BD9329AEFJ Datasheet, PDF (9/16 Pages) Rohm – Simple Step-down Switching Regulator with Built-in Power MOSFET

BD9329AEFJ
Technical Note
(2) Loop Compensation
Choosing compensation capacitor CCMP and resistor RCMP
The current-mode buck converter has 2-poles and 1-zero system. Choosing the compensation resistor and capacitor is
important for a good load-transient response and good stability.
The example of DC/DC converter application bode plot is shown below.
The compensation resistor RCMP will decides the cross over frequency FCRS (the frequency that the total DC-DC
loop-gain falls to 0dB).
Setting the higher cross over frequency achieves good response speed, however less stability. While setting the lower
cross over frequency shows good stability but worse response speed.
The 1/10 of switching frequency for the cross over frequency shows a good performance at most applications.
( i ) Choosing phase compensation resistor RCMP
The compensation resistor RCMP can be on following formula.
RCMP =
2πx VOUT x FCRS x COUT
[Ω]
VFB x GMP x GMA
Where
VOUT ; Output voltage, FCRS ; Cross over frequency, COUT ; Output Capacitor,
VFB ; internal feedback voltage (0.9V(TYP)), GMP ; Current Sense Gain (7.8A/V(TYP)) ,
GMA ; Error Amplifier Trans-conductance (300µA/V(TYP))
Setting VOUT= 3.3V, FCRS= 38kHz, COUT= 20µF;
RCMP =
2πx 3.3 x 38k x 20u
=
0.9 x 7.8 x 300u
7.48k ~= 7.5k
[Ω]
( ii ) Choosing phase compensation capacitor CCMP
For the stability of DC/DC converter, canceling the phase delay that derives from output capacitor COUT and
resistive load ROUT by inserting the phase advance.
The phase advance can be added by the zero on compensation resistor RCMP and capacitor CCMP.
Making Fz= FCRS / 6 gives a first-order estimate of CCMP.
Compensation Capacitor
CCMP=
1
[F]
2π x RCMP x Fz
Setting Fz= FCRS/6 = 6.3kHz;
Compensation Capacitor
CCMP=
1
= 3.54n ~= 3.3n [F]
2π x 7.5k x 6.3k
( iii ) The condition of the loop compensation stability
The stability of DC/DC converter is important. To secure the operating stability, please check the loop compensation
has the enough phase-margin. For the condition of loop compensation stability, the phase-delay must be less than
150 degree where Gain is 0 dB.
Feed forward capacitor CRUP boosts phase margin over a limited frequency range and is sometimes used to
improve loop response. CRUP will be more effective if RUP >> RUP||RDW
VOUT
RUP
RDW
CRUP
FB
－
＋
0.9V
COMP
RCMP
CCMP
Fig.20
A
Gain [dB]
0
PHASE 0
－90
－180
(a)
GBW(b)
F
FCRS
－90°
PHASE MARGIN
－180°
F
Fig.21
www.rohm.com
© 2011 ROHM Co., Ltd. All rights reserved.
9/15
2011.02 - Rev.A