BD9305AFVM Datasheet, PDF (10/15 Pages) Rohm – Single-output Step-up,High-efficiency Switching Regulator(Controller Type)

BD9306AFVM, BD9305AFVM
Technical Note
(9) Phase compensation
Phase Setting Method
The following conditions are required in order to ensure the stability of the negative feedback circuit.
Phase lag should be 150° or lower during gain 1 (0 dB) (phase margin of 30° or higher).
Because DC/DC converter applications are sampled using the switching frequency, the overall GBW should be set to 1/10
the switching frequency or lower. The target application characteristics can be summarized as follows:
Phase lag should be 150° or lower during gain 1 (0 dB) (phase margin of 30° or higher).
The GBW at that time (i.e., the frequency of a 0-dB gain) is 1/10 of the switching frequency or below.
In other words, because the response is determined by the GBW limitation, it is necessary to use higher switching
frequencies to raise response.
One way to maintain stability through phase compensation involves canceling the secondary phase lag (-180°) caused by
LC resonance with a secondary phase advance (by inserting 2 phase advances).
The GBW (i.e., the frequency with the gain set to 1) is determined by the phase compensation capacitance connected to
the error amp. Increase the capacitance if a GBW reduction is required.
(a) Standard integrator (low-pass filter)
(b) Open loop characteristics of integrator
Feedback R
FB
＋
A
－
C
Fig. 28
Point (a) fa =
1
2πRCA
(a)
A
-20 dB/decade
COMP
Gain
[dB]
GBW(b)
0
F
0
[Hz]
P[h°as] e-90
-180
-90°
Phase margin
Point (b) fb = GBW =
Fig. 29
1
2πRC
-180°
F
[Hz]
The error amp performs phase compensation of types (a) and (b), making it act as a low-pass filter.
For DC/DC converter applications, R refers to feedback resistors connected in parallel.
From the LC resonance of output, the number of phase advances to be inserted is two.
Vo
R1
R2
R4
C1
－
＋^A
COMP
R3
C2
1
LC resonant frequency fp =
[Hz]
2π√LC
Phase advance
fz1 =
1
2πC1R1
[Hz]
Phase advance
fz2 =
1
2πC2R3
[Hz]
Fig. 30
Set a phase advancing frequency close to the LC resonant frequency for the purpose of canceling the LC resonance.
(※ )If high-frequency noise is generated in the output, FB is affected through condenser C1.
Therefore, please insert the resistor R4=1kΩ or so, which is in series with condenser C1.
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10/14
2009.05 - Rev.A