E910.27A View Datasheet(PDF) - ELMOS Semiconductor AG
Part Name
Description
MFG CO.
'E910.27A' PDF : 50 Pages View PDF
- Preliminary -
E910.27A
9 PCB Layout Considerations
The PC board layout is very important in all high-frequency switching converter designs. Poor layout can
cause switching noise into the feedback signal and may degrade performance and is general the source of
heavy EMI problems.
VIN
C3
R1
C4
C5
ERR
PWGD
WDI
WDE
CDLY
VSM
AGND
C2
E910.27
Separate analog ground
from power ground
Keep very short
R3
SWD1
SWD2
Connect exposed pad to
analog ground
L1
D1
C1
PGND
Keep feedback wiring away
from choke flux
R2
VOUT
Figure 9-1: Typical Converter Schematic for PCB Layout
For minimal inductance, the wires indicated by heavy lines should be as wide and short as possible. Keep
the ground pin of the input capacitor (C5) as close as possible to the anode of the catch diode (D1), snubber
network (if there is any) and output capacitor (C1). This path carries a large AC current. The switching node,
the node with the catch diode cathode, choke, and internal P-channel MOS transistor drain (SWD1 &
SWD2), should be kept short. This node is one of the main sources for radiated EMI since it is an AC vol-
tage with switching frequency. It is always a good practice to use a ground plane in the design of the PC
board, particularly at high frequency currents.
The three ground pins, AGND, SGND and GND, should be connected by as short a trace as possible. The
ground pins should be tied to the ground plane, or to a large ground trace in close proximity to both the VFB
divider and then should be connected to PGND pin and the output capacitor (C1) grounds.
The VFB pin is a high impedance node and care should be taken to make the VFB trace short to avoid noise
pickup, inaccurate regulation and erratic function of the PWGD output. The feedback resistors should be pla-
ced as close as possible to the E910.27, with the ground of R3 placed as close as possible to the SGND of
the E910.27. Avoid inductive coupling to the choke or the switching node, by keeping the VFB trace away
from these areas. Radiated noise can be decreased by choosing a shielded version for the choke.
Refer to the E910.27 demo board as an example layout.
ELMOS Semiconductor AG
Specification 34 / 50
03SP0357E.00 05.09.2006
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