BD8961NV ROHM Co. Ltd., BD8961NV Datasheet - Page 10

• Manufacturer Part Number: BD8961NV
• Description: Low Noise Step-down Switching Regulator
• Manufacturer: ROHM Co. Ltd.

Phase
Phase
Gain
[deg]
Gain
[deg]
[dB]
[dB]
3. Selection of input capacitor (Cin)
A low ESR 22μF/10V ceramic capacitor is recommended to reduce ESR dissipation of input capacitor for better efficiency.
4. Determination of RITH, CITH that works as a phase compensator
V
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.
-90
-90
CC
A
A
0
0
0
0
Fig.28 Input capacitor
Fig.30 Error amp phase compensation characteristics
I
L
OUT
fp(Min.)
fz(Amp.)
Cin
Min.
Fig.29 Open loop gain characteristics
Co
fp(Max.)
I
OUT
Max.
V
OUT
fz(ESR)
Input capacitor to select must be a low ESR capacitor of the capacitance
sufficient to cope with high ripple current to prevent high transient voltage. The
ripple current IRMS is given by the equation (5):
If V
I
< Worst case > I
When Vcc is twice the V
I
RMS
RMS
CC
=I
=2×
=5.0V, V
OUT
×
√
10/15
fp=
fz
Pole at power amplifier
Zero at power amplifier
√
OUT
3.3(5.0-3.3)
(ESR)
V
RMS(max.)
When the output current decreases, the load resistance Ro
increases and the pole frequency lowers.
Increasing capacitance of the output capacitor lowers the pole
frequency while the zero frequency does not change. (This
is because when the capacitance is doubled, the capacitor
ESR reduces to half.)
OUT
5.0
=3.3V, and I
2π×R
=
(V
V
CC
2π×E
CC
OUT
1
-V
O
fp
fp
fz
, I
×C
=0.947[A
OUT
1
(Min.)
(Max.)
(Amp.)
RMS
OUTmax.=
SR
O
)
×C
=
=
=
=
[A]・・・(5)
O
2π×R
2π×R
2π×R
RMS
I
OUT
2A, (BD9111NV)
2
]
OMax.
OMin.
ITH
1
1
1
×C
×C
×C
ITH
O
O
[Hz]←with lighter load
[Hz] ←with heavier load