ML4769
DESIGN CONSIDERATIONS (Continued)
OUTPUTCAPACITOR
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 inductor discharge current
times the ESR. To minimize these effects, choose an
output capacitor with less than 10nH of ESL and less than
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 Q1 turns off, the current in the
output capacitor ramps quickly to between 0.3A and
Kemet
Sprague
600
500
90
80
70
60
V
= 3V
OUT
V
= 5.5V
OUT
V
= 3V
OUT
400
300
200
100
0
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 5. Efficiency vs. I
Using the Circuit of Figure 8
Figure 4. I
vs. V Using the Circuit of Figure 8
IN
OUT
OUT
350
300
250
200
150
100
50
160
120
V
= 5.5V
OUT
80
40
0
V
= 3V
2.0
OUT
0
1.0
3.0
5.0
7.0
1.0
3.0
(V)
4.0
5.0
V
(V)
V
IN
IN
Figure 6. Input Leakage vs. V in Shutdown
Figure 7. No Load Input Current vs. V
IN
IN
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
