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OPERATING
CONSIDERATIONS
PB51 • PB51A
GENERAL
STABILITY
PleasereadApplicationNote1"GeneralOperatingConsider-
ations" which covers stability, supplies, heat sinking, mounting,
current limit, SOA interpretation, and specification interpreta-
tion. Visit www.apexmicrotech.com for design tools that help
automatetaskssuchascalculationsforstability, internalpower
dissipation, current limit; heat sink selection; Apex’s complete
Application Notes library; Technical Seminar Workbook; and
Evaluation Kits.
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.
Stabilityisthemostdifficulttoachieveinaconfigurationwhere
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.
CURRENT LIMIT
For proper operation, the current limit resistor (RCL) must be
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:
+IL=.65/RCL+ .010, -IL = .65/RCL.
SAFE OPERATING AREA
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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.
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COMPOSITE AMPLIFIER CONSIDERATIONS
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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.
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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.
GAIN SET
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OUTPUT SWING
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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
Vosdriftandboostergainaccuracyshouldbeconsideredwhen
calculating maximum available driver swing.
The booster’s closed-loop gain is given by the equation
above.Thecompositeamplifier’sclosedloopgainisdetermined
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
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.
4APEX MICROTECHNOLOGY CORPORATION • 5980 NORTH SHANNON ROAD • TUCSON, ARIZONA 85741 • USA • APPLICATIONS HOTLINE: 1 (800) 546-2739
PB51U REV D OCTOBER 2004 © 2004 Apex Microtechnology Corp.
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