How to prevent the LED driver output from shorting to ground

In applications such as automotive LED lighting, because the driver is often away from the LED, it is necessary to increase the short-circuit protection. In this article, JOHN RICE describes how to prevent the LED driver output from shorting to ground.
Non-synchronous, boost, and 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 set of series/parallel LED strings. This inductive switching topology generates the compliance voltage necessary to achieve LED current regulation and is commonly used in LCD backlight applications. For example, in LED matrix applications such as automotive interior and exterior lighting away from the driver, catastrophic consequences can occur if there is a risk of shorting the output to ground. Limiting the current and running the protection circuit as an electronic circuit breaker 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 saturates the boost inductor, which causes a current spike sufficient to damage the boost diode. Worse, this short circuit can also interfere with all devices connected to the input, including pulse width modulation (PWM) controllers. Obviously, some type of circuit protection is needed to power remote LEDs when using this topology. Next, consider designing a versatile, low-cost circuit that can be optimized to protect the boost converter and prevent short-circuit loads at the input. In addition, we will verify the required response through an analog circuit.

Current limiter and electronic circuit breaker
The 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 its typical configuration. This particular device integrates an open-drain comparator; this comparator can be set to toggle, latch, and reset on pre-set line currents.

The output of this comparator can be used to control an external MOSFET switch that can interrupt the load short circuit within a few milliseconds. In addition to interrupting the input current when a fault condition occurs on the output, the analog output can also solve the so-called "negative input impedance" problem of the switching regulator, preventing the input current from increasing with decreasing input voltage.
The input is clamped 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 as the input voltage drops below a preset level, limiting the input current.

Circuit operation
Figure 4 shows the circuit implementation of a boost converter LED driver with output short-circuit protection. The LEDs produced by Osram Opto Semiconductors Ostar in the circuit are a device for automotive headlamp applications, actually a single piece, LED on an insulated metal substrate. This device has a surge current rated at 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 fully adjusts the output voltage to regulate the LED current. The LED current is set by the sense resistor (RSNS) and its value is proportional to the internal bandgap reference of the PWM converter (RSNS = VREF/ILED). Using a boost converter with a low reference makes it easier to achieve higher converter efficiency and reduce component thermal stress.

Although the lifetime can be as long as 50,000 hours or more, LEDs are very sensitive to temperature and electrical overstress, and their dynamic impedance characteristics often pose challenges for the selection of switching regulator components and the design of control loops. These selections and design challenges are described in this operating manual. In this way, the circuit simulation shown in Figure 4 was developed to analyze the complexity of the LED driver/protection circuit and predict the operation of the circuit under various operating conditions.
The PWM controller selected for this analysis has a feedback reference voltage of 0.26V. Therefore, when the LED current is 1A, the power dissipation of the LED sensing resistor is only 0.26W. Since the CSM has a gain of 50, a much smaller sense resistor is needed to sense the output current. When the current flowing through the CSM shunt resistor exceeds the limit set by the CSM sense resistor, the CSM gain and the comparator threshold (R, R), the PMOS turn-on transistor interrupts the load current - thereby acting as an electronic circuit breaker .
The latched output can be reset by switching the RESET pin low. However, for the purposes of this article, RESET has been disabled to verify response speed. The 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. These factors together affect the output impedance of the converter. In order to accurately evaluate the mode of operation, we have run the simulation with a maximum time step of 50 ns and a DC relative tolerance set to 0.001%. This analysis runs in TINA-TI, a free Berkeley SPICE 3f5 compatible emulator. The 5ms simulation of the boost converter with a working frequency of 300kHz can be started to a steady state in just 30 seconds.