BD9E301EFJ-LB Datasheet, PDF (23/33 Pages) Rohm – 7.0V to 36V Input, 2.5A Integrated MOSFET Single Synchronous Buck DC/DC Converter

BD9E301EFJ-LB
4. Phase Compensation
A current mode control buck DC/DC converter is a two-pole, one-zero system. The two poles are formed by an error
amplifier and load and the one zero point is added by the phase compensation. The phase compensation resistor RCMP
determines the crossover frequency FCRS where the total loop gain of the DC/DC converter is 0 dB. The high value of this
crossover frequency FCRS provides a good load transient response characteristic but inferior stability. Conversely,
specifying a low value for the crossover frequency FCRS greatly stabilizes the characteristics but the load transient
response characteristic is impaired.
(1) Selection of Phase Compensation Resistor RCMP
The phase compensation resistance RCMP can be determined by using the following equation.
R_CMP
=
2π
× V_OUT × F_CRS × C _OUT
V _FB × G _MP × G _MA
[Ω] 　
where :
V_OUT is the output voltage
F_CRS is the crossover frequency
C _OUT is the output capacitance
V _FB is the feedback reference voltage (1.0 V (Typ))
G _MP is the current sense gain (7 A/V (Typ))
G _MA is the error amplifier transconductance(150 μA/V (Typ))
(2) Selection of phase compensation capacitance CCMP
For stable operation of the DC/DC converter, inserting a zero point under 1/6 of the zero crossover frequency cancels
the phase delay due to the pole formed by the load often, thus, providing favorable characteristics.
The phase compensation capacitance CCMP can be determined by using the following equation.
C _CMP
=
2π
×
1
R_CMP
×
F_Z
[F]
where
F_Z 　 is the Zero point inserted
(3) Loop stability
To ensure the stability of the DC/DC converter, make sure that a sufficient phase margin is provided. Phase margin
of at least 45 degrees in the worst conditions is recommended. The feed forward capacitor CRUP is used for the
purpose of forming a zero point together with the resistor RUP to increase the phase margin within the limited
frequency range. Using a CRUP is effective when the RUP resistance is larger than the combined parallel resistance of
RUP and RDW.
V_OUT
(a)
A
Gain [dB]
R_UP
C_RUP
GBW(b)
FB
CO^【M^dBP^】
0
f
FCRS
R_DW
1.0V
C_CMP
Phase[deg] 0
－90
－90°
Phase R_CMP
【°】
－180
PHASE MARGIN
－180°
f
Figure 49. Phase compensation circuit
Figure 50. Bode plot
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TSZ02201-0J3J0AJ00380-1-2
21.Feb.2014 Rev.002