TLE2662 Datasheet, PDF (31/40 Pages) Texas Instruments – DUAL uPOWER JFET-INPUT OPERATIONAL AMPLIFIER WITH SWITCHED-CAPACITOR VOLTAGE CONVERTER

TLE2662
DUAL µPOWER JFET-INPUT OPERATIONAL AMPLIFIER
WITH SWITCHED-CAPACITOR VOLTAGE CONVERTER
SLOS118B – DECEMBER 1992 – REVISED AUGUST 1994
APPLICATION INFORMATION
switched-capacitor function
A review of a basic switched-capacitor building block is helpful in understanding the operation of the TLE2662.
When the switch shown in Figure 59 is in the left position, capacitor C1 charges to the voltage at V1. The total
charge on C1 is q1 = C1V1. When the switch is moved to the right, C1 is discharged to the voltage at V2. After
this discharge time, the charge on C1 is q2 = C1V2. The charge has been transferred from the source V1 to
the output V2. The amount of charge transferred is as shown in equation 1.
∆q = q1 – q2 = C1(V1 – V2)
(1)
If the switch is cycled f times per second, the charge transfer per unit time (i.e., current) is shown in equation 2.
I = f x ∆q = f x C1(V1 – V2)
(2)
To obtain an equivalent resistance for a switched-capacitor network, this equation can be rewritten in terms of
voltage and impedance equivalence as shown in equation 3.
+ _ń* + ^* I
V1 V2
(1 fC1)
V1 V2
REQUIV
(3)
V1
V2
f
RL
C1
C2
Figure 59. Switched-Capacitor Block
A new variable, R_EQUIV, is defined as R_EQUIV = 1 ÷ fC1. The equivalent circuit for the switched-capacitor network
is as shown in Figure 60. The TLE2662 has the same switching action as the basic switched-capacitor voltage
converter. Even though this simplification does not include finite switch-on resistance and output-voltage ripple,
it provides an insight into how the device operates.
These simplified circuits explain voltage loss as a function of oscillator frequency (see Figure 43). As oscillator
frequency is decreased, the output impedance is eventually dominated by the 1/fC1 term and voltage losses
rise.
Voltage losses also rise as oscillator frequency increases. This is caused by internal switching losses that occur
due to some finite charge being lost on each switching cycle. This charge loss per-unit-cycle, when multiplied
by the switching frequency, becomes a current loss. At high frequency, this loss becomes significant and voltage
losses again rise. The oscillator of the TLE2662 switched-capacitor section is designed to run in the frequency
band where voltage losses are at a minimum.
REQUIV
V1
REQUIV
=
1
fC1
C2
V2
RL
Figure 60. Switched-Capacitor Equivalent Circuit
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