Practice Safe/DC converter

Short-circuit protection is an obvious requirement for a power supply, especially when it 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 could be inadequate. figure 1shows a step-down DC/converter for both VI Deocameras installed on a remotely controlled vehicle. Because the vehicle operates in rather harsh environments, the cables can frequently short-circuit during the system's INS Tallation and normal operation. The cameras require 12V and consume approximately mA each. The converter is based in national Semiconductor's LM2675 chip, which by itself includes good protection against overloads . However, in the case of a shorted output, the catch diode, D_{1, must withstand the maximum from the IC. The according to the data sheet, can is as high as 2.2A, calling for an oversized diode. Also, waiting for the thermal protection to kick in assumes, is 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 P Roblem. Ideally, someone should immediately report the faults-an operator.}

The system In figure 1 uses a Microchip technology pic16f84µc, which receives its power from a separate DC sour Ce. As usual, the I/O pins in theµc is at a premium, because of the multitude of the 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 your first apply power, Theµc starts up. In this condition, any of its I/O lines is high-impedance inputs, so the LM2675 cannot start; Resistor R_{1 ties its   pin low. After the initialization routine, when it was time to turn on the camera, Theµc makes its pin PB0 pin (or any pin that had A totem-pole driver) an output, and sends a high level to that output. The/DC converter starts up. After a short delay, theµc again makes it PB0 pin an input, but the power supply keeps itself on because its output volt Age connects to control input via R2 and d2. This condition prevails and the load is normal. When the output is short-circuits, the bias voltage disappears, and the chip shuts itself down. The level is at PB0 goes low, notifying theµc of this condition. (PB0 is especially useful in this situation, because it can generate an interrupt request.) Theµc 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 Theµc. This pulse should is long enough-usually, approximately msec-to allow the normal load (with its own input capacitors an D Power Converters) to start. The LM2675 with a catch diode in figure 1withstands short circuits for several seconds without overheating or any oth Er problem, so the short-circuit mode is perfectly safe. An added benefit is so theµc can at any time shut down the DC-converter, making it possible to save battery power an D Reduce heat dissipation. For shutdown, Theµc again makes PB0 an output and sends a zero to the I/O pin. The powerful driver in the pic16f84 easily overcomes the bias from R_{2, d2, and R1 and shuts down The LM2675. The divider r2, d2, r1provides A voltage-5v in this case-close to Theµc ' s vdd when the output of voltage is normal. Slight voltage variations cause no harm to theµc, 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 of the divider low enough to don't create a significant voltage drop in R1 by The bias from the LM2675 ' S  pin (37µa maximum). With the values shown, the largest bias current creates a drop this does not exceed 20% of the lowest possible threshold ( 0.8V) for The  pin. (DI #2562)}

Practice Safe/DC converter