SC420A
POWER MANAGEMENT
Applications Information (Cont.)
COMPONENT SELECTION FOR SC420A APPLICATION:
High Side MOSFET (LSFET)
The SC420A is usually used for low duty cycle ( ~ 10% )
applications. So the Rds (ON) of the high side MOSFET is
not a parameter of significant importance. A 10 – 25
mÙ Rds for the HSFET is acceptable depending on the
load current. Minimum Qg for the HSFET is important for
component selection. Typical range is 10 – 25 nC.
Low Side MOSFET (LSFET)
Rds is the critical selection parameter for LSFET. IT should
be as low as possible for reduction of conduction losses
and hence increase efficiency. Typical range is 1 – 3 mÙ.
Rg is another important parameter for LSFET. It should
be as low as possible as this will give better efficiency.
Typical range 0.1 – 2
Ω.
Ratio of Qgd/Qgs is third
parameter of consideration.
As the duty cycle for the application increases, require-
ments for the two FETs become more similar; however,
switching charge will always be more important to the
HSFET since it switches into the full voltage, and the LSFET
always switching into the near zero voltage.
Boost Capacitor (Cbst)
Boost capacitor is important for SC420A application as
shown in the above figure. It is a good design rule to have
boost capacitance at least 100 X the Cgs for the HSFET.
Boost Resistor (Rbst)
Boost resistance is important and depends on the layout.
We recommend always designing with the resistor as shown
in the above circuit to help minimize EMI when the HSFET
turns on. The value required is layout dependant.
Top Gate Resistor (Rtg)
TG resistance is not generally required, as Rbst can take
care of the rising edge. We recommend one Rtg for each
HSFET only when the maximum length of the TG trace > 2
inches. Populate with 0
Ω
initially.
Boost Diode (Dbst)
Boost Diode as shown in the above figure is required and
should have a very low forward voltage drop. This increases
the amount of charge on Cbst capacitor.
Delay Capacitor (Cdly)
Delay capacitor is not added in a typical application. This
option is useful to control the delay between the BG falling
and TG rising edges. Cdly is used for very high capacitance
LSFETs to ensure BG is below Vth of the FET before TG
turns on.
Decoupling capacitors (C1,C3)
These are de-coupling capacitors present in the circuit.
Place as close to SC420A as possible. Typical rating is 1uF/
10V for C1 and 0.1uF /25V for C3.
Bottom Gate Resistor (Rbg)
BG resistance is normally not required, but may be needed
for damping for long BG trace runs. We recommend one
Rbg for each LSFET only when the maximum length of the
BG trace is > 1 inch. Populate with 0
Ω
initially.
©
2004 Semtech Corp.
11
United States Patent No. 6,441,597
www.semtech.com
SEMTECH [ SEMTECH CORPORATION ]
