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ISL6740, ISL6741
40
30
20
10
0
-10
The higher the desired bandwidth of the converter, the more
difficult it is to create a solution that is stable over the entire
operating range. A good rule of thumb is to limit the bandwidth to
about f SW /4, where f SW is the switching frequency of the
converter. However, due to the bandwidth constraints of the opto-
coupler and the LM431 shunt regulator, the bandwidth was
reduced to about 25kHz.
The first pole is placed at the origin by default (C20 is an
integrating capacitor). If the two zeroes are placed at the same
frequency, they should be placed at f LC /2, where f LC is the
resonant frequency of the output L-C filter. To reduce the gain
peaking at the L-C resonant frequency, the two zeroes are often
-20
10
100 1?10 3 1?10 4 1?10 5
FREQUENCY (Hz)
FIGURE 21A. CONTROL-TO-OUTPUT GAIN
1?10 6
separated. When they are separated, the first zero may be placed
at f LC /5, and the second at just above f LC . The second pole is
placed at the lowest expected zero cause by the output capacitor
ESR. The third, and last pole is placed at about 1.5 times the
50
0
-50
-100
cross over frequency.
Some liberties where taken with the generally accepted
compensation procedure described above due to the transfer
characteristics of the opto coupler. The effects of the opto-
coupler tend to dominate over those of the LM431 so the GBWP
effects of the LM431 are not included here.
The gain and phase characteristics of the opto coupler are shown
in Figure 22A.
10
-150
5
-200
10
100
1?10 3
1?10 4
1?10 5
1?10 6
0
FREQUENCY (Hz)
FIGURE 21B. CONTROL-TO-OUTPUT PHASE
The Type 3 compensation configuration has three poles and two
zeros. The first pole is at the origin, and provides the integration
characteristic which results in excellent DC regulation. Referring
to the Typical Application Schematic for the regulated output, the
-5
-10
-15
f p2 = -----------------------------------------
(EQ. 27)
f p3 ≈ -------------------------------------
remaining poles and zeros for the compensator are located at:
1
2 π ? R21 ? C20
1
C19 ? C20
(EQ. 28)
2 π ? R4 ? C22
-20
10
100 1?10 3 1?10 4 1?10 5
FREQUENCY (Hz)
FIGURE 22A. OPTO COUPLER GAIN
1?10 6
90
f z1 = -----------------------------------------
1
2 π ? R21 ? C19
(EQ. 29)
45
f z2 ≈ -----------------------------------------
1
2 π ? R23 ? C22
R23 ? R4
(EQ. 30)
0
From (Equation 26), it can be seen that the control to output
transfer function frequency dependence is a function of the
output load resistance, the value of output capacitor and
inductor, and the output capacitance ESR. These variations must
-45
be considered when compensating the control loop. The worst
case small signal operating point for a voltage mode converter
-90
10
100
1?10 3
1?10 4
1?10 5
1?10 6
tends to be at maximum Vin, maximum load, maximum C OUT ,
and minimum ESR.
23
FREQUENCY (Hz)
FIGURE 22B. OPTO COUPLER
FN9111.6
December 2, 2011
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