MIC4684BM Micrel Inc, MIC4684BM Datasheet - Page 11

• Manufacturer Part Number: MIC4684BM
• Description: IC REG BUCK 2A ADJ VOLT HE 8SOIC
• Manufacturer: Micrel Inc
• Series: SuperSwitcher™r
• Type: Step-Down (Buck)r

Specifications of MIC4684BM

• Mounting Type: Surface Mount
• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adj to 1.235V
• Current - Output: 2A
• Frequency - Switching: 200kHz
• Voltage - Input: 4 ~ 30 V
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 8-SOIC (3.9mm Width)
• No. Of Pins: 8
• Peak Reflow Compatible (260 C): No
• Current Rating: 2A
• Output Voltage Max: 1.235V
• Supply Voltage Max: 30V
• Leaded Process Compatible: No
• Voltage Regulator Type: Switching
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant, Lead free / RoHS Compliant

A worst-case rule of thumb is to assume that 80% of the total
output power dissipation is in the MIC4684 (P
is in the diode-inductor-capacitor circuit.
Calculate the worst-case junction temperature:
where:
The θ
25°C/W.
entry point of pins 2, 6 or 7
for the specific design.
Calculating the maximum junction temperature given a
maximum ambient temperature of 60°C:
January 2010
MIC4684
P
P
P
T
T
P
θ
JC
T
T
T
T
T
JC
A
A(max)
J
J
C
J
J
D(IC)
D(IC)
D(IC)
D(IC)
for the MIC4684’s power-SOP-8 is approximately
= P
= MIC4684 junction temperature
= ambient temperature
= 0.76 × 25°C/W + (41°C – 25°C) + 60°C
= 95°C
= “pin” temperature measurement taken at the
= junction-to-case thermal resistance.
D(IC)
= 0.8 P
= 0.8 × 0.95W
= 0.76W
= MIC4684 power dissipation
= maximum ambient operating temperature
θ
JC
D
+ (T
C
– T
A
) + T
(34V transient)
+4V to +30V
Power
SOP-8
A(max)
V
IN
C
IN
Figure 5. Critical Traces for Layout
D(IC)
3
8
IN
EN
) and 20%
MIC4684BM
2
GND
6
7
SW
BS
FB
11
4
1
5
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
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
allowing the MIC4684 to be used in very low head-room ap-
plications I.E. 5V
D1
68µH
L1
C
OUT
IN
to 3.3V
R1
R2
V
GND
IN
OUT
close to pin 3 and pins 2, 6, and 7.
OUT.
M9999-012610
Micrel, Inc.
SAT
thus