Circuits restraint soft


L'Use of a single tactile button to toggle a circuit ON and OFF requires a circuit containing a logical bistable latch. A latch that can be implemented using a flop flop D or by using a positive feedback around the two amplifiers or inverting logic gates. Eg, a base block ON-OFF switch can be implemented using a double NAND gate, using this circuit:

Fig. 1 Closure to NAND gates with momentary press of the button

A short press of the button activates or deactivates the of this closure device of the NAND output state. You can use any CMOS NAND gate, including Schmitt trigger gate. In fact, any reversal device can be used with slight modifications to the circuit: an inverter, open collector inverter, porte NAND o NOR, transistor, FET or MOSFET.

When power is applied for the first time, the capacitor at the entrance of the first gate ensures that the output of the latch, Switched Power, is initially low. subsequently, pressing the button makes the switch in the top and bottom output.

To turn the latch in a power switch, the NAND output can be used to control a high-side switch P-MOSFET, as shown here:

Fig. 2 NAND to control a MOSFET switch

A drawback of this circuit is that it consumes a small current even in the OFF state. This does not happen with the following circuit that uses two MOSFETs contained in DMC2308 or its equivalent

Fig. 3 Implementation of the previous circuit MOSFET with NAND

This circuit uses only a single active device, double surface mount MOSFET, but on and off currents up to 4,5 Amp.Il load can be virtually any electrical device, from a high-power LED lights to power the vast majority of circuits.

I do not provide a printed possible for its obvious simplicity and because sicuamente it will be a part of a much more complex circuit.

Managing capacitive loads

Place the MOSFET high side switch within the feedback path for the latch introduces a potential malfunction if the pilot output capacitive loads to slow decay, as it might occur in the power bus switches. If the switch applies power to a power supply with large input capacitors and the load does not absorb enough current to allow these capacitors to quickly discharge, the pressure of the transitional button may not turn off the switch reliably. instead, after that the high side switch is turned off, the voltage supported at its output, supported by large capacitors, It retains the lower MOSFET lit, which in turn activates the switch on the high side after the release of the button.One way to prevent such failure modes is to place a Schottky diode in series with the load, come shown in this circuit diagram:

Fig. 4

Pressure ON - Long press for OFF

For the control of intelligent products containing microcontrollori, often you want to make sure that the power supply is not inadvertently removed. Consequently, you want a power switch that can be turned on by pressing a button briefly, but it requires a longer pressure and deliberate the button to turn it off. This prevents accidentally delete the power if the tactile switch is unintentionally touched.

The following shows a NAND gate latch that implements the desired action:

Fig. 5 short press ON lunga-OFF

When power is applied for the first time, the output is kept low for 3 seconds, during which the button has no effect. subsequently, a momentary pressure blocks the exit at the top and a prolonged pressure exceeding 3 seconds relock the lower output. You can use the output to turn on and turn off a variety of devices, including MOSFET switches hight or low-side. The other NAND gate output can be used if it is necessary to invert a logic level.

The following diagram shows a very useful circuit for controlling a high side switch (a P-MOSFET) similarly. The circuit uses only an active component, a dual MOSFET, that provides the logic block and acts as a switch / relay power high side. A momentary press of the button causes the circuit blocks is, providing power to the load, and it is shut off again by a longer press of the button. It can provide power from 5 a 18 It was the fino 3 ampere.

Fig. 6

In the initial state OFF, this circuit does not consume energy – the current absorbed only are the reverse leakage currents of the diode and MOSFET. Even in the ON state, the circuit absorbs very little bias current to maintain its locked state – only the series transistor dissipates a certain power while the current flows to the load. And that power is minimal; P-MOSFET has an ON resistance of only 74 milli-Q.

When in the OFF state, by momentarily pressing the tactile button turns on power switched ON and crashes, providing energy output. While it is switched on, by holding down the button for more than 3 seconds, the power switch is again turned off and remains off until the next momentary press of the button. In order to turn off reliably OFF, the load should absorb sufficient current so that any downstream capacitors to discharge within a few seconds, allowing the decay of the gate drive N-MOSFET.

The RDS-ON of the P-MOSFET used as a high-side switch is only 0,074 ohm, so its container surface mount not very hot, even at current 3 amp. The input voltage is limited by the maximum gate-source voltage, VGS, MOSFETs, which it is 20V, therefore recommend a maximum input voltage of 18 V, which allows a safety margin of 2 V.

The 10uF capacitor should be a variety with low loss – a ceramic capacitor works well.

The operation of the circuit is as follows: When power is applied for the first time, the gate voltage of the P-FET source remains zero, maintaining the high side switch to OFF, or the gate is kept low when the source increases, turning on the MOSFET, depending on the Auto-ON jumper . When the MOSFET is turned off, a momentary touch of the ON button / OFF lowers the gate to tens of milliseconds, turning completely on the MOSFET high side. The increase of the output voltage is transmitted to the gate of N-MOSFET through the capacitor and the resistors 10 μF, lighting it. With the low side FET ON, the gate of the high side FET is lowered, keeping it lit. Consequently, power locks after you release the button. Only a few volts of VGS are enough to keep the MOSFET is completely turned on.

subsequently, a long touch of button turns the circuit. Pressing and holding the button lowers the gate of the low side MOSFET after a few seconds of delay determined by the discharge of the capacitor 10 uF through the resistor 270 kΩ. The low side MOSFET turns off, allowing the gate of the MOSFET high side to reach the source voltage, switching off also. As long as the load voltage falls within about a second (or, It is not supported by any really big output capacitor), feeding hangs on OFF until the next momentary press of the ON button / OFF.

The feedback capacitor enables operation even with moderate capacitive loads (in slow decay of the output voltage). The user holds down the button until you notice that the power is off. At that moment the load is decaying and -dV / dt of the output voltage is transmitted from the feedback capacitor to the gate of N-MOSFET, holding it off even when the output voltage is still above the ignition voltage of the MOSFET.

You can check the startup behavior of the circuit in such a way that turn on automatically when first switched on or remains switched off. In the following circuit, if the jumper Auto-ON connect the capacitor to the ground 0,1μF (or if it is permanently connected in that way), the circuit turns on if the power is suddenly applied to its input. In the other position (or if the 0.1μF capacitor is connected permanently to the power input bar), the circuit remains in the OFF state when the power is applied.

I am hoping to be helpful I leave wishing you my sincerest HAPPY HOLIDAYS


2 replies
  1. Amilcare
    Amilcare says:

    Essentially it will be used in SMD version with dual mosfet, The circuit may quietly behind the same button, obviously on the opposite face of the pcb. The use of logic gates served to simplify the discussion without having to use special formulas. If the logic circuit then the mosfet is it comprises a consequence easily assimilated. Then everyone is free to use the section you want.

  2. Picmicro675
    Picmicro675 says:

    Very interesting information. I was intrigued by the idea of ​​a touch sensor, possibly bistable, for a new program. In the case of NAND I I would have opted for a flip-flop, and use all 4 door.

    Happy holidays to all forum participants.


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