ML4869 Просмотр технического описания (PDF) - Micro Linear Corporation
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ML4869 Datasheet PDF : 8 Pages
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ML4869
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
(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 0.3A and
0.9A. 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 product of inductor
discharge current and 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
600
500
VOUT = 3.3V
400
300
VOUT = 5V
200
100
0.0
1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00
VIN (V)
Figure 4. IOUT vs. VIN Using the Circuit of Figure 8
250
200
150
VOUT = 5V
VOUT = 3.3V
100
50
0
012345678
VIN (V)
Figure 6. Input Leakage vs. VIN in Shutdown
6
95.0
90.0
85.0
VOUT = 5V
VOUT = 3.3V
80.0
75.0
0.1
1
10
100
IOUT (mA)
1000
Figure 5. Efficiency vs. IOUT Using the Circuit of Figure 8
80
70
VOUT = 5V
60
50
40
30
VOUT = 3.3V
20
1
2
3
4
5
VIN (V)
Figure 7. No Load Input Current vs. VIN
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