PA26 查看數據表（PDF） - Apex Microtechnology

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PA26

Power dual operational amplifier

Apex Microtechnology 

OPERATING

CONSIDERATIONS

PA21/25/26 • PA21A/25A

ADDITIONAL PA26 PIN FUNCTIONS

VBOOST

The VBOOST pin is the positive terminal for the load of the

second stage of the amplifier. When that terminal is connected

to a voltage greater than +VS it will provide more drive to the

upper output transistor, which is a darlington connected emit-

ter follower. This will better saturate the output transistor.

When VBOOST is about 5 Volts greater than +VS the positive

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

improvement as is possible.

VBOOST pin requires approximately 10–12mA of current.

Dynamically it represents 1K Ω impedance. The maximum

voltage that can be applied to VBOOST is 40 volts with respect to

–VS . There is no limit to the difference between +VS and VBOOST.

+VS ≤ 20V

DB1

DB2

7 3 10

CB1

PA26A

5

CB2

SPEAKER

PA26B

8

FIGURE 3. SIMPLE BOOTSTRAPPING IMPROVES POSITIVE

OUTPUT SWING. CONNECT PINS 3 AND 10 TO VS IF NOT

USED. TYPICAL CURRENTS ARE 12mA EACH.

Figure 3 shows a bootstrap which dynamically couples the

output waveform onto the VBOOST pin. This causes VBOOST to

swing positive from it's initial value, which is equal to +VS -0.7 V

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

words, if VBOOST was initially 19.3, and the output swings

positive 18 Volts, the voltage on the VBOOST pin will swing to 19.3

-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.

ISENSE

The ISENSE pin is in series with the negative half of the output

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 ISENSE pin will have 1A of current flow, if +1A flows

in the output the ISENSE pin will have 0 current flow).

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.

+VS

PA26

VBIAS

B

R

RL IL

VIN

A

R

RFB

RIN

RS

RS RIN RFB

–VS OR GND

VREF

FIGURE 4. ISENSE TRANSCONDUCTANCE BRIDGING

AMPLIFIER

Figure 4 shows the PA26 ISENSE feature being used to obtain

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

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

sensing resistors RS is applied to the summing network and

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:

IL= (VIN – VREF) *RIN/ RFB/ Rs

In the illustration, resistors RIN, RFB and RS determine gain.

VBIAS should be set midway between +Vs and -Vs, Vref is

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.

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PA21/25/26U REV. G FEBRUARY 2000 © 2000 Apex Microtechnology Corp.

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