LM2662, LM2663

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SNVS002D – JANUARY 1999 – REVISED MAY 2013

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

performance 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.

Table 1. LM2662 Oscillator Frequency Selection

FC

OSC

Oscillator

Open

Open

20 kHz

V+

Open

150 kHz

Open or V+

External Capacitor

See Typical Performance Characteristics

N/A

External Clock (inverter mode only)

External Clock Frequency

Table 2. LM2663 Oscillator Frequency Selection

OSC

Oscillator

Open

150 kHz

External Capacitor

See Typical Performance Characteristics

External Clock (inverter mode only)

External Clock Frequency

SHUTDOWN MODE

For the LM2663, a shutdown (SD) pin is available to disable the device and reduce the quiescent current to 10

μA. Applying a voltage greater than 2V to the SD pin will bring the device into shutdown mode. While in normal

operating mode, the SD pin is connected to ground.

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)

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

Low ESR capacitors (Table 3) are recommended for both capacitors to maximize efficiency, reduce the output

The output resistance varies with the oscillator frequency and the capacitors. In Figure 19, the output resistance

vs. oscillator frequency curves are drawn for four difference capacitor values. 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 100 kHz for the 47

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. Ceramic capacitors can be

chosen for their lower ESR. As shown in Figure 19, in higher frequency range, the output resistance using the 10

μF ceramic capacitors is close to these using higher value tantalum capacitors.

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