ML4872 데이터 시트보기 (PDF) - Micro Linear Corporation
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ML4872 Datasheet PDF : 9 Pages
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ML4872
DESIGN CONSIDERATIONS (Continued)
OUTPUT CAPACITOR
The output capacitor filters the pulses of current from the
switching regulator. Since the switching frequency will
vary with inductance, the minimum output capacitance
required to reduce the output ripple to an acceptable
level will be a function of the inductor used. Therefore, to
maintain an output voltage with less than 100mV of ripple
at full load current, use the following equation:
COUT
=
44 L
VOUT
(2)
The output capacitor’s Equivalent Series Resistance (ESR)
and Equivalent Series Inductance (ESL), also contribute to
the ripple. Just after the NMOS transistor, Q1, turns off,
the current in the output capacitor ramps quickly to
between 0.5A and 1.5A. This fast change in current
through the capacitor’s ESL causes a high frequency (5ns)
spike to appear on the output. After the ESL spike settles,
the output still has a ripple component equal to the
inductor discharge current times the ESR. To minimize
these effects, choose an output capacitor with less than
10nH of ESL and 100mW of ESR.
Suitable tantalum capacitors can be obtained from the
following vendors:
AVX
(207) 282-5111
Kemet
(846) 963-6300
Sprague (207) 324-4140
1000
800
VOUT = 3.3V
600
VOUT = 5V
400
200
0
1.0
2.0
3.0
4.0
5.0
VIN (V)
Figure 4. IOUT vs. VIN Using the Circuit of Figure 8
90
VOUT = 3.3V
80
70
VOUT = 5V
VIN = 2.4V
60
1
10
100
IOUT (mA)
1000
Figure 5. Efficiency vs. IOUT Using the Circuit of Figure 8
90
VOUT = 5V
60
VOUT = 3.3V
30
0
1.0
2.0
3.0
4.0
5.0
VIN (V)
Figure 6. No Load Input Current vs. VIN
6
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