ISL6740 Datasheet, PDF (24/29 Pages) Intersil Corporation – Flexible Double Ended Voltage and Current Mode PWM Controllers

ISL6740, 1SL6741
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 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 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
5
0
-5
-10
-15
-20
10
100
1•10³
1•10⁴
1•10⁵
FREQUENCY (Hz)
FIGURE 22A. OPTO COUPLER GAIN
1•10⁶
90
45
0
-45
-90
10
100
1•10³
1•10⁴
1•10⁵
FREQUENCY (Hz)
FIGURE 22B. OPTO COUPLER
1•10⁶
The following compensation components were selected
R₂₃ = 9.53kΩ
R₂₄ = 2.49kΩ
R₄ = 499Ω
R₂₁ = 4.22kΩ
C₂₂ = 1nF
C₂₀ = 82pF
C₁₉ = 0.22μF
From (Equations 27, 28, 29 and 30), the poles and zeroes
are:
f_z1 = 171Hz
f_z2 = 16.7kHz
f_p2 = 460kHz
f_p3 = 319kHz
The calculated gain and phase plots of the error amplifier
appear below using an ideal op amp.
20
10
0
-10
10
100
1•10³
1•10⁴
1•10⁵
1•10⁶
FREQUENCY (Hz)
FIGURE 23A. IDEAL ERROR AMPLIFIER GAIN
90
45
0
-45
-90
10
100
1•10³
1•10⁴
1•10⁵
1•10⁶
FREQUENCY (Hz)
FIGURE 23B. IDEAL ERROR AMPLIFIER PHASE
24
FN9111.4
July 13, 2007