Charge Pump Symbol
Definition: The Charge Pump symbol represents a switched-capacitor DC-DC converter block — used in power electronics schematics — that uses capacitors as energy-transfer elements switched by clock-controlled MOSFETs to generate an output voltage that is higher than, inverted relative to, or a fraction of the input voltage, without requiring a magnetic transformer or inductor.
Also known as: charge pump, switched-capacitor converter, Dickson charge pump, voltage doubler (capacitor), voltage inverter (switched-capacitor), capacitive DC-DC converter.
What the Charge Pump symbol means
The Charge Pump symbol represents a power conversion block with four pins: Vin (positive supply input, left-top), GND (ground reference, left-bottom), Vout (regulated or multiplied positive output, right-top), and Vout- (inverted or negative output, right-bottom). In the charging phase, capacitors are connected in parallel to the input supply and charged to Vin; in the pumping phase, the switching network reconnects these capacitors in series with the input (or inverted), boosting or inverting the output voltage.
Charge pumps are used in applications where a small, inductor-free voltage conversion is required — particularly in ICs, LCD bias generation, RS-232 level shifters, and flash memory programming. Because they use only capacitors and switches (no inductors), they generate less EMI than inductive converters, but they are limited in output current (typically < 200 mA) and efficiency degrades significantly when the load current is a large fraction of rated output.
How to identify the Charge Pump symbol
The Charge Pump symbol is a rectangular block labelled 'Charge Pump' or 'CP' with four labelled pins: Vin and GND on the left side, and Vout and Vout- on the right side. The designator is U or IC in schematics. Some representations show internal capacitor and switch symbols within the block for clarity; discrete implementations show the Dickson multiplier ladder or the full-wave doubler topology.
Function in a circuit
A charge pump converts the input DC voltage to a different output voltage level using only capacitors and switches — no inductor or transformer. In a voltage-doubling charge pump (Dickson), Vout = 2 × Vin. In an inverting charge pump, Vout- = −Vin. In a fractional charge pump, Vout = Vin/2. The switching frequency (typically 100 kHz to several MHz in ICs) determines the effective output impedance and ripple; higher frequency enables smaller capacitors. Conversion efficiency is limited by capacitor charge redistribution losses and is typically 80–90% at light load, falling at heavy load.
Standards: IEC vs ANSI
| IEC 60617 | IEC 61000-3-2 and related harmonic/EMC standards apply to charge pump power supplies in terms of conducted emissions. IEC 62368-1 covers audio/video and IT equipment safety including switch-mode power converters. The charge pump block symbol follows IEC 60617 functional block representation conventions. |
|---|---|
| ANSI/IEEE 315 | IEEE standards for switch-mode power supply design apply (IEEE Std 1221). ANSI/IEEE 315 / IEEE Std 315-1975 defines the rectangular block symbol with labelled pins used to represent functional modules. UL 60950-1 / UL 62368-1 cover product safety for devices incorporating charge pumps. |
| Key difference | The IEC and ANSI/IEEE symbols for a charge pump are both represented as a labelled rectangular functional block with named input and output pins. There is no fundamentally different glyph between the two standards; the difference is in the level of internal detail shown (IEC may show component symbols; IEEE 315 permits abstract blocks). |
Terminals / pins
| Pin | Name |
|---|---|
| vin | Vin |
| gnd | GND |
| vout | Vout |
| vout_neg | Vout- |
Typical values
Input voltage: 1.5 V to 30 V depending on application. Output current: typically 10 mA to 200 mA (IC charge pumps); up to several amps for discrete designs. Switching frequency: 100 kHz to 10 MHz (IC); 10 kHz to 500 kHz (discrete). Conversion ratios: ×2 (doubler), −1 (inverter), ×1.5, ×3, ÷2 (fractional). Efficiency: 75–90% at rated load. Common ICs: ICL7660 (voltage inverter), MAX1044 (doubler/inverter), LTC1044, TC7660.
