LM2660/LM2661
Pin Description
Pin
1
Name
Voltage Inverter
FC
(LM2660)
Frequency control for internal oscillator:
FC = open, f
OSC
= 10 kHz (typ);
FC = V+, f
OSC
= 80 kHz (typ);
FC has no effect when OSC pin is driven externally.
1
2
3
4
5
6
SD
(LM2661)
CAP+
GND
CAP−
OUT
LV
Shutdown control pin, tie this pin to the ground in
normal operation, and to V+ for shutdown.
Connect this pin to the positive terminal of
charge-pump capacitor.
Power supply ground input.
Connect this pin to the negative terminal of
charge-pump capacitor.
Negative voltage output.
Low-voltage operation input. Tie LV to GND when
input voltage is less than 3.5V. Above 3.5V, LV can
be connected to GND or left open. When driving
OSC with an external clock, LV must be connected
to GND.
Oscillator control input. OSC is connected to an
internal 15 pF capacitor. An external capacitor can
be connected to slow the oscillator. Also, an
external clock can be used to drive OSC.
Power supply positive voltage input.
Same as inverter.
Same as inverter.
Power supply positive voltage input.
Same as inverter.
Power supply ground input.
LV must be tied to OUT.
Function
Voltage Doubler
Same as inverter.
7
OSC
Same as inverter except that OSC cannot be driven
by an external clock.
8
V+
Positive voltage output.
Circuit Description
The LM2660/LM2661 contains four large CMOS switches
which are switched in a sequence to invert the input supply
voltage. Energy transfer and storage are provided by exter-
nal capacitors.
Figure 2
illustrates the voltage conversion
scheme. When S
1
and S
3
are closed, C
1
charges to the sup-
ply voltage V+. During this time interval switches S
2
and S
4
are open. In the second time interval, S
1
and S
3
are open
and S
2
and S
4
are closed, C
1
is charging C
2
. After a number
of cycles, the voltage across C
2
will be pumped to V+. Since
the anode of C
2
is connected to ground, the output at the
cathode of C
2
equals −(V+) assuming no load on C
2
, no loss
in the switches, and no ESR in the capacitors. In reality, the
charge transfer efficiency depends on the switching fre-
quency, the on-resistance of the switches, and the ESR of
the capacitors.
Application Information
SIMPLE NEGATIVE VOLTAGE CONVERTER
The main application of LM2660/LM2661 is to generate a
negative supply voltage. The voltage inverter circuit uses
only two external capacitors as shown in the Basic Applica-
tion 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 cir-
cuitry. This gives the best performance in low voltage appli-
cations. 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/
LM2661 for the LMC7660 Switched Capacitor Voltage Con-
verter.
The output characteristics of this circuit can be approximated
by an ideal voltage source in series with a resistor. The volt-
age 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:
where R
SW
is the sum of the ON resistance of the internal
MOS switches shown in
Figure 2.
DS012911-21
FIGURE 2. Voltage Inverting Principle
High value, low ESR capacitors will reduce the output resis-
tance. Instead of increasing the capacitance, the oscillator
frequency can be increased to reduce the 2/(f
osc
x C
1
) term.
Once this term is trivial compared with R
SW
and ESRs, fur-
ther increasing in oscillator frequency and capacitance will
become ineffective.
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