“In applications such as automotive LED lighting, where drivers are often far away from the LEDs, short-circuit protection needs to be added. In this article, JOHN RICE describes how to protect the LED driver output from shorts to ground.

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In applications such as automotive LED lighting, where drivers are often far away from the LEDs, short-circuit protection needs to be added. In this article, JOHN RICE describes how to protect the LED driver output from shorts to ground.

Asynchronous, boost, power conversion topologies are often used in applications such as LED drivers. In these applications, the input voltage (VIN) is not sufficient to forward bias a series/parallel LED string. This inductive switching topology generates the compliance voltage necessary to achieve LED current regulation and is commonly used in LCD backlighting applications. For example, in LED matrix applications such as automotive interior and exterior lighting away from the driver, the risk of an output shorting to ground can have catastrophic consequences. Limiting current and operating protective circuits as Electronic circuit breakers can prevent these catastrophic failures.

As shown in Figure 1, the input of the boost converter is physically connected to its output through a boost Inductor (L1) and a boost diode (D1). Therefore, a short-circuit condition on the output can saturate the boost inductor, causing a current spike large enough to damage the boost diode. To make matters worse, this short-circuit condition also interferes with all devices connected to the input, including the pulse-width modulation (PWM) controller. Obviously, some type of circuit protection is required to power the remote LEDs when using this topology. Next, consider designing a versatile, low-cost circuit that can be optimized to protect the boost converter from short-circuit load conditions at the input. In addition, we will verify the desired response with an analog circuit.

Figure 1. LED driver circuit based on non-isolated boost topology

Current limiters and electronic circuit breakers

A current shunt monitor (CSM) is a high precision, high gain differential current sense amplifier that is often used to monitor input and output currents. Figure 2 shows a typical configuration. This particular device integrates an open-drain comparator; this comparator can be programmed to trip, latch, and reset on a preset line current.
　

Figure 2. A shunt monitor component adds protection

The output of this comparator can be used to control an external MOSFET switch that interrupts a load short circuit within milliseconds. In addition to interrupting the input current in the event of a fault condition on the output, the analog output addresses the so-called “negative input impedance” problem of switching regulators, preventing the input current from increasing as the input voltage decreases.

Clamping of the input is achieved by connecting the input current to the output current in a logical OR configuration. As shown in Figure 3, the purpose is to generate a composite feedback signal that drives the PWM controller. The CSM then disables the output current feedback and forces the LED current to decrease when the input voltage falls below a preset level, limiting the input current.
　

Figure 3. Input Current Limiter Depends on Sensing Input and Output Current

circuit operation

Figure 4 shows a circuit implementation of a boost converter LED driver with output short-circuit protection. The Osram Opto Semiconductors Ostar LED shown in the circuit is a device for automotive headlight applications and is actually a monolithic, LED on an insulating metal substrate. The device has an inrush current rating of 2A (less than 10 μs) and a typical forward voltage of 18V at 1A. The DC/DC boost converter senses the forward LED current on the FEEDBACK pin and adjusts the output voltage sufficiently to regulate the LED current. The LED current is set by a sense resistor (RSNS) whose value is proportional to the PWM converter’s internal bandgap reference (RSNS = VREF/ILED). Using a boost converter with a low reference voltage makes it easier to achieve higher converter efficiency and reduce component thermal stress.

Based on a circuit design of an automotive LED boost driver with protection function
Figure 4. LED Boost Driver Circuit with Shorted Load Fault Protection

Although lifetimes can be as long as 50,000+ hours, LEDs are sensitive to temperature and electrical overstress, and their dynamic impedance characteristics often present challenges in switching regulator component selection and control loop design. These selection and design challenges are explained in this operating manual. Following this approach, the circuit simulation shown in Figure 4 was developed to analyze the complexity of the LED driver/protection circuit and to predict how the circuit would behave under various operating conditions.

The PWM controller chosen for this analysis has a feedback reference voltage of 0.26V. Therefore, the power dissipation of the LED sense resistor is only 0.26W when the LED current is 1A. Since the CSM has a gain of 50, a much smaller sense resistor is required to sense the output current. When the current through the CSM shunt resistor exceeds the limit set by the CSM sense resistor, the CSM gain and comparator thresholds (R, R), the PMOS pass transistor interrupts the load current – thus acting as an electronic circuit breaker .

The latched output can be reset by toggling the RESET pin low. However, for the purpose of this article, RESET has been disabled to check response speed. Response speed and peak current depend on many variables. These variables include component selection, CSM bandwidth, noise filter, output capacitor, FET selection, and output boost inductor. Together these factors affect the output impedance of the converter. To accurately evaluate the operation, we ran the simulation with a maximum time step of 50ns and a DC relative tolerance set to 0.001%. This analysis was run in TINA-TI, a free Berkeley SPICE 3f5 compliant simulator. A 5ms simulation of a boost converter operating at 300kHz starts to steady state in just 30 seconds.