MIC4681 datasheet(13/19 Pages) MICREL

13 / 19 page

February 2001

MIC4681

Micrel

µH

GND

SHDN

COUT

VOUT

MIC4681BM

GND

CIN

VIN

+4V to +30V

Power

SOP-8

5678

Figure 5. Critical Traces for Layout

µH

GND

SHDN

B340A

SS36

VOUT

GND

U1 MIC4681BM

0.1

µF

50V

µF

35V

VIN

4V to +30V

GND

SOP-8

5–8

JP1

OFF

220

µF

10V

C3* 1800pF / 50V

optional

0.1

µF

50V

3.01k

6.49k

JP2a

1.8V

optional

2.94k

1.78k

976

Ω

JP2b

2.5V

JP2c

3.3V

JP2d

5.0V

* C3 can be used to provide additional stability

and improved transient response.

Figure 6a. Evaluation Board Schematic Diagram

A = ambient temperature

A(max) = maximum ambient operating temperature

for the specific design.

Calculating the maximum junction temperature given a

maximum ambient temperature of 65

°C:

J = 1.064 × 20°C/W + (45°C – 25°C) + 65°C

J = 106.3°C

This value is within the allowable maximum operating junc-

tion temperature of 125

°C as listed in “Operating Ratings.”

Typical thermal shutdown is 160

°C and is listed in “Electrical

Characteristics.”

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, indicated by the heavy lines in Figure 5, as short as

possible. For example, keep D1 close to pin 3 and pins 5

through 8, keep L1 away from sensitive node FB, and keep

IN close to pin 2 and pins 5 though 8. See “Applications

Information: Thermal Considerations” for ground plane lay-

out.

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 Figures 6a

though 6e. Gerber files are available upon request.

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