Datasheet

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−40
−60
−50
0
−30
−20
10
40
20
30
−10
100 100 k10 k1 k 1 M
PWM and LC Response
ESR = 0 Ω
Frequency − Hz
Gain − dB
Overall System
Response
ESR = 0 Ω
f
CO2
f
CO1
Compensation
Response
Overall System
Response
ESR = 0.0095 Ω
GBWP
PWM and LC Response
ESR = 0.0095 Ω
TPS40075
SLUS676A – MAY 2006 – REVISED SEPTEMBER 2007
Desired frequency response and resultant overall system response can be seen in Figure 33 .
Figure 33. Overall System Bode Plot
Using these values and the equations above the resistors and capacitors around the compensation network can
be calculated.
1. Set R
Z1
= 10 k Ω .
2. Calculate R
SET
using Equation 49 ; R
SET
= 8750 Ω . Two resistors in parallel, R
SET1
and R
SET2
, are used to
make up R
SET
. R
SET1
= 9.53 k Ω , R
SET2
= 105 k Ω .
3. Using Equation 54 and f
Z1
= 3559 Hz, C
PZ1
can be calculated to be 4.47 nF; C
PZ1
= 4.7 nF.
4. F
P1
and Equation 52 yields R
P1
to be 677 Ω , R
P1
= 680 Ω .
5. The required gain of 17.6 dB (7.586) and Equation 52 sets the value for R
PZ1
. Note actual gain used for this
calculation was 20 dB (10), this ensures that the gain of the transfer function is high enough, R
PZ1
= 6.2 k Ω .
6. C
Z2
is calculated using Equation 55 and the desired frequency for the second zero, C
Z2
= 6.8 nF.
7. C
P2
is calculated using the second pole frequency and Equation 53 , C
P2
= 150 pF.
Using MathCAD the above values were used to draw the actual Bode plot for gain and phase. From these plots
the crossover frequency, phase margin and gain margin can be recorded.
Table 3. Equivalent Series Resistance
ESR CROSSOVER FREQUENCY PHASE MARGIN GAIN MARGIN
( Ω ) (kHz) ( ° ) (dB)
0 23.1 72 > 46
0.0095 98.6 78.8 > 33
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