BD9673EFJ-E2 ROHM Electronics, BD9673EFJ-E2 Datasheet - Page 13

• Manufacturer Part Number: BD9673EFJ-E2
• Description: Specifications: Type: Step-Down (Buck) ; Output Type: Adjustable ; PWM Type: Current Mode ; Synchronous Rectifier: Yes ; Number of Outputs: 1 ; Voltage - Output: 1 V ~ 29.4 V ; Current - Output: 1.5A ; Frequency - Switching: 300kHz ; Voltage - Input:
• Manufacturer: ROHM Electronics

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BD9673EFJ
© 2012 ROHM Co., Ltd. All rights reserved.
(5) About Adjustment of DC/DC Comparator Frequency Characteristics
Role of Phase compensation element C1,C2,R3 (See P.7 Example of Reference Application Circuit)
Stability and Responsiveness of Loop are controlled through VC Pin which is the output of Error Amp.
The combination of zero and pole that determines Stability and Responsiveness is adjusted by the combination of
resistor and capacitor that are connected in series to the VC Pin.
DC Gain of Voltage Return Loop can be calculated for using the following formula.
Here, VFB is Feedback Voltage (1.0V).A
of Current Detect (typ : 10A/V), and Rl is the Output Load Resistance value.
There are 2 important poles in the Control Loop of this DC/DC.
The first occurs with/ through the output resistance of Phase compensation Capacitor (C1) and Error amplifier.
The other one occurs with/through the Output Capacitor and Load Resistor.
These poles appear in the frequency written below.
Here, G
Here, in this Control Loop, one zero becomes important.
With the zero which occurs because of Phase compensation Capacitor C1 and Phase compensation Resistor R3, the
Frequency below appears.
Also, if Output Capacitor is big, and that ESR (RESR) is big, in this Control Loop, there are cases when it has an
important, separate zero (ESR zero).
This ESR zero occurs due to ESR of Output Capacitor and Capacitance, and exists in the Frequency below.
(ESR zero)
In this case, the 3
(R3) is used in order to correct the ESR zero results in Loop Gain.
This pole exists in the frequency shown below.
(pole that corrects ESR zero)
The target of Phase compensation design is to create a communication function in order to acquire necessary band
and Phase margin.
Cross-over Frequency (band) at which Loop gain of Return Loop becomes “0” is important.
When Cross-over Frequency becomes low, Power supply Fluctuation Response, Load Response, etc worsens.
On the other hand, when Cross-over Frequency is too high, instability of the Loop can occur.
Tentatively, Cross-over Frequency is targeted to be made 1/20 or below of Switching Frequency.
EA
is the trans-conductance of Error amplifier(typ : 220µA/V).
Adc
rd
pole determined with the 2
fp
fp2
fz
fp
fz
1
1
=
3
=
=
=
=
Rl
=
2
2
2
π
2
π

π
π
2
Gcs

π



COUT
COUT
G

C1
C1
1
EA

1
EA
C2
1
A

1
is Voltage Gain of Error amplifier (typ : 77dB), Gcs is the Trans-conductance

EA
A

R3

RESR


EA
nd
Rl
Vout
Phase compensation Capacitor (C2) and Phase Correction Resistor
R3
V
13/18
FB
Technical Note
2012.02 - Rev.C