MAX887HC Просмотр технического описания (PDF) - Maxim Integrated

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MAX887HC

100% Duty Cycle, Low-Noise, Step-Down, PWM DC-DC Converter

Maxim Integrated 

100% Duty Cycle, Low-Noise,

Step-Down, PWM DC-DC Converter

Bypass V+ to GND using a 0.33µF capacitor. Also

bypass VL to GND with a 2.2µF capacitor, and VREF to

GND using a 0.047µF capacitor. These capacitors

should be placed within 0.2in (5mm) of their respective

pins. A small ceramic capacitor (C1) of around 100pF

to 470pF should be added in parallel with R1 to com-

pensate for stray capacitance at the FB pin and output

capacitor ESR.

Output Diode Selection

A 1A external diode (D1) is required as an output recti-

fier to pass inductor current during the second half of

each cycle. This diode operates in PFM mode and dur-

ing transition periods while the synchronous rectifier is

off. Use a Schottky diode to prevent the slow internal

diode of the N-channel MOSFET from turning on.

PC Board Layout and Routing

High switching frequencies and large peak currents

make PC board layout a very important part of design.

Poor design can result in excessive EMI on the feed-

back paths and voltage gradients in the ground plane,

both of which can result in instability or regulation

errors. Power components, such as the MAX887,

inductor, input filter capacitor, and output filter capaci-

tor should be placed as close together as possible,

and their traces kept short, direct, and wide. Connect

their ground pins at a common node in a star-ground

configuration. Keep the extra copper on the board and

integrate into ground as a pseudo-ground plane. The

external voltage-feedback network should be very

close to the FB pin, within 0.2in (5mm). Keep noisy

traces, such as from the LX pin, away from the voltage-

feedback network, and separate using grounded cop-

per. Place the small bypass capacitors (C1, C3, C5,

and C6) within 0.2in (5mm) of their respective pins. The

MAX887 evaluation kit manual illustrates an example

PC board layout, routing, and pseudo-ground plane.

___________________Chip Information

TRANSISTOR COUNT: 2006

SUBSTRATE CONNECTED TO GND

______________________________________________________________________________________ 11

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