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ML4870CS-3

High Current Boost Regulator with Load Disconnect

Micro Linear Corporation 

DEISGN CONSIDERATIONS (Continued)

INPUT CAPACITOR

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

efficiency by reducing the I2R losses during the charge

cycle of the inductor. Again, a low ESR capacitor (such as

tantalum) is recommended.

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.

SHUTDOWN

The input levels of the SHDN pin are CMOS compatible.

To guarantee proper operation, SHDN must be pulled to

within 0.5V of GND or VIN to prevent excessive power

dissipation and possible oscillations. A graph of input

leakage current while in shudown is shown in Figure 6.

LAYOUT

Good layout practices will ensure the proper operation of

the ML4870. Some layout guidelines follow:

• Use adequate ground and power traces or planes

• Keep components as close as possible to the ML4870

• Use short trace lengths from the inductor to the VL1 and

VL2 pins and from the output capacitor to the VOUT 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

• Separate the ground for the converter circuitry from the

ground of the load circuitry and connect at a single

point

ML4870

VIN

100µF

10µH

(SUMIDA CD75)

ML4870

VL1 PWR GND

VIN

NC

GND

VL2

SHDN VOUT

VOUT

100µF

Figure 8. Design Example Schematic Diagram

DESIGN EXAMPLE

In order to design a boost converter using the ML4870,

it is necessary to define the values of a few parameters.

For this example, we have assumed that VIN = 3.0V to

3.6V, VOUT = 5.0V, and IOUT(MAX) = 400mA

First, it must be determined whether the ML4870 is

capable of delivering the output current. This is done

using Equation 1:

  IOUT(MAX) = 0.972 ™

3.0V

5.0V

- 0.144 = 439mA

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.

Finally, the value of the output capacitor is determined

using Equation 3:

COUT

=

44 ™ 10mH

5.0V

= 88mF

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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