MIC4685

Micrel

September 2002

11

MIC4685

BS

L1

39

µH

IN

FB

GND

R1

R2

D1

MIC4685BR

GND

+4V to +30V

(34V transient)

7-lead

SPAK

4, Tab

3

1

2

SW

EN

6

5

Figure 2. Critical Traces for Layout

Layout Considerations

Layout is very important when designing any switching regu-

lator. Rapidly changing currents through the printed circuit

board traces and stray inductance can generate voltage

transients which can cause problems.

To minimize stray inductance and ground loops, keep trace

lengths as short as possible. For example, keep D1 close to

pin 6 and pin 4, and Tab, keep L1 away from sensitive node

FB, and keep C

plane layout.

The feedback pin should be kept as far way from the switching

elements (usually L1 and D1) as possible.

A circuit with sample layouts are provided. See Figure 7.

Gerber files are available upon request.

Bootstrap Diode

The bootstrap diode provides an external bias source directly

to the main pass element, this reduces V

MIC4685 to be used in very low head-room applications i.e.

5V

use if V