PB58 Datasheet, PDF (4/4 Pages) List of Unclassifed Manufacturers – POWER BOOSTER AMPLIFIERS

PB58 • PB58A
OPERATING
CONSIDERATIONS
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_CL) must be
connected as shown in the external connection diagram. The
minimum value is 0.33Ω 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.
SAFE OPERATING AREA (SOA)
3
2
1
.5
.4
.3
steady
stasteteTacdy=
steady
state _Tc
125°C
=
t _{= 100ms}
state _Tc
85°C
=
25°C
.2
.1
10 20 30 40 50 100 200 300
SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE, Vs –Vo (V)
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_G = [ (Av-1) _* 3.1K] – 6.2K
Av = ^{RG + 6.2K} +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
OP07
741
LF155
LF156
TL070
C_CH
-
-
-
-
22p
C_F
C_C FPBW SR
22p 22p 4kHz 1.5
18p 10p 20kHz 7
4.7p 10p 60kHz >60
4.7p 10p 80kHz >60
15p 10p 80kHz >60
For: R_F = 33K, R_I = 3.3K, R_G = 22K
Table 1: Typical values for case where op amp effective gain = 1.
C_F
R_I
+15V R_{F +Vs}
C_CH
R _CL
OP
IN
AMP
OUT
V_IN
COM PB58
COMP
–15V
C_C
R_L
–Vs
GAIN _{R G}
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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PB58U REV. H JANUARY 1998 © 1998 Apex Microtechnology Corp.