ML4871
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:
C
OUT
44
L
=
V
OUT
(2)
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
Kemet
Sprague
(207) 282-5111
(846) 963-6300
(207) 324-4140
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
1000
90
800
VOUT = 3.3V
IOUT (mA)
600
VOUT = 5V
EFFICIENCY (%)
80
VOUT = 3.3V
400
70
VOUT = 5V
200
VIN = 2.4V
1
10
IOUT (mA)
100
1000
0
1.0
2.0
3.0
VIN (V)
4.0
5.0
60
Figure 4. I
OUT
vs. VIN Using the Circuit of Figure 8
Figure 5. Efficiency vs. I
OUT
Using the Circuit of Figure 8
90
VOUT = 5V
60
IIN (µA)
VOUT = 3.3V
30
0
1.0
2.0
3.0
VIN (V)
4.0
5.0
Figure 6. No Load Input Current vs. V
IN
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
