Practice safe dc/dc converter

Short-circuit protection is an obvious requirement for a power supply, especially when its load connects to a cable that's subject to damage. Many modern power-converter ICs include some means of protection, such as thermal shutdown, against the overload condition, but, in some cases, the built-in protection may be inadequate. Figure 1shows a step-down dc/dc converter for two videocameras installed on a remotely controlled vehicle. Because the vehicle operates in rather harsh environments, the cables can frequently short-circuit during the system's installation and normal operation. The cameras require 12V and consume approximately 250 mA each. The converter is based on National Semiconductor's LM2675 chip, which by itself includes good protection against overloads. However, in the case of a shorted output, the catch diode, D₁, must withstand the maximum current from the IC. This current, according to the data sheet, can be as high as 2.2A, calling for an oversized diode. Also, waiting for the thermal protection to kick in assumes that you are willing to allow the device's temperature to rise significantly, heating adjacent components. This situation is undesirable, given the long periods of nonsupervised operation, during which someone should notice the problem. Ideally, someone should immediately report the faults to an operator.

The system in Figure 1 uses a Microchip Technology PIC16F84 µC, which receives its power from a separate dc source. As usual, the I/O pins in the µC are at a premium, because of the multitude of other tasks the controller performs. However, you can obtain reliable short-circuit protection and on/off control of the power supply using just one I/O pin. When you first apply power, the µC starts up. In this condition, all of its I/O lines are high-impedance inputs, so the LM2675 cannot start; resistor R₁ ties its   pin low. After the initialization routine, when it is time to turn on the camera, the µC makes its pin PB0 pin (or any pin that has a totem-pole driver) an output and sends a high level to that output. The dc/dc converter starts up. After a short delay, the µC again makes its PB0 pin an input, but the power supply keeps itself on because its output voltage connects to control input via R₂ and D₂. This condition prevails while the load is normal. When the output short-circuits, the bias voltage disappears, and the chip shuts itself down. The level at PB0 goes low, notifying the µC of this condition. (PB0 is especially useful in this situation, because it can generate an interrupt request.) The µC can then alert the user of the failure, try to restart the converter after a delay in a "hiccup" mode, or both.

The duration of the worst-case short-circuit condition with this scheme is a function of the length of the start-up pulse from the µC. This pulse should be long enough—usually, approximately 10 msec—to allow the normal load (with its own input capacitors and power converters) to start. The LM2675 with a catch diode in Figure 1withstands short circuits for several seconds without overheating or any other problem, so the short-circuit mode is perfectly safe. An added benefit is that the µC can at any time shut down the dc/dc converter, making it possible to save battery power and reduce heat dissipation. For shutdown, the µC again makes PB0 an output but sends a zero to the I/O pin. The powerful driver in the PIC16F84 easily overcomes the bias from R₂, D₂, and R₁ and shuts down the LM2675. The divider R₂, D₂, R₁provides a voltage—5V in this case—close to the µC's V_DD when the output voltage is normal. Slight voltage variations cause no harm to the µC, thanks to the controller's input-diode protection and the fact that R₂ limits the current to the PB0 pin to a safe level. However, you should keep the values in the divider low enough to not create a significant voltage drop in R₁ by the bias current from the LM2675's  pin (37 µA maximum). With the values shown, the largest bias current creates a drop that does not exceed 20% of the lowest possible threshold (0.8V) for the  pin. (DI #2562)