MIC4684_10 Datasheet, PDF (11/16 Pages) Micrel Semiconductor – 2A High-Efficiency SuperSwitcher Buck Regulator

MIC4684
A worst-case rule of thumb is to assume that 80% of the total
output power dissipation is in the MIC4684 (P_D(IC)) and 20%
is in the diode-inductor-capacitor circuit.
P_D(IC) = 0.8 P_D
P_D(IC) = 0.8 × 0.95W
P_D(IC) = 0.76W
Calculate the worst-case junction temperature:
T_J = P_D(IC) θ_JC + (T_C – T_A) + T_A(max)
where:
T_J = MIC4684 junction temperature
P_D(IC) = MIC4684 power dissipation
θ_JC = junction-to-case thermal resistance.
The θ_JC for the MIC4684’s power-SOP-8 is approximately
25°C/W.
T_C = “pin” temperature measurement taken at the
entry point of pins 2, 6 or 7
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 60°C:
T_J = 0.76 × 25°C/W + (41°C – 25°C) + 60°C
T_J = 95°C
Micrel, Inc.
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 Charac-
teristics. Also see SOA curves on pages 7 through 8.
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 as short as possible. For example, keep D1 close
to pin 1 and pins 2, 6, and 7, keep L1 away from sensitive
node FB, and keep C_IN close to pin 3 and pins 2, 6, and 7.
See Applications Information: Thermal Considerations for
ground plane layout.
The feedback pin should be kept as far way from the switch-
ing elements (usually L1 and D1) as possible.
A circuit with sample layouts are provided. See Figure 7.
Gerber files are available upon request.
Feed Forward Diode
The FF diode (feed forward) provides an external bias source
directly to the main pass element, this reduces V_SAT thus
allowing the MIC4684 to be used in very low head-room ap-
plications I.E. 5V_IN to 3.3V_OUT.
V_IN
+4V to +30V
(34V transient)
C_IN
Power
SOP-8
MIC4684BM
3 _IN
BS ⁴
8 _EN
SW ¹
FB ⁵
GND
267
L1
68µH
D1
V_OUT
C_OUT ^R1
R2
GND
Figure 5.  Critical Traces for Layout
January 2010
11
M9999-012610