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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Devices on the MAX869L chip. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer The MAX869L microcircuit is an electronic key based on a p-channel field-effect transistor with a current limiter designed to switch low-voltage power circuits of electronic components, while simultaneously protecting the source from overload. In addition to being used for its intended purpose, it is possible to assemble some other devices useful in amateur radio practice. The microcircuit, which will be discussed, is available in a packageless version (MAX869LC / D) and in a small-sized 16-pin package with dimensions of approximately 5x6,5 mm along with pins (MAX869LEEE). Of course, only the second option is suitable for use in amateur radio designs. It is allowed to supply a positive voltage of 869 ... 1,4 V relative to the common wire (pin 5,12,13,16) to the input of the key available in the MAX8L chip (parallel connected pins 2,7, 5,5). The load is connected to the output of the key - terminals 2, 3, 6, 11, 14, 15, also connecting them in parallel. It is not recommended to partially use the input and output pins, leaving some of them free. This can lead to burnout of thin connecting wires inside the microcircuit. The resistance of the key in the open state does not exceed 0,045 Ohm. The built-in limiter starts to operate when the flowing current reaches the value of Ilimit. The limitation threshold in the range from 0,4 to 2,4 A is set using a resistor with a nominal value of R-8lorp connected between pins 9 and 1,2 of the microcircuit (current - A, resistance - kOhm). Formula error - no more than ±20%. Thanks to the limiter, even if the voltage between the output and the common wire is less than 1,6 V, the current through the key does not exceed 1,4 Ilimit. To open the key, a high logic level signal should be applied to pin 7 of the microcircuit. There is an open drain output (pin 10). A low logic level here indicates that the current limiter of the key has worked or the temperature of the microcircuit crystal has exceeded 135 ° C. In the latter case, the key opens automatically and remains in this state until the crystal cools down to 125 °C. Figure 1 shows the circuit diagram of the electronic fuse on the MAX869L chip. In addition to it, the device has a trigger on transistors VT1 and VT2, which is set by pressing the SB1 button to its initial state: VT1 - closed, VT2 - open. As long as the load current does not exceed the threshold value set using the tuned resistor R7, the transistor inside the microcircuit (its drain is connected to pin 10) is closed and does not shunt the base-emitter section of transistor VT2. The logic level at pin 7 of DA1 is low, the supply voltage is supplied to the load through a closed key. The glowing HL2 LED indicates normal operation, and the HL1 LED is off.
As soon as the current flowing through the key exceeds Iorp, the base of the transistor VT2 through the opened internal transistor of the microcircuit will be connected to a common wire, as a result, the transistor VT2 will close, the HL2 LED will turn off. At the same time, the transistor VT1 will open and, signaling an accident, the HL1 LED will light up. The high logic level at the collector of the transistor VT2 and at pin 7 of DA1 will remain unchanged even after the overload is eliminated, keeping the key open. The load is turned on again by pressing the SB1 button, which returns the trigger to its original state. It should be noted that if the cause of the accident has not been eliminated, it is impossible to maintain a low logic level at pin 7 of the DA1 microcircuit for a long time, since in this case the microcircuit is in the current limiting mode, dissipating power up to 1,4 IlimitΔU, where ΔU is the voltage difference between the input and key exit. Permissible value of dissipated power - 667 mW. In the device under consideration, the duration of a possible overload is limited by the duration of charging the capacitor C2 through the resistor R5 and the HL2 LED. Resistor R3 serves to discharge the capacitor in the intervals between button presses. The fuse can be assembled on a 19x14 mm printed circuit board made of double-sided foil fiberglass, shown in fig. 2 in 2:1 scale. It is designed for surface mounting of most elements located on both sides of the board. The leads of the parts and the connecting wires inserted into the holes of the board should be soldered to the pads on both sides. Insert short pieces of uninsulated wire into the remaining vias that are not occupied, also soldering them on both sides. Fixed resistors - P1-12, tuned - RVG or POZ, capacitors C1 and C3 - K10-17 or similar imported ones. In the case of using transistors of the KT315 series, MLT resistors and other large parts, the dimensions of the board will have to be increased.
On the MAX869L chip according to the circuit shown in fig. 3, you can assemble a timer that turns off the load some time after the supply voltage is applied. At the initial moment, the capacitor C2 is discharged, at the input 7 of the DA1 microcircuit there is a low logic level, therefore the key is open and the supply voltage is supplied to the load. As soon as the capacitor is charged through the resistor R1, the key will be closed, the load will be de-energized. Tests of the timer layout showed that with a supply voltage of 5,5 V, the shutdown occurs abruptly as soon as the voltage across the capacitor C2 exceeds 2 V. The exposure time at the values \u1b\u2bof the elements R4,5 and CXNUMX indicated in the diagram is approximately XNUMX minutes.
After the timer is triggered, the current consumed by it is 15 ... 17 μA and decreases several times more after the capacitor is fully charged. Discharging the capacitor by pressing the SB1 button, the load is turned on again for a specified time. If you need a turn-on delay, and not turn off the load, it is enough to swap the resistor R1 and capacitor C2 (together with the SB1 button). Resistor R2 of the rating indicated in the diagram limits the load current to 2,2 ... 2,4 A. Another device that can be assembled on the MAX869L chip is a simple but powerful pulse generator. Enough, as shown in Fig. 4, between the control input (pin 7) and the key output, install an integrating circuit R1R3C2. As a result, voltage pulses are generated at the load with a frequency determined by the parameters of this circuit and a duty cycle of approximately 3. It should be noted that the generator does not work without load, since the discharge circuit of capacitor C2 is broken. The total resistance of resistors R1 and R3 must be several times greater than the load resistance.
The load current (pulse) can reach 2 A. The generation frequency F is determined by the formula
(frequency - kHz, resistance - kOhm, capacitance - μF). The maximum frequency is 20 kHz. The duration of the front of the pulses (at a load of 10 ohms) is approximately 10 μs, the decay is 5 μs. If the charging and discharging circuits of the capacitor C2 are made separate, as in Fig. 5, we get a variable duty cycle pulse generator, which can serve as a regulator of the average power delivered to the load, for example, an incandescent lamp. If the load is an electric motor or other device with a significant inductive component of resistance, at the moments of switching (when the current is turned off), self-inductance EMF surges occur on it, which can damage the microcircuit. It is protected by diodes VD3, VD4 shown in fig. 5 dashed lines.
Similar devices can be built on MAX893L (maximum current 1,2 A), MAX890L (1 A), MAX891L, MAX894L (0,5 A), MAX892L, MAX895L (0,25 A) microcircuits, and the MAX894L, MAX895L microcircuits contain two identical keys with independent control and setting of the current protection operation threshold. The cases of these microcircuits are eight-pin with a pin pitch of 1,27 and 0,65 mm. Author: I. Nechaev, Kursk
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