MIC4681_05 Datasheet, PDF (13/16 Pages) Micrel Semiconductor – 2A-Peak SuperSwitcher SOP-8 Buck Regulator

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
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.
V_IN
+4V to +30V
C_IN
Power
SOP-8
MIC4681BM
2 _IN
SW ³
1 _SHDN
FB ⁴
GND
5678
L1
68µH
D1
V_OUT
C_OUT R1
R2
GND
Figure 5. Critical Traces for Layout
J1
V_IN
4V to +30V
C1
22µF
35V
J3
GND
C2
0.1µF
OFF
50V
ON _JP1
U1 MIC4681BM
2 _IN
SW ³
1 _SHDN
FB ⁴
GND
SOP-8
5–8
* C3 can be used to provide additional stability
and improved transient response.
L1
68µH
R1
3.01k
C3* 1800pF / 50V
optional
D1
R6
B340A optional
or
1
SS36
2
R2
6.49k
3
JP2a
1.8V
4
R3
2.94k
5
JP2b
2.5V
6
R4
1.78k
7
JP2c
3.3V
8
R5
976Ω
JP2d
5.0V
Figure 6a. Evaluation Board Schematic Diagram
C4
220µF
10V
J2
V_OUT
1A
C5
0.1µF
50V
J4
GND
April 2005
13
MIC4681