Protect LED driver in backlit displays

(Editor's note: : There are several related “Articles of Interest”, below the “About the Author” section at the end.)

LEDs often serve as the light source in a backlighted display, and they usually operate from a low battery voltage such as that produced by two NiCd cells or one lithium-ion cell. An IC from Maxim (MAX1698/A) can simplify these applications by boosting the battery voltage to a level suitable for LEDs. The chip also regulates LED current, and includes brightness-control circuitry for dimming the LEDs. The LED array and IC should always remain connected (Figure 1 ).

Figure 1: This schematic illustrates the application of a typical LED-backlight driver.

(Click on schematic image to enlarge)

If you disconnect the LED array from the IC, the loss of LED current in RFB allows the voltage at FB (pin 6) to drop below the internal current-controller threshold, causing the device to begin increasing its output voltage. Unfortunately, the MAX1698 (like many similar devices) cannot sense the disconnected-LED condition, so its output voltage increases to a level that can destroy the external MOSFET and Schottky diode. This problem is present for any boost converter; not just LED drivers.

The simplest solution is a Zener diode connected across the LEDs (Figure 2 ).

Figure 2: The simplest protection for the Figure 1 circuit adds just a Zener diode.

(Click on schematic image to enlarge)

A 16 V zener works fine in this case (the four white LEDs drop about 12 V), but it must be capable of dissipating power. When the LEDs are drawing 100 mA or more and someone disconnects them, the Zener must dissipate about 1/6 W. A possible alternative to this circuit is shown in Figure 3 .

Figure 3: Adding a Zener diode and transistor to the Figure 1 circuit provides low-power protection for the MOSFET and Schottky diode.

(Click on schematic image to enlarge)

It requires the addition of two resistors and one transistor, but the Figure 3 circuit doesn't dissipate extra power when the LEDs are disconnected. It also saves space–the zener can be a 0.5 W device, and the resistor and bipolar junction transistor (BJT) can be standard low-power devices available in small packages like the SOT23-3, or smaller. The circuit senses output voltage at the MOSFET drain, and deactivates the driver (MAX1698) by controlling its Shutdown input. You can choose a Zener voltage that ensures this voltage is within the MOSFET's operating characteristics.

In other words, the circuit doesn't “work” except when a user removes the LED array. In that event, the output voltage starts to rise, and when it reaches the Zener voltage, the circuit trips and shuts down the IC. As in shutdown mode, the inductor begins to discharge when the driver turns off the external MOSFET, which allows the output voltage to drop below the Zener voltage and bring the driver out of shutdown. The driver re-starts, and if the LED array remains unconnected, the output voltage increases until it exceeds the Zener voltage, and triggers the protection again.

Because the output voltage regulates around the Zener voltage, this circuit does not generate a damaging current spike when the LED array is reconnected. To save battery energy, it also permits external control of the shutdown mode (using a microcontroller, for instance, as shown in Figure 3), to switch off the backlight array.

Another alternative is the circuit of Figure 4 , which requires an additional comparator and three resistors.

Figure 4: Better yet, this tiny comparator protects the Figure 1 circuit, dissipates little power, and requires little space on the pc board.

(Click on schematic image to enlarge)

This approach also uses small, low-cost components and dissipates negligible power. It senses output voltage at the Schottky-diode cathode, and limits circuit operation to a voltage set by the resistor divider and the driver's VREF output (1.25 V typical).

This protection circuit remains inactive until the LED array is removed, and (again) its operating voltages remain well within limits for the chosen MOSFET. The comparator should have an open-drain output (MAX9060/1 or MAX9028) to permit external control of the shutdown mode by a microcontroller as before, to switch off the backlight array when needed.

The circuit also consumes less power, according to values selected for the resistor divider. (Its quiescent current is a few tens of microamps.) Last, but not least, this circuit is smaller than the other two because the comparator comes in a tiny SOT23-5 package (MAX9060/1) or 1×1.52mm UCSP package (MAX9028). All three circuits protect the external MOSFET and diode in an LED-backlight application, when the LED array is disconnected.

About the author
Massimo Caprioli is a Senior Field Application Engineer with Maxim Integrated Products, Italy. He has a diploma in industrial electronics from ITIS Vimercate (Mi), Italy. Before joining Maxim, he spent 16 years as an electronic designer in several fields: aerospace with LABEN, designing ATE to test space electronic equipment; at FIAR, designing FEP; Agusta, where he designed electronics equipment for helicopters; and telecom, together with SAFNAT, designing PABX and key telephone systems. After these R&D experiences, over a period of more than 13 years, he moved on other activities as filed application engineer (FAE) at TEMIC Semiconductors as well as semiconductor-distribution companies such as Newtek/Memec, Future Electronics, and Rutronik.

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