Datasheet

Device description and operation L6743D
10/16
To prevent bootstrap capacitor to extra-charge as a consequence of large negative spikes,
an external series resistance R
BOOT
(in the range of few ohms) may be required in series to
BOOT pin.
Bootstrap capacitor needs to be designed in order to show a negligible discharge due to the
high-side MOSFET turn-on. In fact it must give a stable voltage supply to the high-side driver
during the MOSFET turn-on also minimizing the power dissipated by the embedded Boot
Diode. Figure 5 gives some guidelines on how to select the capacitance value for the
bootstrap according to the desired discharge and depending on the selected MOSFET.
Figure 5. Bootstrap capacitance design
4.4 Power dissipation
L6743D embeds high current drivers for both high-side and low-side MOSFETs: it is then
important to consider the power that the device is going to dissipate in driving them in order
to avoid overcoming the maximum junction operative temperature.
Two main terms contribute in the device power dissipation: bias power and drivers' power.
● Device power (P
DC
) depends on the static consumption of the device through the
supply pins and it is simply quantifiable as follow:
● Drivers' power is the power needed by the driver to continuously switch ON and OFF
the external MOSFETs; it is a function of the switching frequency and total gate charge
of the selected MOSFETs. It can be quantified considering that the total power P
SW
dissipated to switch the MOSFETs dissipated by three main factors: external gate
resistance (when present), intrinsic MOSFET resistance and intrinsic driver resistance.
This last term is the important one to be determined to calculate the device power
dissipation.
The total power dissipated to switch the MOSFETs results:
When designing an application based on L6743D it is recommended to take into
consideration the effect of external gate resistors on the power dissipated by the driver.
External gate resistors helps the device to dissipate the switching power since the same
power P
SW
will be shared between the internal driver impedance and the external resistor
resulting in a general cooling of the device.
0.0
0.5
1.0
1.5
2.0
2.5
0 102030405060708090100
BOOT Cap discharge [V]
High-Side MOSFET Gate Charge [nC]
Cboot = 47nF
Cboot = 100nF
Cboot = 220nF
Cboot = 330nF
Cboot = 470nF
0
500
1000
1500
2000
2500
0.0 0.2 0.4 0.6 0.8 1.0
Bootstrap Cap [uF]
Boot Cap Delta Voltage [V]
Qg = 10nC
Qg = 25nC
Qg = 50nC
Qg = 100nC
P
DC
V
CC
I
CC
V
PVCC
I
PVCC
⋅+⋅=
P
SW
F
SW
Q
GHS
PVCC⋅ Q
GLS
VCC⋅+()⋅=