MIC2168 Datasheet, PDF (12/14 Pages) Micrel Semiconductor – 1MHZ PWM SYNCHRONOUS BUCK CONTROL IC

MIC2168
Figures 7 and 8 show the gain and phase curves for the above
transfer function with R1 = 9.3k, C1 = 1000pF, C2 = 100pF,
and g_m = .005Ω^–1. It can be seen that at 50kHz, the error
amplifier exhibits approximately 45° of phase margin.
60
60
100
71.607
50
Micrel
40
20
.001
1 ^.103
1000
1 ^.104
1 ^.105
f
1 ^.106
1 ^.107
10000000
Figure 7. Error Amplifier Gain Curve
200
215.856
0
42.933
50
100
100
1.10³
1 ^.104
f
1 ^.105
1 ^.106
1000000
Figure 9. Open-Loop Gain Margin
250
269.097
300
220
240
260
270
10
10
100
1.10³
1 ^.104
f
1 ^.105
1 ^.106
1000000
Figure 8. Error Amplifier Phase Curve
Total Open-Loop Response
The open-loop response for the MIC2168 controller is easily
obtained by adding the power path and the error amplifier
gains together, since they already are in Log scale. It is
desirable to have the gain curve intersect zero dB at tens of
kilohertz, this is commonly called crossover frequency; the
phase margin at crossover frequency should be at least 45°.
Phase margins of 30° or less cause the power supply to have
substantial ringing when subjected to transients, and have
little tolerance for component or environmental variations.
Figures 9 and 10 show the open-loop gain and phase margin.
It can be seen from Figure 9 that the gain curve intersects the
0dB at approximately 50kHz, and from Figure 10 that at
50kHz, the phase shows approximately 50° of margin.
350
360
10
10
100
1.10³
1 ^.104
1 ^.105
1 ^.106
f
1000000
Figure 10. Open-Loop Phase Margin
M9999-111803
12
November 2003