MIC2169_09 Datasheet, PDF (12/15 Pages) Micrel Semiconductor – 500kHz PWM Synchronous Buck Control IC

MIC2169
200
215.856
220
240
260
270
10
10
100 1.10³ 1 ^.104 1.10⁵ 1 ^.106
f
1000000
Figure 8. Error Amplifier Phase Curve
Total Open-Loop Response
The open-loop response for the MIC2169 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.
100
71.607
50
Micrel
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
350
360
10
10
100
1.10³
1 ^.104
1 ^.105
1 ^.106
f
1000000
Figure 10. Open-Loop Phase Margin
M9999-032409
12
March 2009