Datasheet

LM2660

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SNVS135D –SEPTEMBER 1999–REVISED MAY 2013

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.

Table 1. LM2660 Oscillator Frequency Selection

FC OSC Oscillator

Open Open 10 kHz

V+ Open 80 kHz

Open or V+ External Capacitor See Typical Performance Characteristics

N/A External Clock External Clock

(inverter mode only) Frequency

CAPACITOR SELECTION

As discussed in the SIMPLE NEGATIVE VOLTAGE CONVERTER section, the output resistance and ripple

voltage are dependent on the capacitance and ESR values of the external capacitors. The output voltage drop is

the load current times the output resistance, and the power efficiency is

(3)

Where I

(V+) is the quiescent power loss of the IC device, and I

OUT

is the conversion loss associated with the

switch on-resistance, the two external capacitors and their ESRs.

Since the switching current charging and discharging C

is approximately twice as the output current, the effect

of the ESR of the pumping capacitor C

is multiplied by four in the output resistance. The output capacitor C

charging and discharging at a current approximately equal to the output current, therefore, its ESR only counts

once in the output resistance. However, the ESR of C

directly affects the output voltage ripple. Therefore, low

ESR capacitors (Table 2) are recommended for both capacitors to maximize efficiency, reduce the output voltage

drop and voltage ripple. For convenience, C

and C

are usually chosen to be the same.

The output resistance varies with the oscillator frequency and the capacitors. In Figure 17, the output resistance

vs. oscillator frequency curves are drawn for three different tantalum capacitors. At very low frequency range,

capacitance plays the most important role in determining the output resistance. Once the frequency is increased

to some point (such as 20 kHz for the 150 μF capacitors), the output resistance is dominated by the ON

resistance of the internal switches and the ESRs of the external capacitors. A low value, smaller size capacitor

usually has a higher ESR compared with a bigger size capacitor of the same type. For lower ESR, use ceramic

capacitors.

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