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
Open
V+
Open or V+
N/A
Open
Open
External Capacitor
External Clock
(inverter mode only)
OSC
20 kHz
150 kHz
See Typical Performance Characteristics
External Clock Frequency
Oscillator
Table 2. LM2663 Oscillator Frequency Selection
OSC
Open
External Capacitor
External Clock
(inverter mode only)
150 kHz
See Typical Performance Characteristics
External Clock Frequency
Oscillator
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)
Where I
Q
(V+) is the quiescent power loss of the IC device, and I
L2
R
OUT
is the conversion loss associated with the
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
voltage drop and voltage ripple. For convenience, C
1
and C
2
are usually chosen to be the same.
The output resistance varies with the oscillator frequency and the capacitors. In
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
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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