5+&
POWER MANAGEMENT
Applications Information (Cont.)
Layout Guidelines
Try to keep the traces from the adaptor input to the VCC
pins as wide as possible, to eliminate any voltage drop
across the device input traces. You want to make sure
the input-to-output voltage differential of the device does
not approach the dropout voltage. A curve of the drop-
out voltage vs. output current is shown on page 15. Any
voltage dropped over the input traces from the adaptor
will reduce the dropout voltage margin.
Make the high current output trace from the VOUT pins
as wide as possible. The BSEN line should be used prop-
erly to compensate for any voltage drops from the out-
put trace to the battery. Make a Kelvin connection with
the BSEN trace to where VOUT connects the battery ter-
minals. This is done by taking the BSEN trace and tying it
to the VOUT trace as close to the battery terminals as
possible. This way, any voltage drop across the trace re-
sistance to the battery will be compensated for because
BSEN will regulate the device output voltage (VOUT) at
the point it connects to the VOUT trace. If you tie the
BSEN line to the VOUT pin at the device you will eliminate
the benefit of its purpose and the trace resistance drop
will not be compensated. Therefore, it is best to have
the BSEN trace follow in parallel the VOUT trace and tie
them together at the contact point of the battery termi-
nal for the best result.
The bottom of the SC801 package has a heat slug and
this slug should be tied to a ground plane of the PCB
through one large via or a series of smaller vias. If there
is no ground plane, an area should be dedicated on the
bottom of the PCB to act as a heat sink. The evaluation
board has 1 square inch of copper and allows an output
current of greater that 1A. The more copper tied to this
slug the greater the output current available before ther-
mal limitations dominate. The two pins that are labeled
NC are not connected to the die. Therefore, tying these
pins to the ground plane offers no aide in heat removal
and has no electrical benefit.
2002 Semtech Corp.
9
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