ml4870 Microsemi Corporation, ml4870 Datasheet - Page 6

• Manufacturer Part Number: ml4870
• Description: High Current Boost Regulator With Load Disconnect
• Manufacturer: Microsemi Corporation

ML4870
DEISGN CONSIDERATIONS
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:
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
6
Figure 4. I
C
OUT
Figure 6. Input Leakage vs. V
1000
350
300
250
200
150
100
800
600
400
200
=
50
0
0
1.0
1.0
44
V
OUT
OUT
™
L
vs. V
V OUT = 3.3V
2.0
IN
3.0
Using the Circuit of Figure 8
V IN (V)
V IN (V)
V OUT = 5V
3.0
5.0
IN
in Shutdown
4.0
(Continued)
7.0
5.0
(3)
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
100m of ESR.
Suitable tantalum capacitors can be obtained from the
following vendors:
Figure 5. Efficiency vs. I
AVX
Sprague
Kemet
Figure 7. No Load Input Current vs. V
90
80
70
60
80
60
40
20
0
1.0
1
V IN = 2.4V
V OUT = 3.3V
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2.0
OUT
V OUT = 5V
10
I OUT (mA)
V IN (V)
Using the Circuit of Figure 8
3.0
V OUT = 5V
100
V OUT = 3.3V
4.0
IN
1000
5.0