APEX MICROTECHNOLOGY CORPORATION • 5980 NORTH SHANNON ROAD • TUCSON, ARIZONA 85741 • USA • APPLICATIONS HOTLINE: 1 (800) 546-2739

OPERATING

CONSIDERATIONS

PB58 • PB58A

GENERAL

Please read the “General Operating Considerations” sec-

tion, which covers stability, supplies, heatsinking, mounting,

current limit, SOA interpretation, and specification interpreta-

tion. Additional information can be found in the application

notes. For information on the package outline, heatsinks, and

mounting hardware, consult the “Accessory and Package

Mechanical Data” section of the handbook.

CURRENT LIMIT

For proper operation, the current limit resistor (R

connected as shown in the external connection diagram. The

minimum value is 0.33

Ω with a maximum practical value of

47

Ω. For optimum reliability the resistor value should be set as

high as possible. The value is calculated as follows: +I

R

SAFE OPERATING AREA (SOA)

NOTE: The output stage is protected against transient flyback.

However, for protection against sustained, high energy flyback, exter-

nal fast-recovery diodes should be used.

COMPOSITE AMPLIFIER CONSIDERATIONS

Cascading two amplifiers within a feedback loop has many

advantages, but also requires careful consideration of several

amplifier and system parameters. The most important of these

are gain, stability, slew rate, and output swing of the driver.

Operating the booster amplifier in higher gains results in a

higher slew rate and lower output swing requirement for the

driver, but makes stability more difficult to achieve.

GAIN SET

R

3.1K] – 6.2K

R

Av =

+1

3.1K

The booster’s closed-loop gain is given by the equation

above. The composite amplifier’s closed loop gain is deter-

mined by the feedback network, that is: –Rf/Ri (inverting) or

1+Rf/Ri (non-inverting). The driver amplifier’s “effective gain”

is equal to the composite gain divided by the booster gain.

Example: Inverting configuration (figure 1) with

R i = 2K, R f = 60K, R g = 0 :

Av (booster) = (6.2K/3.1K) + 1 = 3

Av (composite) = 60K/2K = – 30

Av (driver) = – 30/3 = –10

STABILITY

Stability can be maximized by observing the following guide-

lines:

1. Operate the booster in the lowest practical gain.

2. Operate the driver amplifier in the highest practical effec-

tive gain.

3. Keep gain-bandwidth product of the driver lower than the

closed loop bandwidth of the booster.

4. Minimize phase shift within the loop.

A good compromise for (1) and (2) is to set booster gain from

3 to 10 with total (composite) gain at least a factor of 3 times

booster gain. Guideline (3) implies compensating the driver as

required in low composite gain configurations. Phase shift

within the loop (4) is minimized through use of booster and loop

compensation capacitors Cc and Cf when required. Typical

values are 5pF to 33pF.

Stability is the most difficult to achieve in a configuration

where driver effective gain is unity (ie; total gain = booster

gain). For this situation, Table 1 gives compensation values for

optimum square wave response with the op amp drivers listed.

DRIVER

C

CH

C

F

C

C

FPBW

SR

OP07

-

22p

22p

4kHz

1.5

741

-

18p

10p

20kHz

7

LF155

-

4.7p

10p

60kHz

>60

LF156

-

4.7p

10p

80kHz

>60

TL070

22p

15p

10p

80kHz

>60

For: R

Table 1: Typical values for case where op amp effective gain = 1.

Figure 2. Non-inverting composite amplifier.

SLEW RATE

The slew rate of the composite amplifier is equal to the slew

rate of the driver times the booster gain, with a maximum value

equal to the booster slew rate.

OUTPUT SWING

The maximum output voltage swing required from the driver

op amp is equal to the maximum output swing from the booster

divided by the booster gain. The Vos of the booster must also

be supplied by the driver, and should be subtracted from the

available swing range of the driver. Note also that effects of

Vos drift and booster gain accuracy should be considered

when calculating maximum available driver swing.

t =

100ms

steady

state

Tc

=

25°C

steady

state

Tc

=

85°C

steady

state

Tc

=

125°C

3

2

1

.2

.3

.4

.5

10

20

30 40 50

100

200 300

SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE, Vs –Vo (V)

.1

OP

AMP

PB58

+15V

–15V

IN

COM

+Vs

–Vs

OUT

GAIN

COMP

C

CH

*

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.

PB58U REV. H JANUARY 1998

© 1998 Apex Microtechnology Corp.