Datasheet

TPS40055-EP
SGLS310D –JULY 2005–REVISED FEBRUARY 2012 www.ti.com
The 10-V reference pin, BP10V provides energy for both the synchronous MOSFET and the high-side MOSFET
via the BOOST capacitor. Neglecting any efficiency penalty, the BP10V capacitance is described in Equation 30.
(30)
dv/dt INDUCED TURN-ON
MOSFETs are susceptible to dv/dt turn-on particularly in high-voltage (V
DS
) applications. The turn-on is caused
by the capacitor divider that is formed by C
GD
and C
GS
. High dv/dt conditions and drain-to-source voltage, on the
MOSFET causes current flow through C
GD
and causes the gate-to-source voltage to rise. If the gate-to-source
voltage rises above the MOSFET threshold voltage, the MOSFET turns on, resulting in large shoot-through
currents. Therefore, the SR MOSFET should be chosen so that the C
GD
capacitance is smaller than the C
GS
capacitance.
HIGH SIDE MOSFET POWER DISSIPATION
The power dissipated in the external high-side MOSFET is comprised of conduction and switching losses. The
conduction losses are a function of the I
RMS
current through the MOSFET and the R
DS(on)
of the MOSFET. The
high-side MOSFET conduction losses are defined by Equation 31.
(31)
where:
TC
R
is the temperature coefficient of the MOSFET R
DS(on)
The TC
R
varies depending on MOSFET technology and manufacturer, but typically ranges between
3500 ppm/°C and 10000 ppm/°C.
The I
RMS
current for the high side MOSFET is described in Equation 32.
(32)
The switching losses for the high-side MOSFET are described in Equation 33.
(33)
where:
I
O
is the dc-output current
t
SW
is the switching rise time, typically < 20 ns
f
SW
is the switching frequency
Typical switching waveforms are shown in Figure 16.
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