SNVS420D November 2008 – May 2018 LM7705

PRODUCTION DATA.

- 1 Features
- 2 Applications
- 3 Description
- 4 Revision History
- 5 Pin Configuration and Functions
- 6 Specifications
- 7 Detailed Description
- 8 Application and Implementation
- 9 Power Supply Recommendations
- 10Layout
- 11Device and Documentation Support
- 12Mechanical, Packaging, and Orderable Information

- DGK|8

- DGK|8

This section uses a simplified but realistic equivalent circuit that represents the basic function of the charge pump. The schematic is given in Figure 30.

When the switch is in position A, capacitor C_{FLY} will charge to voltage V_{1}. The total charge on capacitor C_{FLY} is Q_{1} = C_{FLY} × V _{1}. The switch then moves to position B, discharging C_{FLY} to voltage V_{2}. After this discharge, the charge on C_{FLY} will be Q_{2} = C_{FLY} × V_{2}. The charge has been transferred from the source V_{1} to the output V_{2}. The amount of charge transferred is:

Equation 1.

When the switch changes between A and B at a frequency f, the charge transfer per unit time, or current is:

Equation 2.

The switched capacitor network can be replaced by an equivalent resistor, as indicated in Figure 31.

The value of this resistor is dependent on both the capacitor value and the switching frequency as given in Equation 3

Equation 3.

The value for R_{EQ} can be calculated from Equation 3 and is given in Equation 4

Equation 4.

Equation 4 show that the value for the resistance at an increased internal switching frequency, allows a lower value for the used capacitor.