mic44f18 Micrel Semiconductor, mic44f18 Datasheet - Page 10

• Manufacturer Part Number: mic44f18
• Description: Mic44f18,mic44f19,mic44f20 6a High Speed Mosfet Drivers In 2mm ? 2mm Package
• Manufacturer: Micrel Semiconductor

January 2007
Micrel, Inc.
Application Information
Power Dissipation Considerations
Power dissipation in the driver can be separated into two
areas:
Output Driver Stage Power Dissipation
Power dissipation in the output driver stage is mainly
caused by charging and discharging the gate to source
and gate to drain capacitance of the external MOSFET.
Figure 4 shows a simplified equivalent circuit of the
MIC44F18 driving an external MOSFET.
Dissipation During the External MOSFET Turn-On
Energy from capacitor C
capacitance of the MOSFET (C
delivered to the MOSFET is dissipated in the three
resistive components, R
resistance of the upper driver MOSFET in the MIC44F18.
R
the MOSFET. R
MOSFET. R
specifications. The ESR of capacitor C
of the connecting etch can be ignored since they are much
less than R
The effective capacitance of C
calculate since they vary non-linearly with I
Fortunately, most power MOSFET specifications include a
typical graph of total gate charge vs. V
a typical gate charge curve for an arbitrary power
MOSFET. This illustrates that for a gate voltage of 10V,
the MOSFET requires about 23.5nC of charge. The energy
dissipated by the resistive components of the gate drive
circuit during turn-on is calculated as:
G
is the series resistor (if any) between the driver IC and
•
•
Figure 4. Output Driver Stage Power Dissipation
Output driver stage dissipation
Quiescent current dissipation used to supply the
internal logic and control functions.
ON
G_FET
and R
G_FET
is usually listed in the power MOSFET’s
G_FET
is the gate resistance of the
ON
VDD
.
, R
is used to charge up the input
G
and R
GD
GD
and C
and C
G_FET
B
GS
and the resistance
GS
. R
. Figure 5 shows
). The energy
GS
D
, V
ON
is difficult to
GS
is the on
, and V
DS
10
.
The same energy is dissipated by R
when the driver IC turns the MOSFET off. Assuming Ron
is approximately equal to R
dissipated by the resistive drive elements is:
The power dissipated inside the MIC4100/4101 is equal to
the ratio of R
R
the MIC44F18 due to driving the external MOSFET is:
G
and R
Where
E
power
P
MOSFET on and off
Q
V
MOSFET
f
circuit
but
E
where
E
Q
so
Ciss
E
and
P
S
DRIVER
DRIVER
GS
DRIVER
G
DRIVER
=
=
=
is the switching frequency of the GATE drive
is the total GATE charge at V
1/2
C
1
2
is
is the GATE to SOURCE voltage on the
G_FET
×
×
the
×
Ciss
is the power dissipated by switching the
V
is the energy dissipated per switching
Qg
=
=
ON
. Letting R
Q
total
Q
G
×
& R
×
Figure 5. GATE Charge
G
×
V
V
V
×
gate
GS
GS
GS
V
OFF
GS
2
×
capacitanc
to the external resistive losses in
ON
f
S
OFF
= R
, the total energy and power
OFF
, the power dissipated in
e
MIC44F18/19/20
GS
of
M9999-011207
OFF
the
, R
MOSFET
G
and R
G_FET