8 2005 Semtech Corp.
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SC1405D
POWER MANAGEMENT
placement and short, wide traces must be used in layout
of The Drives, DRN, and especially PGND pin. The top
gate driver supply voltage is provided by bootstrapping
the +5V supply and adding it the phase node voltage
(DRN). Since the bootstrap capacitor supplies the charge
to the TOP gate, it must be less than 0.5” away from the
SC1405. Ceramic X7R capacitors are a good choice for
supply bypassing near the chip. The Vcc pin capacitor
must also be less than 0.5” away from the SC1405. The
ground node of this capacitor, the SC1405 PGND pin
and the Source of the bottom FET must be very close to
each other, preferably with common PCB copper land
and multiple vias to the ground plane (if used). The par-
allel Schottky (if used) must be physically next to the
Bottom FET’s drain and source. Any trace or lead induc-
tance in these connections will drive current way from
the Schottky and allow it to flow through the FET’s body
diode, thus reducing efficiency.
Preventing Inadvertent Bottom FET Turn-on
At high input voltages, (12V and greater) a fast turn-on
of the top FET creates a positive going spike on the Bot-
tom FET’s gate through the Miller capacitance, Crss of
the bottom FET. The voltage appearing on the gate due
to this spike is:
issrss
rssin
spike
CC
C*V
V
+
=
Where Ciss is the input gate capacitance of the bottom
FET. This is assuming that the impedance of the drive
path is too high compared to the instantaneous imped-
ance of the capacitors. (since dV/dT and thus the effec-
tive frequency is very high). If the BG pin of the SC1405D
is very close to the bottom FET, Vspike will be reduced
depending on trace inductance, rate if rise of current,
etc.
While not shown in Application Evaluation Board Sche-
matic on page 12, a capacitor may be added from the
gate of the bottom FET to its source, preferably less than
0.5” away. This capacitor will be added to Ciss in the
above equation to reduce the effective spike voltage,
Vspike.
The selection of the bottom FET must be done with at-
tention paid to the Crss/Ciss ratio. A low ratio reduces
Applications Information
the Miller feedback and thus reduces Vspike. Also FETs
with higher Turn-on threshold voltages will conduct at a
higher voltage and will not turn on during the spike. The
FET shown in the schematic has a 2 volt threshold and
will require approximately 5 volts Vgs to be conducting,
thus reducing the possibility of shoot-through. A zero
ohm bottom FET gate resistor will obviously help keeping
the gate voltage low.
Ultimately, slowing down the top FET by adding gate re-
sistance will reduce di/dt which will in turn make the ef-
fective impedance of the capacitors higher, thus allow-
ing the BG driver to hold the bottom gate voltage low.
Ringing on the Phase Node
The top FET source must be close to the bottom FET
drain to prevent ringing and the possibility of the phase
node going negative. This frequency is determined by:
ossst
ring
C*L2
1
F
Π
=
Where:
L
st
= The effective stray inductance of the top FET added
to trace inductance of the connection between top FET’s
source and the bottom FET’s drain added to the trace
resistance of the bottom FET’s ground connection.
Coss=Drain to source capacitance of bottom FET. If there
is a Schottky used, the capacitance of the Schottky is
added to the value.
Although this ringing does not pose any power losses
due to a fairly high Q, it could cause the phase node to
go too far negative, thus causing improper operation,
double pulsing or at worst driver damage. This ringing is
also an EMI nuisance due to its high resonant frequency.
Adding a capacitor, typically 1000-2000pf, in parallel with
Coss can often eliminate the EMI issue. If double puls-
ing is caused due to excessive ringing, placing 4.7-10
ohm resistor between the phase node and the DRN pin
of the SC1405 should eliminate the double pulsing.
The negative voltage spikes on the phase node adds to
the bootstrap capacitor voltage, thus increasing the volt-
age between VBST - VDRN. If the phase node negative
spikes are too large, the voltage on the boost capacitor
