MB39A125 Datasheet, PDF (39/64 Pages) Fujitsu Component Limited. – DC/DC Converter IC for Charging Li-ion Battery

MB39A125/126
• Frequency Characteristics of Error Amp
90
80
70
60
50
40
30
20
10
0
−10
−20
−30
−40
−50
−60
−70
−80
−90
1
Total frequency characteristic
Gain
10 100
180
total gain 160
AMP Open 140
Loop Gain 120
total phase 100
80
60
40
20
0
Phase
−20
−40
−60
−80
−100
−120
−140
−160
−180
1k
10k 100k 1M
Frequency [Hz]
Cut-off frequency
1
f2(Hz) =
2π × Rc × Cc
Rc = 150 kΩ
Cc = 3300 pF
• Frequency Characteristics of DC/DC converter
Total frequency characteristic
90
80
70
60
50
40
30
20
10
0
−10
−20
−30
−40
−50
−60
−70
−80
−90
1
Gain
Phase
10
100 1k
180
total gain 160
AMP Open 140
Loop Gain 120
total phase ¹⁰⁰
80
60
40
20
0
−20
−40
−60
−80
−100
−120
−140
−160
−180
10k 100k 1M
The overview of frequency characteristic
for DC/DC converter can be obtained in
combination between “Frequency
Characteristics of LC filter” and
“Frequency Characteristics of Error Amp ”
as mentioned above.
Please note the following point in order to
stabilize the frequency characteristics of
DC/DC converter .
Cut-off frequency of DC/DC converter
should be set to half or less of the
triangular wave oscillator frequency.
Frequency [Hz]
Triangular wave frequency
Notes : 1) Please review the Error Amp frequency characteristics, when LC filter parameter is modified.
2) When the ceramic capacitor is used as smoothing capacitor Co, phase margin is reduced because ESR
of the ceramic capacitor is extremely small as shown in “Frequency Characteristics of LC filter which is
using low ESR”.
Therefore, change phase compensation of Error Amp or create resistance equivalent to ESR using
pattern.
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