Datasheet

LM2664
SNVS005D –NOVEMBER 1999–REVISED MAY 2013 www.ti.com
Application Information
SIMPLE NEGATIVE VOLTAGE CONVERTER
The main application of LM2664 is to generate a negative supply voltage. The voltage inverter circuit uses only
two external capacitors as shown in the Basic Application Circuits. The range of the input supply voltage is 1.8V
to 5.5V.
The output characteristics of this circuit can be approximated by an ideal voltage source in series with a
resistance. The voltage source equals −(V+). The output resistance R
out
is a function of the ON resistance of the
internal MOSFET switches, the oscillator frequency, the capacitance and ESR of C
1
and C
2
. Since the switching
current charging and discharging C
1
is approximately twice as the output current, the effect of the ESR of the
pumping capacitor C
1
will be multiplied by four in the output resistance. The output capacitor C
2
is charging and
discharging at a current approximately equal to the output current, therefore, its ESR only counts once in the
output resistance. A good approximation of R
out
is:
where R
SW
is the sum of the ON resistance of the internal MOSFET switches shown in Figure 15.
High capacitance, low ESR capacitors will reduce the output resistance.
The peak-to-peak output voltage ripple is determined by the oscillator frequency, the capacitance and ESR of the
output capacitor C
2
:
Again, using a low ESR capacitor will result in lower ripple.
SHUTDOWN MODE
A shutdown (SD ) pin is available to disable the device and reduce the quiescent current to 1µA. Applying a
voltage less than 20% of V+ to the SD pin will bring the device into shutdown mode. While in normal operating
mode, the pin is connected to V+.
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
Where I
Q
(V+) is the quiescent power loss of the IC device, and I
L
2
R
out
is the conversion loss associated with the
switch on-resistance, the two external capacitors and their ESRs.
The selection of capacitors is based on the specifications of the dropout voltage (which equals I
out
R
out
), the
output voltage ripple, and the converter efficiency. Low ESR capacitors are recommended to maximize efficiency,
reduce the output voltage drop and voltage ripple.
Table 1. Low ESR Capacitor Manufacturers
Manufacturer Phone Capacitor Type
Nichicon Corp. (708)-843-7500 PL & PF series, through-hole aluminum electrolytic
AVX Corp. (803)-448-9411 TPS series, surface-mount tantalum
Sprague (207)-324-4140 593D, 594D, 595D series, surface-mount tantalum
Sanyo (619)-661-6835 OS-CON series, through-hole aluminum electrolytic
Murata (800)-831-9172 Ceramic chip capacitors
Taiyo Yuden (800)-348-2496 Ceramic chip capacitors
Tokin (408)-432-8020 Ceramic chip capacitors
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