When the switch is in position A, capacitor C
FLY
will charge to
voltage V
1
. The total charge on capacitor C
FLY
is Q
1
= C
FLY
x
V
1
. The switch then moves to position B, discharging C
FLY
to
voltage V
2
. After this discharge, the charge on C
FLY
will be
Q
2
= C
FLY
x V
2
. Note that the charge has been transferred
from the source V
1
to the output V
2
. The amount of charge
transferred is:
(1)
When the switch changes between A and B at a frequency f,
the charge transfer per unit time, or current is:
(2)
The switched capacitor network can be replaced by an equiv-
alent resistor, as indicated in Figure 5.
20173032
FIGURE 5. Switched Capacitor Equivalent Circuit
The value of this resistor is dependent on both the capacitor
value and the switching frequency as given in Equation 3
(3)
The value for R
EQ
can be calculated from Equation 3 and is
given in Equation 4
(4)
Equation 4 show that the value for the resistance at an in-
creased internal switching frequency, allows a lower value for
the used capacitor.
Key Specification
The key specifications for the LM7705 are given in the fol-
lowing overview:
Supply Voltage
The LM7705 will operate over a supply
voltage range of 3V to 5.25V, and meet
the specifications given in the Electrical
Table. Supply voltage lower than 3.3 Volt
will decrease performance (The output
voltage will shift towards zero, and the
current sink capabilities will decrease) A
voltage higher than 5.25V will exceed the
Abs Max ratings and therefore damage
the part.
Output Voltage/
Line Regulation
The fixed and regulated output voltage of
−0.23 V has tight limits, as indicated in the
Electrical Characteristics table, to guar-
antee a stable voltage level. The usage of
the pre- and post regulator in combination
with the charge pump inverter ensures
good line regulation of 0.29%/V
Output current/
Load regulation
The LM7705 can sink currents > 26 mA,
causing an output voltage shift to −200
mV. A specified load-regulation of 0.14%
mA/V ensures a minor voltage deviation
for load current up to 20 mA.
Quiescent
current
The LM7705 consumes a quiescent
current less than 100 µA. Sinking a load
current, will result in a current conversion
efficiency better than 90%, even for load
currents of 1 mA, increasing to 98% for a
current of 5mA.
In the next section a general amplifier application requiring a
true-zero output, will be discussed, showing an increased
performance using the LM7705.
GENERAL AMPLIFIER APPLICATION
This section will discuss a general DC coupled amplifier ap-
plication. First, one of the limitations of a DC coupled amplifier
is discussed. This is illustrated with two application examples.
A solution is a given for solving this limitation by using the
LM7705.
Due to the architecture of the output stage of general ampli-
fiers, the output transistors will saturate. As a result, the output
of a general purpose op amp can only swing to a few 100 mV
of the supply rails. Amplifiers using CMOS technology do
have a lower output saturation voltage. This is illustrated in
Figure 6. E.g. National Semiconductors LM7332 can swing to
200 mV to the negative rail, for a 10 k
Ω
load, over all tem-
peratures.
www.national.com
10
LM7705
