ISL6741IB Intersil, ISL6741IB Datasheet - Page 24

• Manufacturer Part Number: ISL6741IB
• Description: IC CTRLR PWM DBL-ENDED 16-SOIC
• Manufacturer: Intersil

Specifications of ISL6741IB

• Pwm Type: Current Mode
• Number Of Outputs: 2
• Frequency - Max: 2MHz
• Duty Cycle: 100%
• Voltage - Supply: 9 V ~ 16 V
• Buck: No
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 105°C
• Package / Case: 16-SOIC (3.9mm Width)
• Frequency-max: 2MHz
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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
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
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
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.
-45
-90
90
45
-10
-15
-20
10
0
-5
5
0
10
10
FIGURE 22A. OPTO COUPLER GAIN
FIGURE 22B. OPTO COUPLER
100
LC
100
SW
/5, and the second at just above f
/4, where f
FREQUENCY (Hz)
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FREQUENCY (Hz)
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24
3
3
SW
1•10
1•10
is the switching
4
4
LC
1•10
1•10
/2, where f
5
5
ISL6740, 1SL6741
1•10
1•10
LC
LC
6
.
6
is
The following compensation components were selected
R
R
R
R
C
C
C
From (Equations 27, 28, 29 and 30), the poles and zeroes
are:
f
f
f
f
The calculated gain and phase plots of the error amplifier
appear below using an ideal op amp.
z1
z2
p2
p3
23
24
4
21
22
20
19
= 499Ω
= 171Hz
= 16.7kHz
= 319kHz
= 460kHz
= 9.53kΩ
= 2.49kΩ
= 4.22kΩ
= 1nF
= 82pF
= 0.22μF
-45
-90
90
45
0
-10
FIGURE 23B. IDEAL ERROR AMPLIFIER PHASE
10
20
10
FIGURE 23A. IDEAL ERROR AMPLIFIER GAIN
0
10
100
100
FREQUENCY (Hz)
1•10
FREQUENCY (Hz)
1•10
3
3
1•10
1•10
4
4
1•10
1•10
5
5
1•10
July 13, 2007
1•10
FN9111.4
6
6