AN1082 Intersil Corporation, AN1082 Datasheet

AN1082

Manufacturer Part Number

AN1082

Description

Using the ISL6401 RSLIC PWM Controller Evaluation Board

Manufacturer

Intersil Corporation

Datasheet

1.AN1082.pdf (3 pages)

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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

power). Note that R

which is characteristic of any FET switch. Switching

elements may be connected in parallel to reduce R

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, R

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,

neglible effect in (d).

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

x R

is part of the gain determining network in (c), but has

FIGURE 1. OP AMP SWITCHING APPLICATIONS

(a) LOW ACCURACY

(absolute maximum derated power- quiescent

of the input selector switch adds to R

increases at high current levels,

(b) HIGH ACCURACY

(d) HIGH ACCURACY

Application Note

1-888-INTERSIL or 321-724-7143

CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.

Analog Switch and Multiplexer Applications

charge will change the capacitor voltage at the instant the

switch is opened (Figure 2 (b)).

Charge injection is measured in picocoulombs; the voltage

transferred to the capacitor is computed by:

V = Charge (pC)

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.

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.

FIGURE 2. SWITCHING SPIKES AND CHARGE INJECTION

Capacitance (pF)

SIGNAL

August 2002

Intersil (and design) is a registered trademark of Intersil Americas Inc.

(a)

FIGURE 3. DIFFERENTIAL CIRCUIT

1M , 20pF

SCOPE

R =

CLOSED

OPEN

(b)

DIFFERENTIAL

AMPLIFIER

AN1034

V =

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AN1082 Summary of contents

Page 1

Application Note 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 recommended that instantaneous current be limited ...

Page 2

High Frequency Switching When considering a switching element for RF or video type information, two factors must be watched: attenuation vs. frequency characteristics switch, and feedthrough (isolation) vs. frequency characteristics of the OFF switch. Optimizing the first ...

Page 3

... Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use ...

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