Micrel, Inc.
Layout Considerations
Layout is very important when designing any switching
regulator. 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 6.
Gerber
files
are available upon request.
MIC4685
Bootstrap Diode
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 2.
V
IN
+4V to +30V
(34V transient)
C
IN
7-pin
SPAK
MIC4685_R
2 IN
BS 1
5
6
EN
SW
FB 3
D1
L1
39µH
C
OUT
V
OUT
R1
R2
Load
GND
4,Tab
4, Tab
GND
Figure 2. Critical Traces for Layout
January 2010
12
M9999-012610
MICREL [ MICREL SEMICONDUCTOR ]
