ML4771
DESIGN CONSIDERATIONS (Continued)
OUTPUT CAPACITOR
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.
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:
Suitable tantalum capacitors can be obtained from the
following vendors:
44 L
VOUT
COUT
=
(2)
AVX
(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
current in the output capacitor ramps quickly to between
Kemet
Sprague
1000
800
90
V
= 3.0V
OUT
80
70
60
V
= 3.0V
OUT
600
400
200
0
V
= 5.5V
OUT
V
= 5.5V
OUT
V
= 2.4V
IN
1.5
2.5
3.5
(V)
4.5
5.5
1
10
100
1000
V
I
(mA)
OUT
IN
Figure 4. I
vs. VIN Using the Circuit of Figure 8
Figure 5. Efficiency vs. I
Using the Circuit of Figure 8
OUT
OUT
160
V
= 5.5V
OUT
120
80
40
V
= 3.0V
OUT
0
1.5
2.5
3.5
(V)
4.5
5.5
V
IN
Figure 6. No Load Input Current vs. V
Figure 7. Sample PC Board Layout
IN
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
