Datasheet
LM2660
SNVS135D –SEPTEMBER 1999–REVISED MAY 2013
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APPLICATION INFORMATION
SIMPLE NEGATIVE VOLTAGE CONVERTER
The main application of LM2660 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 1.5V
to 5.5V. For a supply voltage less than 3.5V, the LV pin must be connected to ground to bypass the internal
regulator circuitry. This gives the best performance in low voltage applications. If the supply voltage is greater
than 3.5V, LV may be connected to ground or left open. The choice of leaving LV open simplifies the direct
substitution of the LM2660 for the LMC7660 Switched Capacitor Voltage Converter.
The output characteristics of this circuit can be approximated by an ideal voltage source in series with a resistor.
The voltage source equals −(V+). The output resistance R
out
is a function of the ON resistance of the internal
MOS switches, the oscillator frequency, and the capacitance and ESR of C
1
and C
2
. A good approximation is:
(1)
where R
SW
is the sum of the ON resistance of the internal MOS switches shown in Figure 16.
High value, low ESR capacitors will reduce the output resistance. Instead of increasing the capacitance, the
oscillator frequency can be increased to reduce the 2/(f
osc
× C
1
) term. Once this term is trivial compared with R
SW
and ESRs, further increasing in oscillator frequency and capacitance will become ineffective.
The peak-to-peak output voltage ripple is determined by the oscillator frequency, and the capacitance and ESR
of the output capacitor C
2
:
(2)
Again, using a low ESR capacitor will result in lower ripple.
POSITIVE VOLTAGE DOUBLER
The LM2660 can operate as a positive voltage doubler (as shown in the Basic Application Circuits). The doubling
function is achieved by reversing some of the connections to the device. The input voltage is applied to the GND
pin with an allowable voltage from 2.5V to 5.5V. The V+ pin is used as the output. The LV pin and OUT pin must
be connected to ground. The OSC pin can not be driven by an external clock in this operation mode. The
unloaded output voltage is twice of the input voltage and is not reduced by the diode D
1
's forward drop.
The Schottky diode D
1
is only needed for start-up. The internal oscillator circuit uses the V+ pin and the LV pin
(connected to ground in the voltage doubler circuit) as its power rails. Voltage across V+ and LV must be larger
than 1.5V to insure the operation of the oscillator. During startup, D
1
is used to charge up the voltage at V+ pin to
start the oscillator; also, it protects the device from turning-on its own parasitic diode and potentially latching-up.
Therefore, the Schottky diode D
1
should have enough current carrying capability to charge the output capacitor
at start-up, as well as a low forward voltage to prevent the internal parasitic diode from turning-on. A Schottky
diode like 1N5817 can be used for most applications. If the input voltage ramp is less than 10V/ms, a smaller
Schottky diode like MBR0520LT1 can be used to reduce the circuit size.
SPLIT V+ IN HALF
Another interesting application shown in the Basic Application Circuits is using the LM2660 as a precision voltage
divider. Since the off-voltage across each switch equals V
IN
/2, the input voltage can be raised to +11V.
CHANGING OSCILLATOR FREQUENCY
The internal oscillator frequency can be selected using the Frequency Control (FC) pin. When FC is open, the
oscillator frequency is 10 kHz; when FC is connected to V+, the frequency increases to 80 kHz. A higher
oscillator frequency allows smaller capacitors to be used for equivalent output resistance and ripple, but
increases the typical supply current from 0.12 mA to 1 mA.
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.
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