MIC4600 Datasheet, PDF (15/26 Pages) Microchip Technology – 28V Half-Bridge MOSFET Driver

EQUATION 6-9:
E_DRIVER = Q_G  V _GS
and
EQUATION 6-10:
P_DRIVER = Q_G  V _GS  f _S
Where:
E_DRIVER = Energy dissipated per switching cycle
P_DRIVER = Power dissipated per switching cycle
Q_G = Total gate charge at V_GS
V_GS = Gate to source voltage on the MOSFET
f_S = Switching frequency of the gate drive circuit
The power dissipated inside the driver is equal to the
ratio of R_ON and R_OFF to the external resistive losses in
R_G and R_{G_FET}. Letting R_ON = R_OFF, the power
dissipated in the MIC4600 due to driving the external
MOSFET is illustrated in Equation 6-11.
EQUATION 6-11:
Pdiss_DRIVER
=
P_DRIVER

--------------------^R----^O----^N---------------------
R_ON + R_G + R_{G_FET}
6.8 Total Power Dissipation and
Thermal Considerations
Total power dissipation in the MIC4600 is equal to the
power dissipation caused by driving the external
MOSFETs and the Quiescent current.
EQUATION 6-12:
P_DISSTOTAL = P_DISSIQ + P_DISSDRIVE
The die temperature can be calculated after the total
power dissipation is known, as in Equation 6-13.
EQUATION 6-13:
T _J = T _A + Pdiss_TOTAL  _JA
Where:
T_A = Maximum ambient temperature
T_J = Junction temperature (°C)
Pdiss_TOTAL = Power dissipation of the MIC4600
_JA = Thermal resistance from junction to ambient air
MIC4600
6.9 Decoupling and Bootstrap
Capacitor Selection
Decoupling capacitors are required for both the low
side (VDD) and high side (BST) supply pins. These
capacitors supply the charge necessary to drive the
external MOSFETs and also minimize the voltage
ripple on these pins. The capacitor from BST to SW has
two functions: it provides decoupling for the high-side
driver and is the supply voltage to the high-side circuit
while the external MOSFET is on. Ceramic capacitors
are recommended because of their low impedance and
small size. Z5U type ceramic capacitor dielectrics are
not recommended because of the large change in
capacitance over temperature and voltage. A minimum
value of 0.1 μF is required for each of the capacitors,
regardless of the MOSFETs being driven. Larger
MOSFETs may require larger capacitance values for
proper operation.
Placement of the decoupling capacitors is critical. The
bypass capacitor for VDD should be placed as close as
possible between the VDD and PGND pins. The
bypass capacitor (C_B) for the BST supply pin must be
located as close as possible between the BST and SW
pins. The etch connections must be short, wide, and
direct. The use of a ground plane to minimize
connection impedance is recommended (refer to the
Grounding, Component Placement, and Circuit Layout
section for more information).
6.10 Grounding, Component
Placement, and Circuit Layout
Nanosecond switching speeds and ampere peak
currents in and around the MIC4600 drivers require
proper placement and trace routing of all components.
Improper placement may cause degraded noise
immunity, false switching, excessive ringing, or circuit
latch-up.
Figure 6-3 shows the critical current paths when the
driver outputs go high and turn on the external
MOSFETs. It also helps demonstrate the need for a low
impedance ground plane. Charge needed to turn-on
the MOSFET gates comes from the decoupling
capacitors C_VDD and C_B. Current in the low-side gate
driver flows from C_VDD through the internal driver, into
the MOSFET gate, and out the source. The return
connection back to the decoupling capacitor is made
through the ground plane. Any inductance or
resistance in the ground return path causes a voltage
spike or ringing to appear on the source of the
MOSFET. This voltage works against the gate drive
voltage and can either slow down or turn off the
MOSFET during the period when it should be turned
on.
Current in the high-side driver is sourced from
capacitor C_B and flows into the BST pin and out the DH
pin, into the gate of the high side MOSFET. The return
 2016 Microchip Technology Inc.
DS20005584A-page 15