ML4761
500
ML4761(V
OUT
= 3.3V)
L = 10µH
L = 15µH
500
ML4761(V
OUT
= 5.0V)
L = 27µH
L = 15µH
400
400
I
OUT
MAX (mA)
I
OUT
MAX (mA)
300
L = 27µH
L = 10µH
300
L = 56µH
200
200
L = 56µH
100
100
0
1.0
2.0
V
IN
(V)
3.0
0
1.0
2.0
V
IN
(V)
3.0
4.0
Figure 4. Output Current vs. Input Voltage.
95%
90%
ML4761-(V
OUT
= 3.3V)
95%
ML4761-(V
OUT
= 5.0V)
L = 57µH
L = 56µH
EFFICIENCY AT I
OUT
MAX
90%
L = 27µH
85%
L = 15µH
80%
EFFICIENCY AT I
OUT
MAX
85%
L = 27µH
80%
L = 15µH
75%
L = 10µH
70%
75%
L = 10µH
70%
0
1.0
2.0
V
IN
(V)
3.0
4.0
65%
0
1
2
V
IN
3
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 10µH, the
efficiency drops to between 70% and 75%. With 56µH,
the efficiency exceeds 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.
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:
I
L(PEAK)
T
×
V
IN(MAX)
=
ON(MAX)
L
MIN
(3)
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 ML4761 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 Application
Note 29.
Suitable inductors can be purchased from the following
suppliers:
Coilcraft
Coiltronics
Dale
Sumida
(708) 639-6400
(407) 241-7876
(605) 665-9301
(708) 956-0666
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
