Charge pump
As a charge pump , English Charge Pump , be several different electrical circuits summarizes which electrical voltages larger in value or reverse DC voltages in polarity. The output voltage of a charge pump is always a direct voltage . If the input voltage is also a DC voltage, the charge pump is one of the DC voltage converters . As an essential feature, charge pumps manage without magnetic components such as coils or transformers .
Charge pumps transport the electrical charge with the help of electrical capacitors and through periodic switching with switches, with which different electrical output voltages can be generated. The procedures are similar to when water is carried with buckets from a lower place to a higher place and there is collected with higher potential energy .
Charge pumps are used as voltage converters where no large output currents are required or where no suitable magnetic components such as coils can be used.
Feed
AC voltage
If charge pumps are fed with alternating voltage, they are also counted among the rectifiers with voltage doubling, such as the Greinacher circuit . If the switching elements - usually diodes that switch due to potential differences - are cascaded several times , very high DC voltages can be generated and the circuit is referred to as a high-voltage cascade . Applications are in the area of tube televisions / monitors, laser printers for high voltage generation for the application of the toner to the paper, in high voltage laboratories or as part of particle accelerators such as the Cockcroft-Walton accelerator .
DC voltage
The charge pump as a DC-DC converter ( English DC-DC converter ) is fed with DC voltage and generates depending on the circuit type, either a higher DC voltage than the input voltage with the same polarity or a negative output voltage. To periodically switch the switches, these charge pumps require an oscillator or an externally supplied periodic switchover pulse.
Combinations of actively controlled transistors - metal-oxide-semiconductor field-effect transistors are common - and diodes as potential-controlled switches are used as switches. With low powers, these components can be accommodated directly in the integrated circuits together with the capacitors . The switching frequencies are between 100 kHz and a few megahertz.
In the following, exemplary circuits of charge pumps with DC voltage supply for positive or negative output voltages are described.
Positive output voltages
Voltage doubling
In the adjacent circuit diagram, a charge pump for doubling the DC voltage is shown. The DC voltage U e supplied on the left is converted into a positive DC voltage U a with the value
transformed. The voltage U D is the forward voltage of a diode and is approx. 0.7 V for silicon diodes. The schematically drawn switch S is actually a push-pull output stage ( CMOS inverter ); it is periodically switched between the two switching states at a specific switching frequency supplied by an oscillator.
In the first state the switch is S in the illustrated position and loads the pump capacitor C 1 through the diode D 1 to the input voltage U E to. Because of the voltage drop across the diode (forward voltage), the pump capacitor is charged to slightly less than the full input voltage. Then the switch is switched up. Then the input voltage and the voltage at C 1 are in series, so that D 1 is in the reverse direction and D 2 is conductive. As a result, output capacitor C 2 is charged to a little less than 2 · U E. The cycle then repeats itself.
Voltage multiplication
The above circuit can also be used by cascading to supply higher than double the input voltage, as shown in the adjacent figure. The output voltage for this circuit is:
The function of the left part of the circuit - i.e. up to the junction at C 2 and D 2 - is identical to the circuit for simple voltage doubling. The additionally inserted pump capacitor C 3 is charged via D 3 to approximately double the input voltage and in the next cycle the input voltage U e is added to the voltage at C 3 to approximately three times the input voltage.
Dickson charge pump
A slight expansion of the cascaded charge pump leads to the circuit known as a Dickson charge pump , the switching principle of which is particularly important in the area of integrated circuits. Instead of a changeover switch S or a CMOS stage, two phase-shifted clock signals, as shown in the adjacent sketch as Φ1 and Φ2, are used.
The Dickson charge pump is mainly used in the field of integrated circuits that operate from a low battery voltage, for example from 1 V to 1.5 V, and generate the higher voltages such as 3.3 V required in the IC. The disadvantage of the basic shape is the forward voltage dropping per diode, which prevents operation at these low voltages. Therefore, the diodes are replaced by metal-oxide-semiconductor field effect transistors (MOSFETs) as in the second circuit.
The voltage drop across the MOSFETs is lower than the forward voltage of around 0.7 V for silicon diodes, but in this case the MOSFETs are operated in the linear range, which still leads to voltage losses of around 0.3 V per MOSFET. For example, with a Dickson charge pump with five MOSFETs and an input voltage of 1.5 V, only an actual output voltage of just over 2 V is achieved. By connecting additional MOSFETs in parallel, which serve as switches, as shown in the figure below and whose gate voltage is obtained from higher voltage branches, an output voltage of around 4 V can be achieved in this case. In this case, the voltage drop per stage is around 0.15 V. This is usually sufficient for the operation of conventional CMOS circuits.
In addition, there are extensions beyond the Dickson charge pump, such as the Mandal-Sarpeshkar charge pump, which further reduces the effect of the threshold voltage of the MOSFETs, which is disturbing in this case . The Umeda charge pump and the Nakamoto charge pump completely avoid the effect of the threshold voltage of the MOSFETs, but in the first case require an additional, externally supplied higher voltage, which is not available in all applications. The Nakamoto charge pump generates this higher auxiliary voltage internally, but is associated with a significantly higher level of circuit complexity.
