ML4790
Figure 7 shows efficiency under the conditions used to
create Figure 6. 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 47µH,
the efficiency reaches approximately 90% and there is
little room for improvement. At values greater than 47µH,
the operation of the synchronous rectifier becomes
unreliable at low input voltages because the inductor
current is so small that it is difficult for the control circuitry
to detect as shown for the 5.5V output.
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
ON(MAX)
×
V
IN(MAX)
L
MIN
(2)
It is important to note that for reliable operation, make
sure that I
L(PEAK)
does not exceed the 1A maximum switch
current rating. In the two cell application previously
described, a maximum input voltage of 3V would give a
peak current of 880mA. 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 ML4790 to
determine which inductor is the best choice. Check
200
180
160
V
OUT =
5.5V
L = 22µH
180
160
140
V
OUT =
4.5V
L = 22µH
I
OUT
MAX (mA)
120
100
80
60
40
20
0
1.0
2.0
3.0
V
IN
(V)
4.0
5.0
L = 10µH
I
OUT
MAX (mA)
140
L = 10µH
120
100
80
60
40
20
0
1.0
L = 47µH
L = 47µH
1.5
2.0
2.5
V
IN
(V)
3.0
3.5
4.0
140
120
100
80
60
V
OUT =
3.5V
L = 10µH
L = 22µH
35
30
25
I
OUT
MAX (mA)
V
OUT =
2.5V
L = 10µH
L = 22µH
I
OUT
MAX (mA)
20
L = 47µH
15
10
5
0
1.0
L = 47µH
40
20
0
1.0
1.5
2.0
V
IN
(V)
2.5
3.0
1.2
1.4
1.6
V
IN
(V)
1.8
2.0
Figure 6. Output Current versus Input Voltage.
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
