MIC4681
Micrel
T = ambient temperature
C
close to pin 2 and pins 5 though 8. See “Applications
A
IN
Information: Thermal Considerations” for ground plane lay-
out.
T
= maximum ambient operating temperature
A(max)
for the specific design.
Thefeedbackpinshouldbekeptasfarwayfromtheswitching
elements (usually L1 and D1) as possible.
Calculating the maximum junction temperature given a
maximum ambient temperature of 65°C:
A circuit with sample layouts are provided. See Figures 6a
though 6e. Gerber files are available upon request.
T = 1.064 × 20°C/W + (45°C – 25°C) + 65°C
J
T = 106.3°C
J
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
VIN
MIC4681BM
L1
VOUT
COUT R1
+4V to +30V
2
1
3
4
IN
SW
68µH
CIN
SHDN
GND
FB
D1
R2
Power
SOP-8
5
6 7 8
GND
Figure 5. Critical Traces for Layout
J1
VIN
J2
VOUT
1A
U1 MIC4681BM
L1
4V to +30V
2
1
3
4
IN
SW
C2
0.1µF
50V
68µH
C3* 1800pF / 50V
optional
R1
C1
22µF
35V
J3
GND
3.01k
OFF
ON
SHDN
FB
C4
220µF
10V
C5
0.1µF
50V
JP1
D1
R6
R2
R3
2.94k
R4
1.78k
R5
976Ω
GND
5–8
B340A optional 6.49k
or
SOP-8
1
3
5
SS36
JP2a
1.8V
JP2b
JP2c7 JP2d
3.3V 5.0V
2.5V
J4
GND
8
2
4
6
* C3 can be used to provide additional stability
and improved transient response.
Figure 6a. Evaluation Board Schematic Diagram
April 2005
13
MIC4681
MICREL [ MICREL SEMICONDUCTOR ]
