APEX MICROTECHNOLOGY CORPORATION • TELEPHONE (520) 690-8600 • FAX (520) 888-3329 • ORDERS (520) 690-8601 • EMAIL prodlit@apexmicrotech.com

ADDITIONAL PA26 PIN FUNCTIONS

This data sheet has been carefully checked and is believed to be reliable, however, no responsibility is assumed for possible inaccuracies or omissions. All specifications are subject to change without notice.

PA21/25/26U REV. G FEBRUARY 2000

© 2000 Apex Microtechnology Corp.

Figure 3 shows a bootstrap which dynamically couples the

output waveform onto the V

swing positive from it's initial value, which is equal to +V

(one diode drop), an amount equal to the output. In other

words, if V

positive 18 Volts, the voltage on the V

-0.7 + 18 or 36.6. The capacitor needs to be sized based on a

1K

Ω impedance and the lowest frequency required by the

circuit. For example, 20Hz will require > 8uF.

The I

stage only. Current will flow through this pin only when nega-

tive current is being outputted. The current that flows in this pin

is the same current that flows in the output (if –1A flows in the

output, the I

in the output the I

The resistor choice is arbitrary and is selected to provide

whatever voltage drop the engineer desires, up to a maximum

of 1.0 volt. However, any voltage dropped across the resistor

will subract from the swing to rail. For instance, assume a +/–

12 volt power supply and a load that requires +/–1A. With no

current sense resistor the output could swing +/–10.2 volts. If

a 1

Ω resistor is used for current sense (which will drop 1 Volt

at 1 Amp) then the output could swing +10.2, –9.2 Volts.

Figure 4 shows the PA26 I

a Transconductance function. In this example, amplifier "A" is

the master and amplifier "B" is the slave. Feedback from

sensing resistors R

scaled to the inverting input of amplifier "A" where it is com-

pared to the input voltage. The current sensing feedback

imparts a Transconductance feature to the amplifiers transfer

function. In other words, the voltage developed across the

sensing resistors is directly proportional to the output current.

Using this voltage as a feedback source allows expressing the

gain of the circuit in amperes vs input voltage. The transfer

funcion is approximately:

I

In the illustration, resistors R

usually ground in dual supply systems or used for level

translation in single supply systems.

MOUNTING PRECAUTIONS

1. Always use a heat sink. Even unloaded, the PA26 can

dissipate up to 3.6 watts. A thermal washer or thermal

grease should always be used.

2. Avoid bending the leads. Such action can lead to internal

damage.

3. Always fasten the tab to the heat sink before the leads are

soldered to fixed terminals.

4. Strain relief must be provided if there is any probability of

axial stress to the leads.

10

3

7

5

8

PA26A

PA26B

+V

D

B1

D

B2

C

B1

SPEAKER

C

B2

FIGURE 3. SIMPLE BOOTSTRAPPING IMPROVES POSITIVE

The V

second stage of the amplifier. When that terminal is connected

to a voltage greater than +V

upper output transistor, which is a darlington connected emit-

ter follower. This will better saturate the output transistor.

When V

output can swing 0.5 Volts closer to the rail. This is as much

improvement as is possible.

V

BOOST

pin requires approximately 10–12mA of current.

Dynamically it represents 1K

Ω impedance. The maximum

voltage that can be applied to V

–V

OPERATING

CONSIDERATIONS

PA21/25/26 • PA21A/25A

A

B

+VS

VIN

VREF

–VS OR GND

RS

RS

IL

RL

RIN

RIN

R

PA26

R

RFB

AMPLIFIER