BD9329AEFJ_15 Datasheet, PDF (11/23 Pages) Rohm – Synchronous Buck Converter with Integrated FET

BD9329AEFJ
(a) Choosing phase compensation resistor RCMP
The compensation resistor RCMP can be calculated using the following formula:
R_CMP
=
2π
× V_OUT × f_CRS × C_OUT
V_FB × G_MP × G_MA
[Ω]
Where:
V_OUT is the Output Voltage
fCRS is the Cross Over Frequency
C_OUT is the Output Capacitor
V_FB is the Internal Feedback Voltage (0.9V(TYP))
G_MP is the Current Sense Gain (7.8A/V(TYP))
G_MA is the Error Amplifier Transconductance (300µA/V(TYP))
Setting VOUT= 3.3V, fCRS= 38kHz, COUT= 20µF;
R_CMP
=
2π × 3.3 × 38k × 20µ
0.9 × 7.8 × 300µ
= 7482.5 ≈
7.5k
[Ω]
(b) Choosing phase compensation capacitor CCMP
For the stability of the DC/DC converter, cancellation of the phase delay that is drawn from the output capacitor COUT
and resistive load ROUT is possible 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
1
C_CMP = _{2π × RCMP × fZ}
[F ]
Setting fZ= fCRS/6 = 6.3kHz;
Compensation Capacitor
C _CMP
=
2π
1
× 7.5k
× 6.3k
= 3.368 × 10^−9
≅ 3.3 × 10^−9
[F]
However, the best values for zero and fCRS differ between applications. Decide the values accordingly after
calculation using the formula above and confirmation on the actual application.
(c) The condition of the loop compensation stability
The stability of DC/DC converter is important. To secure operation stability, check if the loop compensation has
enough phase-margin. For the condition of loop compensation stability, the phase-delay must be less than 150
degrees at 0 dB Gain.
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
A
Gain [dB]
0
P_PHha_As_Se_{E 0}
－90
－180
(a)
GBW(b)
F
F_fC_CR_RS_S
－90°
PPHhAaSsEe MMAaRrgGinIN
－180°
F
Figure 21
(3) Design of Feedback Resistance constant
Set the feedback resistance as shown below.
Figure 22
VOUT
R1
R2
0.9V
＋
ERR
－
FB
Figure 23
R1 + R2
V_OUT = _R2 × 0.9
[V ]
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TSZ22111・15・001
11/19
TSZ02201-0323AAJ00010-1-2
16.Feb.2015 Rev.003