Datasheet

   
   
SLUS419C − AUGUST 1999 − REVISED NOVEMBER 2001
26
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23 dB. Therefore the compensation circuitry needs to have a gain of −23 dB at the desired crossover frequency.
For this example R35 is calculated at approximately 18.2 kΩ. Capacitor C14 is estimated to be approximately
10 nF and C15 is calculated at approximately 180 pF.
H(s) + 20 log
ƪ
V
REF
V
OUT
ƫ
R35 + R31 10
ǒ
*G
CO
(s) dB)G
OPTO
(s) dB)H(s) dB
Ǔ
C14 +
1
ǒ
2p R35 f
C
Ǔ
C15 +
1
ǒ
2p R35
f
SW
2
Ǔ
Figure 16 shows the frequency response of the compensation network G
C
(s) and Figure 17 shows the
measured frequency response of the loop gain T(s). The frequency response characteristics in Figure 17 show
that f
C
is approximately 1.5 kHz with a phase margin of about 55 degrees. The gain margin is approximately
50 dB.
Figure 16
FEEDBACK CONTROL TRANSFER FUNCTION
(GAIN AND PHASE)
vs.
FREQUENCY
−20
0
100
20
−60
−40
60
40
60
180
120
0
−60
−180
−120
1 k 10 k 100 k
COMPENSATION
GAIN
COMPENSATION
PHASE
G
OPTO
− Gain − dB
Phase − Degrees
f − Frequency − Hz
−20
0
20
−60
−40
60
40
100 1 k 10 k 100 k
60
180
120
0
−60
−180
−120
Figure 17
G
OPTO
− Gain − dB
Phase − Degrees
TOTAL LOOP TRANSFER FUNCTION
(GAIN AND PHASE)
vs.
FREQUENCY
f − Frequency − Hz
LOOP GAIN
LOOP PHASE
(46)
(47)
(48)
(49)