SNVS002D – JANUARY 1999 – REVISED MAY 2013
APPLICATION INFORMATION
SIMPLE NEGATIVE VOLTAGE CONVERTER
The main application of LM2662/LM2663 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 LM2662/LM2663 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
. 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
is 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 is:
(1)
where R
SW
is the sum of the ON resistance of the internal MOS switches shown in
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 LM2662/LM2663 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 start-up, 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 LM2662/LM2663 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
For the LM2662, the internal oscillator frequency can be selected using the Frequency Control (FC) pin. When
FC is open, the oscillator frequency is 20 kHz; when FC is connected to V+, the frequency increases to 150 kHz.
A higher oscillator frequency allows smaller capacitors to be used for equivalent output resistance and ripple, but
increases the typical supply current from 0.3 mA to 1.3 mA.
8
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