SC420A
POWER MANAGEMENT
Applications Information (Cont.)
result, run the TG and BG connections with a
minimum aspect ratio (length to width) of 20:1. This
results in a 50 mil trace for a one inch connection.
In addition, minimize the loop area of the gate drive
loop. This is easy with BG, since the return path for
the current is GND. In the case of TG, the return
path for drivercurrent is DRN, so run these traces
together, as closely as possible.
Fig 3: Typical Layout schematic for SC420A
Layout Guidelines
As shown in the above layout the traces used for
interconnections are not identical to each. The layout
using the above traces has significant advantages.
The traces used to interconnect C1, Dbst, Cbst, Rbst,
Vcc, GND, and Vbat are wider and heavy. This is done to
reduce the resistance and inductance of the current path.
As a result the voltage drop of the traces is significantly
reduced. This arrangement charges the Cbst capacitor
faster. Because of this, the FET gate capacitances are
also charged and discharged faster, improving the
efficiency of the system. Ceramic X7R capacitors are a
good choice for supply bypassing near the chip.
Vias represent significant inductance and are to be
avoided wherever possible. BG is especially important
because when the HSFET switches off, the high dV/
dt of the DRN node will force current into the LSFET
gate via Cgd. A large inductance in the BG trace will
prevent the driver from holding BG down at this time.
The signal level traces are not critical because the
current levels are much smaller.
Wider traces are also used for TG and BG connects. This
is essential to decrease the delay of signal through the
trace and allow rapid charge and discharge of the FET
capacitance. Inductance is usually the dominant
impedance in the time range of interest (~10ns). As a
We can also see vias (circular dots) present in the
layout. The vias are important for interconnection
between different layers of the PCB. Also they are
important in heat transfer and aid in running the
system cooler.
2004 Semtech Corp.
13
United States Patent No. 6,441,597
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