ML4870
DEISGN CONSIDERATIONS (Continued)
INPUT CAPACITOR
10µH
(SUMIDA CD75)
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
ML4870 control circuitry, and also improves the
ML4870
V
V
PWR GND
NC
L1
IN
V
IN
2
efficiency by reducing the I R losses during the charge
cycle of the inductor. Again, a low ESR capacitor (such as
tantalum) is recommended.
GND
V
100µF
L2
V
OUT
SHDN
V
OUT
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 ML4870 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.
100µF
Figure 8. Design Example Schematic Diagram
DESIGN EXAMPLE
SHUTDOWN
The input levels of the SHDN pin are CMOS compatible.
To guarantee proper operation, SHDN must be pulled to
In order to design a boost converter using the ML4870,
it is necessary to define the values of a few parameters.
within 0.5V of GND or V to prevent excessive power
For this example, we have assumed that V = 3.0V to
IN
IN
dissipation and possible oscillations. A graph of input
leakage current while in shudown is shown in Figure 6.
3.6V, V
= 5.0V, and I
= 400mA
OUT(MAX)
OUT
First, it must be determined whether the ML4870 is
capable of delivering the output current. This is done
using Equation 1:
LAYOUT
3.0V
ꢁ5.0Vꢄ
Good layout practices will ensure the proper operation of
the ML4870. Some layout guidelines follow:
ꢀ ꢃ - 0.144 = 439mA
IOUT(MAX) = 0.972
ꢂ ꢅ
• Use adequate ground and power traces or planes
• Keep components as close as possible to the ML4870
Next, select an inductor:
As previously mentioned, it is 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.
• Use short trace lengths from the inductor to the V and
L1
OUT
V
L2
pins and from the output capacitor to the V
pin
• Use a single point ground for the ML4870 PWR GND
pin and the input and output capacitors, and connect
the GND pin to PWR GND using a separate trace
Finally, the value of the output capacitor is determined
using Equation 3:
44 10 H
COUT
=
= 88 F
• Separate the ground for the converter circuitry from the
ground of the load circuitry and connect at a single
point
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.
The complete circuit is shown in Figure 8. As mentioned
previously, the use of an input supply bypass capacitor is
strongly recommended.
7
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