Datasheet

R
EQ
=
1
f C
FLY
¹
·
©
§
I =
R
EQ
1
f C
FLY
¹
·
©
§
V1 ± V2
V1 ± V2
=
V
1
V
2
R
L
R
EQ
C
RES
LM7705
www.ti.com SNVS420B –NOVEMBER 2008–REVISED MARCH 2013
Figure 26. 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 increased internal switching frequency, allows a lower
value for the used capacitor.
Key Specification
The key specifications for the LM7705 are given in the following 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 ensure 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 application. 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 amplifiers, 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 27. E.g. Texas
Instruments' LM7332 can swing to 200 mV to the negative rail, for a 10 kΩ load, over all temperatures.
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