| Types of Switched Capacitor Voltage Converters | | | | required in addition to the primary positive voltage. |
| In the above circuit capacitor C1 is charged to the | | | | This may occur in single power supply system |
| input voltage during first half of switching cycle. In | | | | where only a few high performance parts require |
| the second half of switching cycle its voltage is | | | | the negative voltage. |
| inverted and applied to capacitor C2 and load. The | | | | |
| output voltage is negative of the input voltage. | | | | Voltage Doubler |
| The Duty cycle – defined as ratio of charging | | | | |
| time of capacitor C1 to the entire switching cycle | | | | Voltage Doubler works similarly to the Inverter, |
| time – is usually 50% because that generally | | | | however, the pump capacitor is placed in series |
| yields the optimal charge transfer efficiency. | | | | with the input voltage during its discharge cycle, |
| | | | | thereby, accomplishing the voltage doubling |
| During the time period of transient conditions | | | | function. In voltage doubler, average input current |
| start-up and steady state condition the capacitor | | | | is approximately twice the average output |
| C1 has to supply only a small amount of charge | | | | current. |
| to the output capacitor on each switching cycle. | | | | |
| The amount of charge transferred depends upon | | | | Voltage doublers are used in low current |
| the load current and the switching frequency. | | | | applications where a voltage greater than the |
| Capacitor C1 is also known as charge pump | | | | primary supply voltage is required. |
| capacitor. | | | | |
| | | | | Regulated Output Switched Capacitor Voltage |
| During the time the charge pump capacitor is | | | | Converters |
| being charged by the input voltage, the output | | | | |
| capacitor C2 must supply the load current. The | | | | Adding regulation to the switched capacitor |
| load current flowing out of C2 causes a droop in | | | | voltage converters increases its usefulness in |
| the output voltage which corresponds to a | | | | many applications. The most straightforward is to |
| component of output voltage ripple. Higher | | | | follow the switched capacitor converter with a |
| switching frequency allow smaller capacitors for | | | | low dropout linear regulator (LDO). The LDO |
| the same amount of droop. Thus the switching | | | | provides the regulated output and also reduces |
| frequency impacts the size of the external | | | | the ripple of the switched capacitor converter. |
| capacitor required. Higher switching frequency | | | | This approach however, adds the complexity and |
| allows the use of smaller capacitor. The Switching | | | | reduces the available output voltage by the |
| frequencies are generally limited to few hundred | | | | dropout voltage of the LDO. |
| Khz. | | | | |
| | | | | Another approach to regulation is to vary the |
| Switched capacitor inverters are low cost, | | | | duty cycle of the switch control signal with the |
| compact and efficiency achieved are greater than | | | | output of an error amplifier which compares the |
| 90%. Typical switched capacitor inverters have | | | | output voltage with a reference. However, this |
| maximum output current of 150mA maximum. | | | | approach is highly non linear and requires long time |
| | | | | constants in order to maintain good regulation |
| The Voltage inverters are used in applications | | | | control. |
| where relatively low current negative voltage is | | | | |