bd91361muv ROHM Co. Ltd., bd91361muv Datasheet - Page 13

• Manufacturer Part Number: bd91361muv
• Description: Single-chip Type With Built-in Fet Switching Regulators
• Manufacturer: ROHM Co. Ltd.

BD91361MUV
© 2010 ROHM Co., Ltd. All rights reserved.
www.rohm.com
4. Determination of RITH, CITH that works as a phase compensator
Phase
Phase
[deg]
[deg]
Gain
Gain
[dB]
[dB]
As the Current Mode Control is designed to limit a inductor current, a pole (phase lag) appears in the low frequency area
due to a CR filter consisting of a output capacitor and a load resistance, while a zero (phase lead) appears in the high
frequency area due to the output capacitor and its ESR. So, the phases are easily compensated by adding a zero to the
power amplifier output with C and R as described below to cancel a pole at the power amplifier.
Stable feedback loop may be achieved by canceling the pole fp (Min.) produced by the output capacitor and the load
resistance with CR zero correction by the error amplifier.
-90
-90
A
A
0
0
0
0
fz
(Amp.)
Fig.34 Error amp phase compensation characteristics
= fp
V
2π×R
Fig.33 Open loop gain characteristics
OUT
I
OUT
(Min.)
fp(Min.)
fz(Amp.)
Min.
V
CC
ITH
1
fp(Max.)
R2
×C
R1
I
OUT
ITH
Max.
R
C
Cin
ITH
ITH
=
fz(ESR)
2π×R
EN
ADJ
ITH
VID<1> VID<0>
V
CC
Fig.35 Typical application
OMax.
1
V
CC
PV
×C
CC
13/17
fp=
fz
Pole at power amplifier
Zero at power amplifier
O
GND,PGND
(ESR)
Increasing capacitance of the output capacitor lowers the
(This is because when the capacitance is doubled, the capacitor
ESR reduces to half.)
R
When the output current decreases, the load resistance
Ro increases and the pole frequency lowers.
pole frequency while the zero frequency does not change.
f
2π×R
=
V
2π×E
CC
1
O
SW
fp
fp
fz
×C
1
(Amp.)
(Min.)
(Max.)
SR
C
O
f
×C
=
=
=
C
BST
O
2π×R
2π×R
2π×R
L
ESR
C
O
OMax.
OMin.
ITH
1
1
1
×C
×C
×C
R
ITH
O
O
O
V
OUT
[Hz]←with lighter load
[Hz] ←with heavier load
Technical Note
2010.06 - Rev.A