MIC4680 Micrel Semiconductor, MIC4680 Datasheet - Page 12

• Manufacturer Part Number: MIC4680
• Description: 1A 200kHz SuperSwitcher Buck Regulator Final Information
• Manufacturer: Micrel Semiconductor

MIC4680
Calculating the maximum junction temperature given a
This value is less than the allowable maximum operating
junction temperature of 125 C as listed in “Operating Rat-
ings.” Typical thermal shutdown is 160 C and is listed in
“Electrical Characteristics.”
Increasing the Maximum Output Current
The maximum output current at high input voltages can be
increased for a given board layout. The additional three
MIC4680
T
T
T
T
A
A(max)
J
J
= 1.064
= 106.3 C
at the ambient temperature (T
measured.
= ambient temperature at which T
for the specific design.
maximum ambient temperature of 65 C:
4V to +34V
= maximum ambient operating temperature
GND
V
J1
J3
IN
15 F
35V
C1
* C3 can be used to provide additional stability
20 C/W + (45 C – 25 C) + 65 C
and improved transient response.
OFF
ON
Figure 4. Increasing Maximum Output Current at High Input Voltages
S1
NKK G12AP
0.1 F
50V
C2
+4V to +34V
Power
SOP-8
Figure 6a. Evaluation Board Schematic Diagram
V
IN
2
1
A
SOP-8
IN
SHDN
C
U1 MIC4680BM
) at which T
IN
Figure 5. Critical Traces for Layout
C
GND
is measured.
IN
SHDN
2
1
5–8
MIC4680BM
IN
SHDN
MIC4680BM
SW
5 6 7 8
FB
GND
5 6 7 8
GND
C
3
4
is
SW
FB
SW
D1
B260A
or
SS26
FB
3
12
3
4
68 H
1N4148
L1
2.2nF
components shown in Figure 4 will reduce the overall loss in
the MIC4680 by about 20% at high V
Even higher output current can be achieved by using the
MIC4680 to switch an external FET. See Figure 9 for a 5A
supply with current limiting.
Layout Considerations
Layout is very important when designing any switching regu-
lator. Rapidly changing switching currents through the printed
circuit board traces and stray inductance can generate volt-
age 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
C
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 Figure 6a
though 6e.
R6
optional
IN
D1
68 H
close to pin 2 and pins 5 though 8. See “Applications
L1
82
1
2
R1
3.01k
R2
6.49k
JP1a
1.8V
D1
3
4
C
OUT
R3
2.94k
JP1b
2.5V
C3*
optional
R1
R2
5
6
GND
V
OUT
R4
1.78k
JP1c
3.3V
7
8
R5
976
JP1d
5.0V
C4
220 F
10V
IN
and high I
V
GND
C5
0.1 F
50V
1A
J2
OUT
J4
June 2000
OUT
Micrel
.