mic4682 Micrel Semiconductor, mic4682 Datasheet - Page 12

• Manufacturer Part Number: mic4682
• Description: Mic4682 Precision Current Limit So-8 Superswitcher? Buck Regulator
• Manufacturer: Micrel Semiconductor

A worst-case rule of thumb is to assume that 80% of the
total output power dissipation is in the MIC4682 (P
and 20%is in the diode-inductor-capacitor circuit.
Calculate the worst-case junction temperature:
where:
The
approximately 20°C/W.
4V to 34V
Micrel, Inc.
June 2007
GND
V
J1
J2
IN
10µF
50V
θ
C1
JC
P
P
P
P
T
T
P
θ
T
the entry point of pins 6 or 7.
T
T
temperature for the specific design.
P
P
JC
J
J
C
A
A(max)
D
D(IC)
D(IC)
D(IC)
D(IC)
D
D
= P
= MIC4682 junction temperature
= ambient temperature
= 1.17W
= “pin” temperature measurement taken at
= junction-to-case thermal resistance.
=
=
10µF
for
50V
C2
= 0.8 P
= 0.8 × 1.17W
= 0.936W
= MIC4682 power dissipation
P
0.81
5W
D(IC)
OUT
η
=
the
0.1µF
θ
−
50V
−
JC
C3
5
D
P
W
+ (T
maximum
OUT
MIC4682’s
C
OFF
ON
– T
1
3
JP1
2
A
JP1 1-2=OFF
JP1 2-3-ON
) + T
2
1
+4V to +34V
R8
option
Power
SOIC-8
JP3c
2.0A
ambient
A(max)
V
Figure 5a. Evaluation Board Schematic Diagram
power-SOIC-8
IN
4
3
C
R7
16.2k
JP3b
1.0A
IN
R9
10M
Figure 4. Critical Traces for Layout
6
5
5
4
R6
25k
operating
JP3a
0.6A
IN
SHDN
MIC4682BM
GND
2 6 7
5
4
3
D(IC)
IN
SHDN
ISET
U1 MIC4682BM
GND
I
is
SET
)
2
3
SW
FB
GND
8
1
GND
SW
6
12
FB
D1
8
7
1
Calculating the maximum junction temperature given a
maximum ambient temperature of 65°C:
This value is within the allowable maximum operating
junction 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
regulator. 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 4, as
short as possible. For example, D1 should be close to
pin 7 and pin 8. C
See “Applications Information: Thermal Considerations”
for ground 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 Figures
5a though 5e. Gerber files are available upon request.
68µH
1
2
L1
1
D1
B340A
2
1
68µH
L1
R5
6.49k
JP2a
1.8V
T
T
3.01k
J
J
C
R1
= 0.936 × 20°C/W + (45°C – 25°C) + 65°C
= 103.7°C
OUT
2
4
3
R4
2.94k
JP2b
2.5V
R1
R2
GND
V
OUT
6
5
C4
Option
IN
R3
1.78k
JP2c
3.3V
should be close to pin 5 and pin 6.
8
7
R2
976
JP2d
5.0V
C5
220µF
10V
C6
Option
M9999-061507
MIC4682
C7
0.1µF
50V
J3
V
J4
GND
OUT