MB39A119 Datasheet, PDF (27/53 Pages) Fujitsu Component Limited. – DC/DC Converter IC of Synchronous Rectification for charging Li-ion battery

MB39A119
• Method to obtain frequency characteristic of Error Amp
90
80
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0
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1
Total frequency characteristic
180
total gain ₁₆₀
AMP Open 140
Loop Gain ₁₂₀
total phase ₁₀₀
80
60
Gain
40
20
Phase
0
−20
−40
−60
−80
−100
−120
−140
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−180
10 100 1k 10k 100k 1M
Frequency [Hz]
Cut-off frequency
1
f2 (Hz) =
2π × Rc × Cc
Rc = 150 kΩ
Cc = 3300 pF
• Method to obtain frequency characteristic of DC/DC converter
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Total frequency characteristic
Gain
Phase
total gain
AMP Open
Gain
Loop
180
160
140
120
total phase
100
80
60
40
20
0
−20
−40
−60
−80
−100
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−140
−160
−180
10 100 1k 10k 100k 1M
Frequency [Hz]
Triangular wave oscillation frequency.
The overview of frequency characteristic
for DC/DC converter can be obtained in
combination between LC filter and fre-
quency characteristic of Error Amp as
mentioned above.
Note the following point in order to stabi-
lize frequency characteristic of DC/DC
converter.
Cut-off frequency of DC/DC converter
should be set to half or less of the triangu-
lar wave oscillator frequency.
Note1) Review frequency characteristic of Error Amp when LC filter constant is changed.
Note2) When the ceramic capacitor is used as smoothing capacitor CO, phase margin is reduced because ESR
of the ceramic capacitor is extremely small as frequency characteristic of LC filter at low ESR.
Therefore, change phase compensation of Error Amp or create resistance equivalent to ESR using pattern.
• Method to obtain frequency characteristic of LC filter at low ESR
Frequency characteristic of power output LC filter
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(DC gain is included.)
Gain
Phase
10 100 1k 10k 100k
gain
phase
180
160
140
120
100
80
60
40
20
0
−20
−40
−60
−80
−100
−120
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−180
1M 10M
Frequency [Hz]
Cut-off frequency
f1 (Hz) =
2π
1
(Ro + ESR)
Lo × Co ×
(Ro + RL)
Lo = 15 µH
Co = 14.1 µF
Ro = 4.2 Ω
RL = 30 mΩ
ESR = 100 mΩ
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