ML4872
DESIGN CONSIDERATIONS (Continued)
DESIGN EXAMPLE
INPUTCAPACITOR
In order to design a boost converter using the ML4872, it
is necessary to define a few parameters. For this example,
Due to the high input current drawn at startup and
possibly during operation, it is recommended to decouple
the input with a capacitor with a value of 47µF to 100µF.
This filtering prevents the input ripple from affecting the
ML4872 control circuitry, and also improves the
efficiency by reducing the I squared R losses during the
charge cycle of the inductor. Again, a low ESR capacitor
(such as tantalum) is recommended.
assume that V = 3.0V to 3.6V, V
= 5.0V, and
IN
OUT
I
= 500mA.
OUT(MAX)
First, it must be determined whether the ML4871 is
capable of delivering the output current. This is done
using Equation 1:
3.0V
ꢁ5.0Vꢄ
ꢀ ꢃ
IOUT(MAX) = 1.25
0.7A = 0.53A
ꢂ ꢅ
It is also recommended that low source impedance
batteries be used. Otherwise, the voltage drop across the
source impedance during high input current situations will
cause the ML4872 to fail to start-up or to operate
unreliably. In general, for two cell applications the source
impedance should be less than 200mW, which means that
small alkaline cells should be avoided.
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:
LAYOUT
44 10 H
COUT
=
= 88 F
Good layout practices will ensure the proper operation of
the ML4872. Some layout guidelines follow:
5.0V
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.
• Use adequate ground and power traces or planes
• Keep components as close as possible to the ML4872
• Use short trace lengths from the inductor to the V and
The complete circuit is shown in Figure 8. As mentioned
previously, the use of an input supply bypass capacitor is
highly recommended.
L1
V
pins and from the output capacitor to the V
pin
L2
OUT
• Use a single point ground for the ML4872 ground pin,
and the input and output capacitors
• Separate the ground for the converter circuitry from the
ground of the load circuitry and connect at a single
point
A sample layout is shown in Figure 7.
10µH
(Sumida CD75)
ML4872
V
V
PWR GND
NC
L1
IN
V
IN
GND
V
100µF
L2
V
OUT
SHDN
V
OUT
100µF
Figure 7. Sample PC Board Layout
Figure 8. Typical Application Circuit
7
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