MIC4681
T
A
= ambient temperature
T
A(max)
= maximum ambient operating temperature
for the specific design.
Calculating the maximum junction temperature given a
maximum ambient temperature of 65°C:
T
J
= 1.064
×
20°C/W + (45°C – 25°C) + 65°C
T
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
V
IN
+4V to +30V
C
IN
Micrel
C
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.
MIC4681BM
2
IN
SW
3
L1
68µH
C
OUT
V
OUT
R1
Power
SOP-8
GND
5 6 7 8
D1
R2
GND
Figure 5. Critical Traces for Layout
J1
V
IN
4V to +30V
C1
22µF
35V
J3
GND
C2
0.1µF
50V
JP1
Load
1
SHDN
FB
4
2
U1
MIC4681BM
IN
SW
3
L1
68µH
R1
3.01k
R2
6.49k
JP2a
1.8V
3
J2
V
OUT
1A
C3* 1800pF / 50V
optional
R3
2.94k
JP2b
2.5V
5
OFF
ON
1
SHDN
GND
SOP-8
5–8
FB
4
D1
R6
B340A optional
or
1
SS36
2
R4
1.78k
JP2c
3.3V
7
R5
976Ω
JP2d
5.0V
C4
220µF
10V
C5
0.1µF
50V
J4
GND
* C3 can be used to provide additional stability
and improved transient response.
4
6
8
Figure 6a. Evaluation Board Schematic Diagram
April 2005
13
MIC4681
MICREL [ MICREL SEMICONDUCTOR ]
