Datasheet

11/16
Technical Note
Phase compensation setting procedure
R
Feedback
C
FB
-
+
A
(1) Typical integrator (Low pass filter) (2) Open loop characteristics of integrator
0
A
0
-90
- 90˚
- 180˚
-180
Phase
[˚]
Gain
[dB]
f
f
GBW(b)
- 20dB/decade
Phase
margin
(a)
The following section describes the stability conditions of the negative feedback system.
Since the DC/DC converter application is sampled according to the switching frequency, GBW (frequency at 0-dB gain)
of the overall system should be set to 1/10 or less of the switching frequency. The following section summarizes the targeted
characteristics of this application.
Ă At a 1 (0-dB) gain, the phase delay is 150˚ or less (i.e., the phase margin is 30˚ or more).
Ă The GBW for this occasion is 1/10 or less of the switching frequency.
Responsiveness is determined with restrictions on the GBW. To improve responsiveness, higher switching frequency
should be provided.
Replace a secondary phase delay (-180˚) with a secondary phase lead by inserting two phase leads, to ensure the stability
through the phase compensation. Furthermore, the GBW (i.e., frequency at 0-dB gain) is determined according to phase
compensation capacitance provided for the error amplifier. Consequently, in order to reduce the GBW,
increase the capacitance value.
1. Application stability conditions
Since the error amplifier is provided with (1) or (2) phase compensation, the low pass filter is applied. In the case of
the DC/DC converter application, the R becomes a parallel resistance of the feedback resistance.
ŢMqagjj_rgmldpcosclaw%qep_nft_jscgq Rwnga_jt_jsc*
mqagjj_rgmldpcosclawgqlcacqq_pwrmamlqgbcp ĺ 0.#_qbgqncpqgml,
Fig.35 RT vs fOSC &BD9001F'
50
100
150
200
250
300
100 50 200 300 400 500 600 700 800
OSCILATING FREQUENCY SETTING RESISTANCE : RT [kΩ]
OSCILATING FREQUENCY : fosc[kHz]
Fig.34 RT vs fOSC (BD9781HFP/BD9778F/HFP)
800
50
100
150
200
250
300
350
400
450
500
100 0200 300 400 500 600 700
OSCILATING FREQUENCY SETTING RESISTANCE : RT[kΩ]
OSCILATING FREQUENCY : fosc[kHz]
Fig.32 BD9001F reference layout pattern Fig.33 BD9781HFP reference layout pattern
Ţ As shown above, design the GND pattern as large area as possible
within inner layer.
Ţ Gray zones indicate GND.
Co
L
C
Cin
Di
RT
R3
Cx1
R1
C3
Cx2
R2
CT
Co
L
C
Cin
Di
Cx1
R3
C3
Cx2
R2
R1
RT
CT
2010.02 - Rev. B
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BD9778F, BD9778HFP, BD9001F, BD9781HFP