ML4871
DESIGN EXAMPLE
In order to design a boost converter using the ML4871, it
is necessary to define a few parameters. For this example,
assume that V
IN
= 3.0V to 3.6V, V
OUT
= 5.0V, and
I
OUT(MAX)
= 500mA.
First, it must be determined whether the ML4871 is
capable of delivering the output current. This is done using
Equation 1:
I
OUT( MAX)
=
125
.
.
30V
0.7A
=
0.53A
50V
.
VIN
The complete circuit is shown in Figure 9. As mentioned
previously, the use of an input supply bypass capacitor is
highly recommended.
10µH
(Sumida CD75)
ML4871
VL1
VIN
100µF
GND
DETECT
PWR GND
RESET
VL2
VOUT
100µF
VOUT
Next, select an inductor. As previously mentioned, the
recommended inductance is 10µH. Make sure that the
peak current rating of the inductor is at least 1.5A, and
that the DC resistance of the inductor is in the range of 50
to 100mW.
Finally, the value of the output capacitor is determined
using Equation 2:
C
OUT
=
44
10
m
H
=
88
m
F
5.0V
Figure 9. Typical Application Circuit
The closest standard value would be a 100µF capacitor
with an ESR rating of 100mW. If such a low ESR value
cannot be found, two 47µF capacitors in parallel could
also be used.
I
OUT(MAX)
(mA)
V
IN
(V)
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
4.8
I
OUT
(mA)
V
IN
= 2.4V, V
OUT
= 3.3V
1.0
2.0
5.0
10.0
20.0
50.0
100.0
200.0
586.0
V
IN
= 2.4V, V
OUT
= 5.0V
1.0
2.0
5.0
10.0
20.0
50.0
100.0
200.0
485.0
EFFICIENCY PERCENTAGE
82.0
84.4
87.0
87.6
87.9
88.3
88.6
88.2
65.1
84.4
87.0
87.7
88.4
88.9
89.1
88.9
87.5
71.6
V
OUT
= 3.3V
386.2
451.9
521.5
585.9
651.0
716.5
782.0
V
OUT
= 5.0V
286.2
332.1
379.1
430.0
479.0
525.4
571.8
618.5
665.0
711.7
758.7
805.3
851.9
899.0
946.1
992.7
Table 1. Typical I
OUT
and Efficiency vs. V
IN
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
