ML4870CS-3 Просмотр технического описания (PDF) - Micro Linear Corporation
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ML4870CS-3 Datasheet PDF : 8 Pages
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ML4870
DEISGN 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
(3)
The output capacitor’s Equivalent Series Resistance (ESR)
and Equivalent Series Inductance (ESL), also contribute to
the ripple. Just after the Q1 turns off, the current in the
output capacitor ramps quickly to between 0.5A and
1.3A. 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 less than
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
600
VOUT = 3.3V
400
VOUT = 5V
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
350
300
250
200
150
100
50
0
1.0
3.0
5.0
7.0
VIN (V)
Figure 6. Input Leakage vs. VIN in Shutdown
6
90
VOUT = 3.3V
VOUT = 5V
80
70
VIN = 2.4V
60
1
10
100
IOUT (mA)
1000
Figure 5. Efficiency vs. IOUT Using the Circuit of Figure 8
80
VOUT = 5V
60
40
VOUT = 3.3V
20
0
1.0
2.0
3.0
4.0
5.0
VIN (V)
Figure 7. No Load Input Current vs. VIN
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