ML4890
For example, a two cell to 5V application requires 40mA
of output current while using an inductor with 15%
tolerance. The output current should be derated by 25%
to 50mA to cover the combined inductor and ON-time
tolerances. Assuming that 2V is the end of life voltage of a
two cell input, Figure 5 shows that with a 2V input, the
ML4890-5 delivers 58mA with a 22µH inductor.
Figure 6 shows efficiency under the conditions used to
create Figure 5. 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 33µH,
the efficiency reaches approximately 85% 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. The data used to generate Figures 5 and 6 is
provided in Table 1.
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 ML4890 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 4.
95
95
ML4890-T
Efficiency at I
OUT
MAX (%)
L = 68µH
ML4890-3
Efficiency at I
OUT
MAX (%)
90
90
L = 47µH
85 L = 33µH
80 L = 22µH
L = 15µH
75
L = 10µH
70
L = 68µH
L = 47
µH
85
L = 33
µH
80
L = 22
µH
L = 15
75
µ
H
L=1
70
0
µ
H
65
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
65
1.0
1.5
2.0
V
IN
(V)
2.5
3.0
V
IN
(V)
95
ML4890-5
L = 68µH
L = 47
µ
H
Efficiency at I
OUT
MAX (%)
90
85
80
75
L = 10µH
70
65
60
1.0
L = 15
µ
H
L = 22
µ
L = 33
µ
H
H
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Figure 6. Typical Efficiency as a Function of V
IN
.
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
