TLE2682 Datasheet, PDF (43/49 Pages) Texas Instruments – HIGH-SPEED JFET-INPUT DUAL OPERATIONAL AMPLIFIER WITH SWITCHED-CAPACITOR VOLTAGE CONVERTER

TLE2682
HIGHĆSPEED JFETĆINPUT DUAL OPERATIONAL AMPLIFIER
WITH SWITCHEDĆCAPACITOR VOLTAGE CONVERTER
SLOS127 − JUNE 1993
APPLICATION INFORMATION
switched-capacitor section
A review of a basic switched-capacitor building block is helpful in understanding the operation of the TLE2682.
When the switch shown in Figure 101 is in the left position, capacitor C1 charges to the voltage at V1. The total
charge on C1 is q1 = C1 × V1. 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 = C1 × V2. 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 as 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.
I
+
V1 * V2
(1ńf C1)
+
V1 * V2
REQUIV
(3)
V1
V2
f
RL
C1
C2
Figure 101. Switched-Capacitor Block
A new variable, R_EQUIV, is defined as R_EQUIV = 1 ÷ f × C1. The equivalent circuit for the switched-capacitor
network is as shown in Figure 102. The TLE2682 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 66). As oscillator
frequency is decreased, the output impedance is eventually dominated by the 1/f × C1 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 TLE2682 switched-capacitor section is designed to run in the frequency band where voltage
losses are at a minimum.
REQUIV
V1
REQUIV
=1
f × C1
C2
V2
RL
Figure 102. Switched-Capacitor Equivalent Circuit
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