Datasheet
P
SW(fsw)
+
ǒ
V
IN
I
OUT
t
SW
Ǔ
f
SW
(Watts)
I
RMS
+ I
O
d
Ǹ
ǒ
Amperes
RMS
Ǔ
P
COND
+
ǒ
I
RMS
Ǔ
2
R
DS(on)
ǒ
1 ) TC
R
ƪ
T
J
* 25
O
C
ƫ
Ǔ
(W)
R2
(MIN)
+
V
C (max)
I
SOURCE (min)
(W) +
3.45 V
2.0 mA
+ 1.725 kW
AMOD(f) + AMOD
ǒ
f
LC
f
C
Ǔ
2
and G +
1
AMOD(f)
TPS40060
TPS40061
www.ti.com
SLUS543F –DECEMBER 2002–REVISED JUNE 2013
The modulator gain as a function of frequency at f
C
, is described in Equation 27.
(27)
Care must be taken not to load down the output of the error amplifier with the feedback resistor, R2, that is too
small. The error amplifier has a finite output source and sink current which must be considered when sizing R2.
Too small a value does not allow the output to swing over its full range.
(28)
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. A 2-Ω to 5-Ω resistor in the upper MOSFET gate lead shapes the turn-on and dv/dt of the SW node
and helps reduce the induced turn-on.
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 29.
where:
• TC
R
is the temperature coefficient of the MOSFET R
DS(on)
(29)
The TC
R
varies depending on MOSFET technology and manufacturer but is typically ranges between 3500
ppm/°C and 1000 ppm/°C.
The I
RMS
current for the high side MOSFET is described in Equation 30.
(30)
The switching losses for the high-side MOSFET are described in Equation 31.
where:
• I
O
is the DC output current
• t
SW
is the switching rise time, typically < 20 ns
• f
SW
is the switching frequency (31)
Typical switching waveforms are shown in Figure 12.
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