ML4775
ML4775 (VOUT = 5V)
ML4775 (VOUT = 3.3V)
L = 15µH
400
300
200
100
0
400
300
200
100
L = 15µH
L = 27µH
L = 27µH
L = 56µH
L = 56µH
0
0
0
1.0
2.0
3.0
(V)
4.0
5.0
1.0
2.0
3.0
(V)
4.0
5.0
V
IN
V
IN
Figure 4. Output Current vs Input Voltage.
ML4775 (VOUT = 3.3V)
ML4775 (VOUT = 5V)
100
90
80
70
60
50
100
90
80
70
60
50
L = 56µH
L = 56µH
L = 27µH
L = 27µH
L = 15µH
L = 15µH
0
1.0
2.0
(V)
3.0
0
1.0
2.0
3.0
(V)
4.0
5.0
V
V
IN
IN
Figure 5. Typical Efficiency as a Function of V .
IN
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
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 ML4775 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, “Choosing an Inductor for Your ML4861
Application.”
unreliable because the inductor current is so small that it
is difficult for the control circuitry to detect.
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:
Suitable inductors can be purchased from the following
suppliers:
T
× V
IN (MAX)
ON (MAX)
Coilcraft
Coiltronics
Dale
(708) 639-6400
(407) 241-7876
(605) 665-9301
(708) 956-0666
I
=
(3)
L(PEAK)
L
MIN
Sumida
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
