MIC4680 Datasheet, PDF (12/16 Pages) Micrel Semiconductor – 1A 200kHz SuperSwitcher™ Buck Regulator Final Information

MIC4680
Micrel
at the ambient temperature (T_A) at which T_C is
measured.
T_A = ambient temperature at which T_C is measured.
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 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
MIC4680BM
IN
SW ³
components shown in Figure 4 will reduce the overall loss in
the MIC4680 by about 20% at high V_IN and high I_OUT.
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_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 Figure 6a
though 6e.
SHDN
1N4148 82Ω ^D1
FB
GND
5678
2.2nF
Figure 4. Increasing Maximum Output Current at High Input Voltages
V_IN
+4V to +34V
C_IN
Power
SOP-8
MIC4680BM
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 +34V
C1
15µF
35V
J3
GND
C2
0.1µF
OFF
50V
ON
S1
NKK G12AP
U1 MIC4680BM
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*
optional
D1
R6
B260A optional
or
1
SS26
2
R2
6.49k
3
JP1a
1.8V
4
R3
2.94k
5
JP1b
2.5V
6
R4
1.78k
7
JP1c
3.3V
8
R5
976Ω
JP1d
5.0V
Figure 6a. Evaluation Board Schematic Diagram
C4
220µF
10V
J2
V_OUT
1A
C5
0.1µF
50V
J4
GND
MIC4680
12
June 2000