Datasheet

B
fzc fpc1
FREQUENCY
GAIN (dB)
fpc
(0Hz)
-20dB/dec
-
20
dB
/
dec
0dB/dec
-60
-40
-20
0
20
0.01 0.1 1 10 100
FREQUENCY (kHz)
GAIN (dB)
1000
0
-45
-90
-135
-180
PHASE (º)
fp
fz
fn
LM26003
www.ti.com SNVS576D –AUGUST 2008–REVISED MARCH 2013
COMPENSATION
The purpose of loop compensation is to ensure stable operation while maximizing dynamic performance. Stability
can be analyzed with loop gain measurements, while dynamic performance is analyzed with both loop gain and
load transient response. Loop gain is equal to the product of control-output transfer function (power stage) and
the feedback transfer function (the compensation network).
For stability purposes, our target is to have a loop gain slope that is -20dB /decade from a very low frequency to
beyond the crossover frequency. Also, the crossover frequency should not exceed one-fifth of the switching
frequency, i.e. 60 kHz in the case of 300 kHz switching frequency.
For dynamic purposes, the higher the bandwidth, the faster the load transient response. The downside to high
bandwidth is that it increases the regulators susceptibility to board noise which ultimately leads to excessive
falling edge jitter of the switch node voltage.
A large DC gain means high DC regulation accuracy (i.e. DC voltage changes little with load or line variations).
To achieve this loop gain, the compensation components should be set according to the shape of the control-
output bode plot. A typical plot is shown in Figure 20 below.
Figure 20. Control-Output Transfer Function
The control-output transfer function consists of one pole (fp), one zero (fz), and a double pole at fn (half the
switching frequency).
Referring to Figure 20, the following should be done to create a -20dB /decade roll-off of the loop gain:
1. Place a pole at 0Hz (fpc)
2. Place a zero at fp (fzc)
3. Place a second pole at fz (fpc1)
The resulting feedback (compensation) bode plot is shown below in Figure 21. Adding the control-output
response to the feedback response will then result in a nearly continuous -20db/decade slope.
Figure 21. Feedback Transfer Function
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