FC

OSC

LM2660

LV

V+

CAP+

OUT

GND

OSCILLATOR

Switch Array

Switch Drivers

CAP-

LM2660

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SNVS135E – SEPTEMBER 1999 – REVISED DECEMBER 2014

8 Detailed Description

8.1 Overview

The LM2660 contains four large CMOS switches which are switched in a sequence to invert the input supply

voltage. Energy transfer and storage are provided by external capacitors. Figure 13 illustrates the voltage

switches, and no ESR in the capacitors. In reality, the charge transfer efficiency depends on the switching

frequency, the on-resistance of the switches, and the ESR of the capacitors.

Figure 13. Voltage Inverting Principle

8.2 Functional Block Diagram

8.3 Feature Description

8.3.1 Changing Oscillator Frequency

The internal oscillator frequency can be selected using the Frequency Control (FC) pin. When FC is open, the

oscillator frequency is 10 kHz; when FC is connected to V+, the frequency increases to 80 kHz. A higher

oscillator frequency allows smaller capacitors to be used for equivalent output resistance and ripple, but

increases the typical supply current from 0.12 mA to 1 mA.

The oscillator frequency can be lowered by adding an external capacitor between OSC and GND. (See Typical

Characteristics.) Also, in the inverter mode, an external clock that swings within 100 mV of V+ and GND can be

used to drive OSC. Any CMOS logic gate is suitable for driving OSC. LV must be grounded when driving OSC.

The maximum external clock frequency is limited to 150 kHz.

The switching frequency of the converter (also called the charge pump frequency) is half of the oscillator

frequency.

NOTE

OSC cannot be driven by an external clock in the voltage-doubling mode.

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Product Folder Links: LM2660