Datasheet
C
BP10
+
ǒ
Q
gHS
) Q
gSR
Ǔ
DV
(Farads)
C
BOOST
+
Q
g
DV
(Farads)
R2
(MIN)
+
V
C (max)
I
SOURCE (min)
+
3.5 V
2 mA
+ 1750 W
TPS40054
TPS40055
TPS40057
SLUS593H –DECEMBER 2003–REVISED JULY 2012
www.ti.com
Minimum Load Resistance
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.
(30)
CALCULATING THE BOOST AND BP10 BYPASS CAPACITOR
The BOOST capacitance provides a local, low impedance source for the high-side driver. The BOOST capacitor
should be a good quality, high-frequency capacitor. The size of the bypass capacitor depends on the total gate
charge of the MOSFET and the amount of droop allowed on the bypass capacitor. The BOOST capacitance is
described in Equation 31.
(31)
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 32.
(32)
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 Q
GD
charge is smaller than the Q
GS
charge.
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