ML4875CS-T datasheet(8/10 Pages) MICRO-LINEAR

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ML4875

INPUT CAPACITOR

Unless the input source is a very low impedance battery,

it will be necessary to decouple the input with a capacitor

with a value of between 47µF and 100µF. This provides

the benefits of preventing input ripple from affecting the

ML4875 control circuitry, and it also improves efficiency

by reducing I-squared R losses during the charge and

discharge cycles of the inductor. Again, a low ESR

capacitor (such as tantalum) is recommended.

DRIVING THE SHDN INPUT

Unlike other boost regulators which use external Schottky

diodes, the ML4875 has the ability to isolate the load from

the battery input when the SHDN pin is high. Since there

may be no other voltage available when the regulator is in

shutdown, the SHDN input threshold is set well below the

minimum VIN voltage. SHDN can be driven directly from

an open collector device with a high value pull-up resistor

to VIN. If SHDN is driven from a TTL or CMOS output

device, a resistor divider should be used to prevent the

SHDN input high level from exceeding VIN, and to ensure

the SHDN input low level is below the 200mV threshold.

SETTING THE RESET THRESHOLD

To use the RESET comparator as an input voltage monitor,

it is necessary to use an external resistor divider tied to the

DETECT pin as shown in the block diagram. The resistor

values RA and RB can be calculated using the following

equation:

IN MIN

()

=×

(5)

The value of RB should be 100ký or less to minimize bias

current errors. RA is then found by rearranging the

equation:

IN MIN

=×

−









()

(6)

LAYOUT

Good PC board layout practices will ensure the proper

operation of the ML4875. Important layout considerations

include:

• Use adequate ground and power traces or planes

• Keep components as close as possible to the ML4875

• Use short trace lengths from the inductor to the VL pin

and from the output capacitor to the VOUT pin

• Use a single point ground for the ML4875 ground pins,

and the input and output capacitors

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