OPERATING
CONSIDERATIONS
PB50
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
OP07
-
22p
22p
741
-
18p
10p
LF155
-
4.7p
10p
LF156
-
4.7p
10p
TL070
22p
15p
10p
For: R
F
= 33K, R
I
= 3.3K, R
G
= 22K
FPBW
4kHz
20kHz
60kHz
80kHz
80kHz
SR
1.5
7
>60
>60
>60
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
CL
) must be
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
L
=
.65/R
CL
+ .010, –I
L
= .65/R
CL
.
Table 1:
Typical values for case where op amp effective gain = 1.
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.
Figure 2. Non-inverting composite amplifier.
GAIN SET
R
G
= [ (Av-1) • 3.1K] – 6.2K
R + 6.2K
Av =
G
+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
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.
This data sheet has been carefully checked and is believed to be reliable,
SHANNON ROAD •
is assumed
ARIZONA 85741
omissions. All specifications are subject
1 (800) 546-2739
APEX MICROTECHNOLOGY CORPORATION • 5980 NORTH
however, no responsibility
TUCSON,
for possible inaccuracies
•
or
USA • APPLICATIONS HOTLINE:
to change without notice.
4
PB50U REV I OCTOBER 2006 © 2006 Apex Microtechnology Corp.
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