156869 Datasheet, PDF (6/9 Pages) Texas Instruments – Switched Capacitor Voltage Converter

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 Cir-
cuits. 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 C₁ and C₂. Since the switching current charging and
discharging C₁ is approximately twice as the output current,
the effect of the ESR of the pumping capacitor C₁ will be
multiplied by four in the output resistance. The output ca-
pacitor C₂ is charging and discharging at a current approxi-
mately 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 2.
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 C₂:
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 depen-
dent 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_L2R_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 volt-
age ripple, and the converter efficiency. Low ESR capacitors
(Table 1) are recommended to maximize efficiency, reduce
the output voltage drop and voltage ripple.
Again, using a low ESR capacitor will result in lower ripple.
Low ESR Capacitor Manufacturers
Manufacturer
Nichicon Corp.
AVX Corp.
Sprague
Sanyo
Murata
Taiyo Yuden
Tokin
Phone
(708)-843-7500
(803)-448-9411
(207)-324-4140
(619)-661-6835
(800)-831-9172
(800)-348-2496
(408)-432-8020
Capacitor Type
PL & PF series, through-hole aluminum electrolytic
TPS series, surface-mount tantalum
593D, 594D, 595D series, surface-mount tantalum
OS-CON series, through-hole aluminum electrolytic
Ceramic chip capacitors
Ceramic chip capacitors
Ceramic chip capacitors
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