What is Charge Pump : Working & Its Applications

A charge pump is a switch type mode power supply, which creates discrete multiples of the input voltage using a capacitor. In low power electronics at certain conditions, where we have a low voltage say 3.3V, but we require 5V. In order to overcome this situation, we use a boost converter. These converters are inefficient at low powers because they consume lots of power while operating, they are noisy device and does not work in reverse operation. Hence in order to overcome this problem we use a switch type mode power supply called charge pump.

What is a Charge Pump?

Definition: A charge pump is a DC to DC converter, that produces a high-efficient output. They usually operate at a higher frequency. It is also named as flying capacitor converter.

Characteristics

The following are the general characteristics of the charge pump they are

• Zero usage of inductors in the circuit
• EMI (Electro-Magnetic Induction) radiated is minimum
• Simple in construction (one input capacitor and two output capacitor).

Charge Pump Circuit Diagram

The folowing circuit usually consists of a switch “S” or a diode “D” and a capacitor “C” at the output, as shown below,

The below circuit shows the construction of a two-stage charge pump where the output of the first stage is given as input to the second stage and the output from the second stage is cascaded with the output load stage. This construction allows the pump to generate a high output voltage from the lower input voltage.

Working

The working of a charge pump can be explained using a capacitor. The basic function of a capacitor is to store or charge and discharge the charge whenever necessary. For example, we have a capacitor of capacitance 9V, where we charge the capacitor up to 9V and measure using a multimeter, where the reading in multimeter gives 9V. Similarly, on cascading another capacitor of 9V with the previous capacitor we get a higher voltage of 18V, not only 2 capacitors are cascaded but a number of capacitors can also be cascaded to achieve higher output voltage. This is how this pump works usually.

The Building of the Practical Circuit

A 3-stage charge pump consists of 3 charge pump stages which are cascaded one after the other along with a 555 IC timer. This construction increases the output voltage.

Components Used

The 2 main components used are 555 timer IC and pump circuit

555 Timer

A 555 IC consist of 8 pins, GND, Trigger, Output, Reset, Power Supply, Discharge Capacitor, Threshold, Control Voltage as shown below.

Components Used in a 555 IC: Bipolar variant 555 timers, 10µF electrolytic capacitor (decoupling), 2 in number 100 nF decoupling ceramic capacitor, 100pF timing ceramic capacitor, 1K timing resistor, 10K resistor.

Function: The timer used here generates a frequency up to 500KHz, which helps the pump capacitor to refresh periodically so that output voltage is not rippled.

Charge Pump Circuit

Components Used in this circuit are 6 in number IN4148 diodes (or UF4007), 5 in number 10 µF electrolytic capacitors, 100 µF electrolytic capacitors. The circuit diagram is shown below, the input to this circuit is taken from the output pin 3 of 555 IC. The input allows the capacitor to charge using the diode. From the circuit we can observe that the negative end of the capacitor is grounded, when the output of the circuit goes high the capacitor negative pin also goes high. But as we know that the capacitor stores charge inside it already, the voltage when measured across it displays a double input voltage.

But the output voltage obtained consists of 50% of ripple, hence in order to overcome this ripple effect at the output, we add an addition circuit called peak detector as shown below.

Charge Pump as Voltage Inverter

A charge pump not only produces high output voltage but can invert the output voltage. The circuit diagram is similar to a voltage doubler where the diode in the circuit is connected in reverse as shown below,

Working

When the output of 555 IC goes high the capacitor charges and when the IC output goes low the capacitor discharges through the 2nd capacitor in a backward direction. Hence generating a negative voltage at the out of the circuit.

Advantages of Charge Pump

The following are the advantages

• Low cost
• Occupies less area
• Compactable
• Can be used in invert voltage polarity
• Generates high output voltage from the low input voltage.

Limitations of Charge Pump

The following are the limitations

• The current obtained at the output is very low, but in certain cases, if compatible IC is used, we can obtain 100mA current at the output but with less efficiency.
• The output is indirectly proportional to the input stages. i,e. if these pumps are added in every stage from starting to ending in order to get high output voltage. This condition only increases the complexity of the system and does not generate a high output voltage.
•  Efficiency depends on the output voltage.

Applications

The applications of the charge pump include the following.

• They are used in RS-232 level shifters, which drive both positive (+10V) and negative voltage (-10V) from 5V or 3V power supply rail.
• Application in LCD or white LED drivers, which generate high bias voltage from a single low voltage supply
• Used in NMOS memories and microprocessor to a negative voltage VBB
• Used in H bridges in high-speed drivers
• Voltage doubler
• Op-amps
• PLL -Phase Lock Loop circuits.

Thus, Charge Pumps are one of the applications in low power electronics, which generates high output voltage from a low input voltage. It is also named as flying capacitor converter. A single-stage charge pump circuit consists of a capacitor, a switch, or a diode connected to a voltage source. In some conditions, the output voltage generated may consist of ripples, which can be eliminated using a peak detector at the output stage. These circuits can also generate inverted output voltage by connecting the diode in reverse polarity. The main advantage of the charge pump is that they are highly efficient, simple in construction.