resistor to adjust the LED operating current. In simple terms, a JFET can be considered as a voltage-controlled resistor. By simply adjusting the gate-source voltage, a relatively constant current can flow out from the source contact. The source can then be a current source for the series connected LED(s). As the drain voltage is connected to the unregulated switched battery connection, it can then provide a relatively constant current and, therefore, be more effective than a standard resistor.

The list of potential interior lighting LED applications include:

• instrument cluster backlighting;
• switch cluster backlighting;
• dome lighting;
• convenience lighting; and
• RGB mood lighting.

Each application requires specific attention to light output and optical design, LED circuit topology, driver current requirements, and thermal management. Nearly all automotive LED lighting applications incorporate the circuit as shown in Figure 1. In particular, if the current is below 100 mA (the majority of interior lighting applications are for some type of back-lighting or switch illumination, and currents are typically 30 mA). The resistor value is calculated to take into account the Vfwd across the series connected LED string. If a specific supply voltage such as 13.5 V is used, a specific resistor can be chosen:

Vsupply - Vsw_bat - Vrpp -I_led*R1 - 2 Vfwd = 0 V

Vsw_bat = 0 V

Vsupply = 13.5 V (typical)

Vrpp = 0.8 V

Vfwd = 3.5 V

I_led = 30 mA

R1 = 13.5 - 0.8 - 2*(3.5) = 190 Ω/30

This method for setting the current with a specific resistor is well known and by knowing the LED's worst case Vfwd drop, a specific range of resistor value can be chosen. However, as the supply voltage varies from 9 V to 18 V, the current changes in the LED. With the same 190 Ω resistor and 9 V, rearranging equation for the I_led value yields 6.3 mA. Assuming all the parameters remain constant and the supply is elevated to 18 V yields and I-led value is 53 mA.