Negative output voltages
Stress inversion
Charge pumps can also be used for voltage inversion. In this way, a negative direct voltage with a voltage value below the reference potential is obtained for the symmetrical supply of operational amplifiers , for example .
In the adjacent circuit diagram of the switch is S initially in the illustrated initial position and loads the pump capacitor C 1 through the diode D 1 to the input voltage. Then the positive side of C 1 is switched to reference potential (ground) by switching S , so that the other capacitor plate assumes a voltage that is negative with respect to ground. As a result, D 1 blocks while the output capacitor C 2 is charged to the negative output voltage via D 2 . The output voltage in this case is:
The ICL7660 integrated circuit from Intersil is an integrated charge pump for generating −5 V from +5 V, which can be described as classic, since it has been on the market since the mid-1980s .
Inverted voltage multiplication
As with positive voltage multiplication, the circuit for inverted voltage multiplication can also be cascaded in order to obtain output voltages that are higher in magnitude than the input voltage. The circuit is again identical to the simple inverter in the left part and is supplemented by an additional pump capacitor C 2 . The output voltage in this case is:
In order to avoid the losses due to the forward voltages U D of the diodes, all switches in integrated circuits are typically designed in the form of controlled field effect transistors with the smallest possible R DS (on) . In order to keep the recharging losses in the capacitors as small as possible, the capacitances are as large as possible, if necessary in the case of integrated circuits in the form of external capacitors, and the switching frequencies are selected as high as possible.
Application examples
RS232 level conversion
Signal levels of ± 12 V are required for communication via a serial interface according to EIA-232 (RS232). However, many digital circuits work with a supply voltage of 5 V or less. In order to be able to do without an additional voltage source of ± 12 V, charge pumps are used for voltage multiplication and voltage inversion in the interface module. In this case, 10 V is obtained from the 5 V operating voltage by doubling it and then −10 V is generated by voltage inversion. The generated voltages of ± 10 V are within the tolerance range of the RS232 and are therefore sufficient.
Such level converter ICs usually contain all the components of the charge pump with the exception of the capacitors, which cannot be integrated directly on the substrate with the necessary capacitance in all integrated circuits. A common circuit of this type is the MAX232 from Maxim Integrated Products and its successors.
Programming voltage generation
Another application of charge pumps is to generate the programming voltage in the range of 10 V to 15 V necessary for programming flash memories . To write to the floating gates of the individual memory cells, higher voltages are required than the 3, 3, which are usually supplied from the outside to the memory chip V. This higher programming voltage is generated directly on the memory chip in the form of a small charge pump with integrated capacitors. The capacities of the capacitors are comparatively small and are in the range of a few pF - however, only low currents are required for the write process. The circuitry advantage is that the memory chips can be supplied with only a single supply voltage with the aid of the integrated charge pumps.
Voltage generation in the DRAM
Synchronous dynamic memories such as SDRAMs are usually operated with only one supply voltage, which depending on the standard at z. B. 1.5 V for DDR3 . In the SDRAM chip, several different voltages are required to control the memory field, some of which are above the supply voltage or are negative. In particular, these are the voltages for the word lines, V PP and V NWL (V 'pumped' and V 'negative word line'). These voltages are in the range of approx. 2.5–3.5 V for V PP and approx. −1 V to 0 V for V NWL (depending on the process and manufacturer). For both voltages, currents of up to 10–100 mA are required. In particular for the V PP charge pumps, which have to be two- or three-stage due to the voltage conditions in DDR2 or DDR3 memories, a relatively large amount of chip area is therefore required compared to other voltage generators.
Driver and Bootstrap Circuits
Charge pumps are included in many circuits in which higher voltages than the input voltage are required or these voltages must have a variable reference potential, e.g. B. in driver circuits for controlling power semiconductor switches (keywords level shifter, high side switches ).
This is e.g. B. the case when an NPN or NMOS transistor is to be used in the upper branch of a bridge circuit . The charge pump is often fed from the output alternating voltage that is already present and then consists only of a diode and a capacitor. It is also referred to as a bootstrap circuit or the associated capacitor as a bootstrap capacitor.
Individual evidence
- ↑ DC-DC Conversion Without Inductors , company publication (Application Note) 725, Maxim-IC, July 22, 2009, engl.
- ↑ a b Mingliang Liu: Demystifying Switched-Capacitor Circuits . Newnes, 2006, ISBN 0-7506-7907-7 .
- ↑ Data sheet (PDF; 442 kB) of the ICL7660 charge pump from Intersil / MAXIM (English)
- ↑ + 5V-Powered, Multichannel RS-232 Drivers / Receivers ( Memento of the original from February 1, 2010 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.9 MB), data sheet from Maxim-IC, January 2006, engl.
- ↑ Brent Keeth, R. Jacob Baker, Brian Johnson: DRAM Circuit Design: Fundamental and High-Speed Topics. 2nd Edition. Wiley & Sons, 2007, ISBN 0-470-18475-2 .
literature
- Ulrich Tietze, Christoph Schenk: Semiconductor circuit technology . 12th edition. Springer, 2002, ISBN 3-540-42849-6 .