Micrel, Inc.
MIC4685
Layout Considerations
Bootstrap Diode
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.
The bootstrap diode provides an external bias source
directly to the main pass element, this reduces VSAT thus
allowing the MIC4685 to be used in very low head-room
applications i.e., 5VIN to 3.3VOUT with high efficiencies.
Bootstrap diode not for use if VIN exceeds 16V, VIN. See
Figure 2.
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 CIN 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.
VIN
+4V to +30V
MIC4685_R
(34V transient)
2
5
1
6
IN
EN
BS
L1
VOUT
R1
SW
CIN
39µH
COUT
3
FB
7-pin
SPAK
GND
D1
4,Tab
R2
4, Tab
GND
Figure 2. Critical Traces for Layout
M9999-012610
January 2010
12
MICREL [ MICREL SEMICONDUCTOR ]
