Principle of generating circuit diagram of negative voltage
We often need to use negative voltage in electronic circuits. For example, we often need to establish a negative voltage when using operational amplifiers. Let's briefly take the positive 5V voltage to negative 5V voltage as an example to talk about its circuit.
Usually, when negative voltage is needed, special negative voltage generation chips are usually used, but these chips are expensive, such as ICL7600, LT1054, etc. I almost forgot about MC34063. This chip is used the most. I won't talk about the negative pressure generating circuit of 34063 here, but it's available in datasheet. Let's take a look at two kinds of negative voltage generating circuits commonly used in electronic circuits of single chip microcomputer.
Nowadays, many single-chip microcomputers have PWM outputs, and PWM is often not used when using single-chip microcomputers. It is a good choice to use it to generate negative pressure.
The above circuit is the simplest negative pressure generating circuit. The use of the original is the least, just provide it with a square wave of about 1kHz, which is quite simple. Here, it should be noted that the load capacity of this circuit is very weak, and at the same time, the voltage drops greatly after the load is added.
The following circuit is produced due to the above reasons:
Analysis of negative voltage generation circuit
Definition of voltage: Voltage, also known as potential difference or potential difference, is a physical quantity that measures the energy difference of unit charge in electrostatic field due to different potentials. Its magnitude is equal to the work done by the unit positive charge moving from point A to point B due to electric field force, and the direction of voltage is defined as the direction from high potential to low potential.
To put it bluntly, the voltage at a certain point is the difference between the potentials relative to a reference point. V =E, a-e. Generally, the negative pole of the power supply is taken as the reference point. The voltage is Vcc=E power plus -E power minus.
If you want to generate negative voltage, let it have a lower potential relative to the negative pole of the power supply. To get lower, another power supply must be involved. The fundamental principle is to use the series connection of two power supplies. After the positive pole of power supply 2 is connected in series with the negative pole of reference power supply 1, the negative pole of power supply 2 is negative voltage.
A negative voltage generating circuit: a new power supply is equivalent by charging the capacitor. After the capacitor is connected in series with GND, it is equivalent to power supply 2, which generates negative voltage.
1. Capacitor charging: When PWM is at low level, Q2 is turned on and Q1 is turned off, and VCC charges C1 through Q2. The charging loop is VCC-Q2-C1-D2-GND, and the Zuo Zheng on C1 is negative to the right.
2. Capacitor C1 is fully charged.
3. Capacitor C1 is used as the power supply, and the high potential pole of C1 is connected in series at the reference point. C1 discharges and continues to flow from C2, resulting in negative voltage.
When PWM is at low level, Q2 turns off, Q1 turns on, and C1 begins to discharge. The discharge loop is C1-C2-D1, which is actually the process of charging C2. After C2 is fully charged, it will go down positive and up negative. If the potential of VCC is 5: 00 volts, it can output a voltage of -5V.
Scheme of generating negative voltage (-5V).
The output capacity of 760 and MAX232 is limited, so it is very difficult to make oscilloscope with high-speed operational amplifier, so it is necessary to expand the current by four parallel chips. The first edition is 7660 with two parallel pieces.Ordinary DC/DC chips can generate negative voltage, and the voltage accuracy is the same as the positive voltage, and the driving ability is also very strong, which can reach more than 300mA.General switching power supply chips can generate negative voltage, but it is really impossible to push the charge pump with PWM output by switching power supply, and it can also generate large current with low cost. I don't know how much ripple is required, and the ripple of the charge pump is quite small after LC filtering. 760 is a charge pump, so the current is very small.In the design of the whole oscilloscope, +5V of digital power supply and+5V of analog power supply are supplied separately, but what should be done with digital ground and analog ground?Digital ground and analog ground must be connected together, otherwise the circuit can't work.Part of the digital ground return current cannot flow through the analog part of the ground, and the two grounds should be connected together at a stable ground reference point.
Meaning of negative voltage
For example, the telephone system is powered by -48V, which can avoid the electrochemical corrosion of the telephone line. Of course, answering the phone backwards can also work, it's just a change of voltage reference point.
Communication interface needs
For example, RS232 interface, negative voltage must be used. -3V ~-15V means 1, and+3 ~ +3～+15V means 0. This is the protocol when the communication interface was originally designed, so it can only be observed. The interface chip such as PS:MAX232 comes with a charge pump, which can generate negative voltage by itself
Provide power rail for (non-rail-to-rail) operational amplifier
The old operational amplifier has no rail-to-rail input/output capability, such as OP07. The input voltage range is always 1V smaller than the power supply voltage range, and the output is 2V smaller. In this way, if VEE uses 0V, the input voltage must exceed 1V, and the output voltage will not be lower than 2V. This may not meet the design requirements of some circuits. In order to work under the input/output condition close to 0V, it is necessary to supply negative voltage to the operational amplifier, such as -5V, so that the operational amplifier can work normally around 0V. However, with the popularity of rail-to-rail operational amplifiers, this situation is becoming less and less common.
Generally speaking, the protection circuit inside the chip is undefended for negative voltage, so as long as the current is slightly larger and the voltage is not applied to the chip with high negative voltage, the chip can be successfully destroyed.
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