ML4871CS-3 datasheet(7/9 Pages) MICRO-LINEAR

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ML4871

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

ML4871 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.

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 ML4871 to fail to start-up or to operate

unreliably. In general, for two cell applications the source

impedance should be less than 200m

W, which means that

small alkaline cells should be avoided.

BATTERY MONITORING

The condition of the batteries can be monitored using the

DETECT pin. For primary batteries, the comparator can be

used to signal that the batteries will soon need to be

replaced. For rechargeable batteries, the comparator can

be used to signal the start of a charging cycle.

For input voltages greater than the minimum operating

voltage, the RESET pin can be set to go low at a specified

battery voltage by connecting a resistor divider across the

battery stack and to the DETECT pin of the ML4871 as

shown in Figure 7. The low battery trip voltage is

determined by first choosing a minimum battery voltage,

VIN(MIN), and then calculating the values of RA and RB:

IN MIN

()

125 16

(3)

The values of RA and RB should be sufficiently large to

minimize the power dissipation in the divider. Also, use

care when selecting the low battery trip point. Too high a

trip voltage can lead to memory effects in the battery,

while too low a trip point can lead to reduced service life

or polarity reversal. Refer to the manufacturer’s data sheets

for more information on selecting and designing battery

systems.

LAYOUT

Good layout practices will ensure the proper operation of

the ML4871. Some layout guidelines follow:

• Use adequate ground and power traces or planes

• Keep components as close as possible to the ML4871

• 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 ML4871 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 8.

DESIGN CONSIDERATIONS (Continued)

Figure 8. Sample PC Board Layout

Figure 7. Battery Monitoring Circuit

VIN

DETECT

RESET

VREF

FROM

START-UP

CIRCUITRY

–

COMP

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