P r o d u c t I n n o v a t i o n F r o m

PB51

4

PB51U

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.

SLEWRATE

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.

OUTPUTSWING

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.

GENERAL

Please readApplication Note 1 "General Operating Consider-

ations" which covers stability, supplies, heat sinking, mounting,

current limit, SOA interpretation, and specification interpretation.

Visit www.Cirrus.com for design tools that help automate tasks

such as calculations for stability, internal power dissipation,

current limit; heat sink selection; Cirrus’s complete Application

Notes library;Technical SeminarWorkbook;and Evaluation Kits.

CURRENTLIMIT

For proper operation, the current limit resistor (R

connected as shown in the external connection diagram. The

minimum value is 0.33 with a maximum practical value of 47.

For optimum reliability the resistor value should be set as high

as possible. The value is calculated as follows:

+I

SAFEOPERATINGAREA

NOTE: The output stage is protected against transient flyback.

However, for protection against sustained, high energy fly-

back, external fast-recovery diodes should be used.

COMPOSITEAMPLIFIERCONSIDERATIONS

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.

GAINSET

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

t =

100ms

steady

state

T

C

= 25°C

steady

state

T

C

= 85°C

steady

state

T

C

= 125°C

3

2

1

.2

.3

.4

.5

10

20

30 40 50

100

200 300

SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE, V

.1

SOA

OP

AMP

PB51

C

F

R

F

R

I

+15V

–15V

IN

COM

+Vs

–Vs

R

CL

OUT

C

C

R

G

R

L

V

IN

GAIN

COMP

C

CH

FIGURE 2. NON-INVERTING COMPOSITE AMPLIFIER.

Av =

+1

3.1K

DRIVER

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

TABLE 1.TYPICAL VALUES FOR CASE WHERE OP AMP

EFFECTIVE GAIN = 1.