Datasheet

-20dB/dec
(f
p1
is at zero frequency)
-20
dB
/
dec
FREQUENCY
GAIN (dB)
B
f
z1
f
z2
f
p2
10 100
1
k
10
k
100
k
1M
FREQUENCY
(Hz)
-60
-40
-20
0
20
GAIN
(dB)
0
-45
-90
-135
-180
PHASE
(
°
)
Asymptoti
c
Gain
Phas
e
LM5642, LM5642X
SNVS219K –JUNE 2003–REVISED APRIL 2013 www.ti.com
(28)
When using FETs in parallel, the same guidelines apply to the top FET as apply to the bottom FET.
Loop Compensation
The general purpose of loop compensation is to meet static and dynamic performance requirements while
maintaining stability. Loop gain is what is usually checked to determine small-signal performance. Loop gain is
equal to the product of control-output transfer function and the feedback transfer function (the compensation
network transfer function). Generally speaking it is desirable to have a loop gain slope that is roughly -20dB
/decade from a very low frequency to well beyond the crossover frequency. The crossover frequency should not
exceed one-fifth of the switching frequency. The higher the bandwidth, the faster the load transient response
speed will be. However, if the duty cycle saturates during a load transient, further increasing the small signal
bandwidth will not help. Since the control-output transfer function usually has very limited low frequency gain, it is
a good idea to place a pole in the compensation at zero frequency, so that the low frequency gain will be
relatively large. A large DC gain means high DC regulation accuracy (i.e. DC voltage changes little with load or
line variations). The rest of the compensation scheme depends highly on the shape of the control-output plot.
Figure 33. Control-Output Transfer Function
As shown in Figure 33, the control-output transfer function consists of one pole (fp), one zero (fz), and a double
pole at fn (half the switching frequency). The following can be done to create a -20dB /decade roll-off of the loop
gain: Place the first pole at 0Hz, the first zero at fp, the second pole at fz, and the second zero at fn. The
resulting feedback transfer function is shown in Figure 34.
Figure 34. Feedback Transfer Function
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