Texas Instruments DIYAMP-SOT23-EVM, User Manual, Page: 22 / 35

iso Load iso Load
1
1 1
6R C 10R C
C
R R
´ ´
£ £
iso
ZERO Load
1
R
2 f C
=
p ´ ´
Schematic and PCB Layout
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22 SBOU191 – July 2017
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DIYAMP-SOT23-EVM
In situations where stability is affected by capacitive loads, the R
iso
dual-feedback configuration has the
ability to stabilize the circuit by compensating the contribution of the capacitive load to circuit instability.
This capacitive load compensation technique uses an isolation resistor that compensates the circuit by
adding a zero to cancel the pole from the output impedance and capacitive load. Refer to the
TI Precision
Labs - Op Amps: Stability 5 video for detailed information on this technique.
The design steps for the R
iso
method follow:
1. Use TINA-TI™ to find the zero frequency, f
ZERO
, where A
OL_Loaded
= 20 dB (example shown in Figure 37 ).
Figure 37. Example of f
ZERO
, Where A
OL_Loaded
= 20 dB
2. Calculate R
iso
to set the zero at f
ZERO
– this will yield between 60° and 90° of phase margin
where
• R
iso
= R3
• C
Load
= C4 (25)
While the R
iso
circuit is both simple to implement and design, it has a big disadvantage in precision
circuits. The voltage drop from R
iso
is dependent on the output current or output load, and may be
significant compared to the desired signal.
The second capacitive load compensation technique uses the R
iso
with dual-feedback stability
compensation method. The R
iso
dual-feedback circuit solves the voltage drop disadvantage of the
previously stated R
iso
. Refer to the TI Precision Labs - Op Amps: Stability 6 video for detailed information
on this technique.
Design steps for the R
iso
method follow:
1. R
iso
using Method 1: R
iso
techniques
2. Set R1: R
1
≥
(R
iso
× 100)
3. Set C
1
:
Using this range ensures that the two feedback paths, R
2
and C
3
, will never create a resonance that would
cause instability. Smaller values of C3 will result in faster settling time at the expense of overshoot for
certain load ranges. While the R
iso
dual-feedback circuit solves the dc accuracy issue with the R
iso
circuit, it
has some disadvantages as well. The disadvantage of this method is that the circuit is not as tolerant to
changes in the output capacitance and can quickly become unstable. Therefore, the R
iso
dual-feedback
circuit is best for situations where the output capacitance is known and will not vary significantly. This
method generally results in a slower settling time than the R
iso
circuit as well.