MIC4685
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 6 and pin 4, and Tab, keep L1 away from sensitive node
FB, and keep C
IN
close to pin 2 and pin 4, and Tab. 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 Figure 7.
Gerber files are available upon request.
Bootstrap Diode
Micrel
The bootstrap diode provides an external bias source directly
to the main pass element, this reduces V
SAT
thus allowing the
MIC4685 to be used in very low head-room applications i.e.
5V
IN
to 3.3V
OUT
with high efficiencies. Bootstrap diode not for
use if V
IN
exceeds 16V, V
IN
. See Figure 3.
V
IN
+4V to +30V
(34V transient)
C
IN
7-lead
SPAK
2
5
MIC4685BR
IN
BS
EN
SW
FB
GND
4, Tab
1
6
L1
39µH
C
OUT
D1
V
OUT
R1
R2
GND
Figure 2. Critical Traces for Layout
September 2002
11
Load
3
MIC4685
MICREL [ MICREL SEMICONDUCTOR ]
