Datasheet
Pin Description
Pin Name Function
Voltage Inverter Voltage Doubler
1 FC Frequency control for internal oscillator: Same as inverter.
FC
=
open, f
OSC
=
10 kHz (typ);
FC
=
V+, f
OSC
=
80 kHz (typ);
FC has no effect when OSC pin is driven externally.
2 CAP+ Connect this pin to the positive terminal of
charge-pump capacitor.
Same as inverter.
3 GND Power supply ground input. Power supply positive voltage input.
4 CAP− Connect this pin to the negative terminal of
charge-pump capacitor.
Same as inverter.
5 OUT Negative voltage output. Power supply ground input.
6 LV 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.
LV must be tied to OUT.
7 OSC 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.
Same as inverter except that OSC cannot be driven
by an external clock.
8 V+ Power supply positive voltage input. Positive voltage output.
Circuit Description
The MAX660 contains four large CMOS switches which are
switched in a sequence to invert the input supply voltage.
Energy transfer and storage are provided by external capaci-
tors.
Figure 2
illustrates the voltage conversion scheme.
When S
1
and S
3
are closed, C
1
charges to the supply volt-
age 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 MAX660 is to generate a negative
supply voltage. The voltage inverter circuit uses only two ex-
ternal capacitors as shown in the TypicalApplication 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 sup-
ply 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 MAX660 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 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
.
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
xC
1
) term.
Once this term is trivial compared with R
SW
and ESRs, fur-
ther 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
:
DS100898-21
FIGURE 2. Voltage Inverting Principle
MAX660
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