ml4872 Microsemi Corporation, ml4872 Datasheet - Page 6

• Manufacturer Part Number: ml4872
• Description: High Current Boost Regulator With Shutdown
• Manufacturer: Microsemi Corporation

ML4872
DESIGN 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 NMOS transistor, Q1, turns off,
the current in the output capacitor ramps quickly to
6
Figure 4. I
C
OUT
1000
800
600
400
200
=
0
1.0
44
V
OUT
OUT
™
L
vs. VIN Using the Circuit of Figure 8
V OUT = 3.3V
2.0
V OUT = 5V
V IN (V)
3.0
4.0
Figure 6. No Load Input Current vs. V
(Continued)
90
60
30
0
1.0
V OUT = 3.3V
5.0
2.0
(2)
V OUT = 5V
V IN (V)
3.0
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 100m of ESR.
Suitable tantalum capacitors can be obtained from the
following vendors:
Figure 5. Efficiency vs. I
AVX
Kemet
Sprague
4.0
90
80
70
60
1
V IN = 2.4V
V OUT = 3.3V
IN
[phone]
[phone]
[phone]
5.0
OUT
10
I OUT (mA)
Using the Circuit of Figure 8
100
V OUT = 5V
1000