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

4

OPERATING

CONSIDERATIONS

PB50

GENERAL

Please read Application Note 1 "General Operating Con-

siderations" which covers stability, supplies, heat sinking,

mounting, current limit, SOA interpretation, and specification

interpretation. Visit www.apexmicrotech.com for design tools

that help automate tasks such as calculations for stability,

internal power dissipation, current limit; heat sink selection;

Apex’s complete Application Notes library; Technical Seminar

Workbook; and Evaluation Kits.

CURRENT LIMIT

For proper operation, the current limit resistor (R

connected as shown in the external connection diagram. The

minimum value is 0.27Ω with a maximum practical value of

47Ω. For optimum reliability the resistor value should be set

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

.65/R

SAFE OPERATING AREA (SOA)

NOTE: The output stage is protected against transient fly-

back. However, for protection against sustained, high energy

flyback, external 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

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 determined

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

guidelines:

1. Operate the booster in the lowest practical gain.

2. Operate the driver amplifier in the highest practical effective

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.

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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.

PB50U REV H OCTOBER 2004 © 2004 Apex Microtechnology Corp.

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