Application Information
This applications section will give a description of the func-
tionality of the LM7705. The LM7705 is a switched capacitor
voltage inverter with a low noise, −0.23V fixed negative bias
output. The part will operate over a supply voltage range of 3
to 5.25 Volt. Applying a logical low level to the SD input will
activate the part, and generate a fixed −0.23V output voltage.
The part can be disabled; the output is switched to ground
level, by applying a logical high level to the SD input of the
part.
FUNCTIONAL DESCRIPTION
The LM7705, low noise negative bias generator, can be used
for many applications requiring a fixed negative voltage. A key
application for the LM7705 is an amplifier with a true zero
output voltage using the original parts, while not exceeding
the maximum supply voltage ratings of the amplifier.
The voltage inversion in the LM7705 is achieved using a
switched capacitor technique with two external capacitors
(C
FLY
and C
RES
). An internal oscillator and a switching net-
work transfers charge between the two storage capacitors.
This switched capacitor technique is given in Figure 1.
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FIGURE 1. Voltage Inverter
The internal oscillator generates two anti-phase clock signals.
Clock 1 controls switches S1 and S2. Clock 2 controls switch-
es S3 and S4. When Switches S1 and S2 are closed, capac-
itor C
FLY
is charged to V
+
. When switches S3 and S4 are
closed (S1 and S2 are open) charge from C
FLY
is transferred
to C
RES
and the output voltage OUT is equal to -V
+
.
Due to the switched capacitor technique a small ripple will be
present at the output voltage, with a frequency of the oscilla-
tor. The magnitude of this ripple will increase for increasing
output currents. The magnitude of the ripple can be influenced
by changing the values of the used capacitors.
In the next section a more detailed technical description of the
LM7705 will be given.
TECHNICAL DESCRIPTION
As indicated in the functional description section, the main
function of the LM7705 is to supply a stabilized negative bias
voltage to a load, using only a positive supply voltage. A gen-
eral block diagram for this charge pump inverter is given in
Figure 2. The external power supply and load are added in
this diagram as well.
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FIGURE 2. LM7705 Architecture
The architecture given in Figure 2 shows that the LM7705
contains 3 functional blocks:
•
Pre-regulator
•
Charge pump inverter
•
Post-regulator
The output voltage is stabilized by:
•
Controlling the power supplied from the power supply to
the charge pump input by the pre-regulator
•
The power supplied from the charge pump output to the
load by the post-regulator.
A more detailed block diagram of the negative bias generator
is given in Figure 3. The control of the pre-regulator is based
on measuring the output voltage of the charge pump. The goal
of the post-regulator is to provide an accurate controlled neg-
ative voltage at the output, and acts as a low pass filter to
attenuate the output voltage ripple. The voltage ripple is a re-
sult of the switching behavior of the charge pump and is
dependent of the output current and the values of the used
capacitors.
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FIGURE 3. Charge Pump Inverter with Input/Output
Control
In the next section a simple equation will be derived, that
shows the relation between the ripple of the output current,
the frequency of the internal clock generator and the value of
the capacitor placed at the output of the LM7705.
Charge Pump Theory
This section uses a simplified but realistic equivalent circuit
that represents the basic function of the charge pump. The
schematic is given in Figure 4.
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FIGURE 4. Charge Pump
9
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LM7705
