PB51 Datasheet, PDF (4/4 Pages) Cirrus Logic – POWER OPERATIONAL AMPLIFIER

PB51
Product Innova tionFrom
GENERAL
Please read Application 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.
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 0.33 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_L=.65/R_CL+ .010, -I_L = .65/R_CL.
SAFE OPERATING AREA
SOA
3
2
1
.5
.4
.3
steady
stasteteTady
C₌
steady
state _T
125°C^{C =}
state
85°C
t _{= 100ms}
T
C _{= 25°C}
.2
.1
10 20 30 40 50 100 200 300
SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE, V_S –V_O (V)
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.
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 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_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
C_CH
+Vs
R_CL
OP
IN
AMP
OUT
V_IN
COM ^PB51 _COMP
–15V
C_C
R_L
–Vs
GAIN _RG
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.
4
PB51U