Datasheet

FB
LM5085
PGATE
GND
Q1
L1
C
OUT
R
FB2
R
FB1
V
OUT
GND
D1 R4
LM5085, LM5085-Q1
SNVS565G –NOVEMBER 2008–REVISED MARCH 2013 www.ti.com
Figure 34. Lowest Cost Ripple Generating Configuration
PC Board Layout
In most applications, the heat sink pad or tab of Q1 is connected to the switch node, i.e. the junction of Q1, L1
and D1. While it is common to extend the PC board pad from under these devices to aid in heat dissipation, the
pad size should be limited to minimize EMI radiation from this switching node. If the PC board layout allows, a
similarly sized copper pad can be placed on the underside of the PC board, and connected with as many vias as
possible to aid in heat dissipation.
The voltage regulation, over-voltage, and current limit comparators are very fast and can respond to short
duration noise pulses. Layout considerations are therefore critical for optimum performance. The layout must be
as neat and compact as possible with all the components as close as possible to their associated pins. Two
major current loops conduct currents which switch very fast, requiring the loops to be as small as possible to
minimize conducted and radiated EMI. The first loop is that formed by C
IN
, Q1, L1, C
OUT
, and back to C
IN
. The
second loop is that formed by D1, L1, C
OUT
, and back to D1. The connection from the anode of D1 to the ground
end of C
IN
must be short and direct. C
IN
must be as close as possible to the VIN and GND pins, and C
VCC
must
be as close as possible to the VIN and VCC pins.
If the anticipated internal power dissipation of the LM5085 will produce excessive junction temperatures during
normal operation, a package option with an exposed pad must be used (HVSSOP-8 or WSON-8). Effective use
of the PC board ground plane can help dissipate heat. Additionally, the use of wide PC board traces, where
possible, helps conduct heat away from the IC. Judicious positioning of the PC board within the end product,
along with the use of any available air flow (forced or natural convection) also helps reduce the junction
temperature.
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