Instrument clusters need to be backlit such that the gauges and indicators can be read in low ambient light conditions. Dimming capability is paramount and should provide a 100: 1 dimming ratio. In addition, to backlighting several telltale lights are required for driver diagnostics such as air bag check out, powertrain diagnostics, fluid levels, etc. Typically, as many as 30 LEDs could be used in a cluster application.

A multi-element extension of Figure 1 is shown driving six strings of LEDs with a low side set of transistors to provide pulse-width modulated (PWM) dimming. The resistor calculated previously is used to set the LED's forward current and, therefore, at a given supply voltage, the total current is fixed in the LED strings. Again, as the supply voltage varies from 9 V to 18 V, the current changes in the LED. For instance, at 9 V supply, sufficient light needs to be generated to safely read the instrument cluster. At 18 V, thermal issues on the printed circuit board (PCB) may arise, and is where worst-case conditions need to be assessed.

Depending on the color of the LED used for the backlighting, the forward drop of 2.4 V per LED in the red, orange, green, and amber colors to as high as 3.8 V for blue and white LEDs. It should be appreciated that a third LED could be placed in series if the former colors are used. For this example, we will assume a typical white light for cluster backlight, and will limit our series string to two LEDs. A reverse polarity diode is required for the misapplication of the vehicle's battery terminals during maintenance, and may be as high as -15 V. LEDs have typically max reverse rating of -5 V and, therefore, need a blocking diode to protect the LEDs during this reverse polarity condition.

A method for dimming the LED series string is provided on the low side of the circuit. The use of bias resistor transistors or digital transistors such as the MMUN2211 family, help provide a simple interface from the host controller within the cluster. The transistor has integrated Rb and Rbe resistors such that a logic level signal can adequately drive the base emitter circuit. Using such a transistor, and controlling the PWM duty cycle at a single frequency, provides a wide dimming range for the cluster.

A need exists for a low-cost, solid-state constant current regulator for LED drive as well as other applications. Hence, factors favoring a solid-state current regulator include:

• low cost;
• maintained current regulation over wide forward voltage;
• low dropout behavior for low forward voltage operation;
• power-limiting behavior at extreme high forward voltage;
• behaves like an ideal two-terminal current source for parallel application;
• provides high-frequency PWM control for LED dimming;
• resists susceptibility to direct-injected RF energy; and
• maintains a high level of ESD immunity.

The circuit in Figure 3 is identical to Figure 1, except the 190 V resistor is replaced with depletion mode N channel JFET. Simply shorting the gate to the source and biasing the drain to source by greater than 1 V causes a current to flow in the series LED circuit. The unique part of using a JFET in place of a resistor to set the LED's forward current is that as the drain source voltage increases (battery voltage varying), the current stays relatively constant. The JFET's constant current behavior is depicted in Figure 4, and can be best understood by examining its current-voltage characteristic over the supply voltage's normal operating range.