ML4875
ML4875-3.3
ML4875-5.0
L = 15µH
L = 27µH
400
300
200
100
0
400
300
200
100
0
L = 15µH
L = 27µH
L = 56µH
L = 56µH
0
1.0
2.0
3.0
(V)
4.0
5.0
0
1.0
2.0
3.0
(V)
4.0
5.0
V
IN
V
IN
ML4875-3.0
400
L = 15µH
300
200
100
0
L = 27µH
L = 56µH
0
1.0
2.0
V
3.0
(V)
4.0
IN
Figure 4. Output Current vs Input Voltage.
tolerances. Assuming that 2V is the end of life voltage of a
two cell input, Figure 4 shows that with a 2V input, the
ML4875-5 delivers 99mA with a 27µH inductor.
In the two cell application previously described, a
maximum input voltage of 3V would give a peak current
of 1.2A. When comparing various inductors, it is
important to keep in mind that suppliers use different
criteria to determine their ratings. Many use a
conservative current level, where inductance has dropped
to 90% of its normal level. In any case, it is a good idea to
try inductors of various current ratings with the ML4875 to
determine which inductor is the best choice. Check
efficiency and maximum output current, and if a current
probe is available, look at the inductor current to see if it
looks like the waveform shown in Figure 3. For additional
information, see Applications Note 29.
Figure 5 shows efficiency under the conditions used to
create Figure 4. It can be seen that efficiency is mostly
independent of input voltage and is closely related to
inductor value. This illustrates the need to keep the
inductor value as high as possible to attain peak system
efficiency. As the inductor value goes down to 15µH, the
efficiency drops to between 70% and 75%. With 56µH,
the efficiency approaches 90% and there is little room for
improvement. At values greater than 100µH, the operation
of the synchronous rectifier becomes unreliable because
the inductor current is so small that it is difficult for the
control circuitry to detect. The data used to generate
Figures 4 and 5 is provided in Table 1.
Suitable inductors can be purchased from the following
suppliers:
Coilcraft
Coiltronics
Dale
(708) 639-6400
(407) 241-7876
(605) 665-9301
(708) 956-0666
After the appropriate inductor value is chosen, it is
necessary to find the minimum inductor current rating
required. Peak inductor current is determined from the
following formula:
Sumida
T
× V
IN(MAX)
ON(MAX)
I
=
(3)
L(PEAK)
L
MIN
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