Where the Charge Pump symbol is used
- RS-232 / UART level shifters — generating ±12 V from a single 5 V or 3.3 V supply for legacy serial port interfaces
- LCD and OLED display bias — generating negative gate bias voltage for TFT LCD pixel control from a single supply
- Flash and EEPROM programming voltage — on-chip charge pumps generate the 12–18 V programming voltage from a 5 V supply inside flash memory ICs
- Op-amp supplies — providing a split ±5 V supply from a single 5 V USB or battery supply for audio circuits
- Low-power microcontroller peripherals — generating a low-current high-voltage supply for sensors or analog circuits
- Charge-coupled device (CCD) image sensors — clock and substrate bias voltage generation inside the sensor IC
- Embedded energy harvesting — voltage doubling of low-energy harvested sources (piezo, thermoelectric) before storage
Example
In a microcontroller-based RS-232 interface circuit, an ICL7660 charge pump receives 5 V at its Vin pin and GND at its GND pin, and outputs +10 V at Vout and −10 V at Vout-; these voltages power the MAX232 RS-232 transceiver line drivers, which require ±10 V swing to drive the serial port without a separate bipolar power supply.
Key facts
- A charge pump is a switched-capacitor DC-DC converter that steps up, inverts, or fractions the input voltage using only capacitors and switches — no inductor or transformer required.
- The symbol has four pins: Vin (input supply), GND (ground), Vout (converted positive output), and Vout- (inverted or negative output).
- Common conversion ratios: ×2 voltage doubler (Vout = 2Vin), ×−1 inverter (Vout- = −Vin), ×1.5 fractional multiplier.
- Charge pump efficiency is limited by capacitor charge redistribution losses; efficiency is typically 80–90% at light load and degrades under heavy load, making inductive converters preferable for higher power.
- Switching frequency in IC charge pumps is typically 100 kHz to several MHz; higher frequency reduces capacitor size but increases switching losses.
- Common IC implementations: ICL7660, MAX1044 (voltage inverter/doubler from 1.5 V to 10 V); LTC1044, TC7660 (CMOS equivalents).
- Charge pumps generate less EMI than inductive (buck/boost) converters because there is no switching magnetic field, making them preferred in noise-sensitive analog and RF circuits.
Frequently asked questions
What does the charge pump symbol mean in a circuit diagram?
The charge pump symbol represents a switched-capacitor power conversion block that converts the input voltage to a higher, inverted, or fractional output voltage using only capacitors and switches. The Vin pin is the supply input, GND is the reference, Vout is the positive converted output, and Vout- is the negative or inverted output.
What does the charge pump symbol look like?
The charge pump symbol is a rectangle labelled 'Charge Pump' or 'CP' with four pins: Vin and GND on the left side, Vout and Vout- on the right side. It follows the standard IC functional block symbol convention per ANSI/IEEE 315 and IEC 60617.
What is the difference between a charge pump and a boost converter?
A charge pump uses only capacitors and switches to step up voltage, while a boost converter uses an inductor as the energy storage element. Boost converters can deliver much higher output currents (amps to tens of amps) and maintain high efficiency across a wide load range. Charge pumps are simpler, smaller, and generate less EMI, but are limited to low output currents (typically < 200 mA) and their efficiency degrades significantly under heavy load.
How does a Dickson charge pump work?
A Dickson charge pump uses a series of diodes (or MOSFET switches) and capacitors arranged in a ladder. A two-phase clock alternately charges and dumps capacitors in series with the input voltage; each stage adds one additional Vin to the output. With N stages, the ideal output voltage is (N+1) × Vin. Practical output is lower due to diode forward voltage drops and capacitor parasitic resistance.
What is the charge pump used for in flash memory?
Flash memory cells require programming voltages of 12–18 V to tunnel electrons through the gate oxide, but the device operates from a 1.8 V or 3.3 V supply. An on-chip charge pump multiplies the supply voltage to generate the programming voltage internally, eliminating the need for an external high-voltage supply and enabling flash memory to be programmed from a standard logic supply.
What is the efficiency of a charge pump?
Charge pump efficiency is typically 80–90% at light load but degrades at higher load currents due to the I²R losses in the capacitors and switches. Efficiency is maximised when the output voltage is at a fixed integer multiple of the input voltage; fractional conversion ratios typically result in lower efficiency. For loads above ~200 mA, an inductive (buck or boost) converter is generally more efficient.
What standard governs charge pump design?
There is no single dedicated standard for charge pump design. EMC conducted emissions are governed by IEC 61000-3-2 (for mains-connected equipment) and CISPR 32 (for multimedia equipment). Product safety standards IEC 62368-1 and UL 62368-1 cover switch-mode converters including charge pumps in consumer electronics. IEEE Std 1221 provides guidance on switch-mode power supply design principles.
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