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PDF AN1082 Data sheet ( Hoja de datos )
| Número de pieza | AN1082 | |
| Descripción | Using the ISL6401 RSLIC PWM Controller Evaluation Board | |
| Fabricantes | Intersil Corporation | |
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® Analog Switch and Multiplexer Applications
Application Note
August 2002
AN1034
High Current Switching
Analog switches are sometimes required to conduct
appreciable amounts of current, either continuous, or
instantaneous - such as charging or discharging a capacitor.
For best reliability, it is recommended that instantaneous
current be limited to less than 80mA peak and that average
power over any 100 millisecond period be limited to
≤ I2 x RON (absolute maximum derated power- quiescent
power). Note that RON increases at high current levels,
which is characteristic of any FET switch. Switching
elements may be connected in parallel to reduce RON.
Op Amp Switching Applications
When analog switches are used either to select an op amp
input, or to change op amp gain, minor circuit
rearrangements can frequently enhance accuracy. In
Figure 1, RON of the input selector switch adds to R1,
reducing gain and allowing gain to change with temperature.
By switching into a non inverting amplifier (b), gain change
becomes negligible. Similarly, in a gain switching circuit,
RON is part of the gain determining network in (c), but has
neglible effect in (d).
-
+
(a) LOW ACCURACY
+
-
+
-
(b) HIGH ACCURACY
+
-
(c) LOW ACCURACY
(d) HIGH ACCURACY
FIGURE 1. OP AMP SWITCHING APPLICATIONS
Switching Spikes And Charge Injection
Transient effects when turning a switch off or on are of
concern in certain applications. Short duration spikes are
generated (Figure 2 (a)) as a result of capacitive coupling
between digital signals and the analog output. These have
the effect of creating an acquisition time interval during
which the output level is invalid even when little or no steady
state level change is involved. The total net energy (charge
injection) coupled to the analog circuit is of concern when
switching the voltage on a capacitor, since the injected
charge will change the capacitor voltage at the instant the
switch is opened (Figure 2 (b)).
V =QC
SCOPE
1MΩ, 20pF
SW
CLOSED
OPEN
(a) (b)
FIGURE 2. SWITCHING SPIKES AND CHARGE INJECTION
Charge injection is measured in picocoulombs; the voltage
transferred to the capacitor is computed by:
V = Charge (pC)
Capacitance (pF)
Both of these effects are, in general, considerably less for
CMOS switches than for equivalent resistance JFET or
PMOS devices, since the gate drive signals for the two
switching transistors are of opposite polarity. However,
complete cancellation is not possible, since the N and P
channel switches do not receive gate signals quite
simultaneously, and their geometries are necessarily
different to achieve the desired DC resistance match.
In applications where transients create a problem, it is
frequently possible to minimize the effect by cancellation in a
differential circuit, similar to Figure 3.
SIGNAL
RS
R=
RS
DIFFERENTIAL
AMPLIFIER
FIGURE 3. DIFFERENTIAL CIRCUIT
Among the analog switches, the Hl-201 is the best from the
transient standpoint, having turn-on spikes of about 100mV
peak, 50ns width at the 50% point, and charge injection at
turn-off of about 20 picocoulombs. Transients of the HI-504X
series are several times higher.
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 321-724-7143 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright © Intersil Americas Inc. 2002. All Rights Reserved
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