mic4102 Micrel Semiconductor, mic4102 Datasheet - Page 12

• Manufacturer Part Number: mic4102
• Description: 100v Half Bridge Mosfet Driver With Anti-shoot Through Protection
• Manufacturer: Micrel Semiconductor

Figure 6 shows a simplified equivalent circuit of the
MIC4102 driving an external MOSFET.
Dissipation during the external MOSFET Turn-On
Energy from capacitor C
capacitance of the MOSFET (Cgd and Cgs).
energy delivered to the MOSFET is dissipated in the
three resistive components, Ron, Rg and Rg_fet. Ron is
the on resistance of the upper driver MOSFET in the
MIC4102. Rg is the series resistor (if any) between the
driver IC and the MOSFET.
resistance of the MOSFET. Rg_fet is usually listed in
the power MOSFET’s specifications. The ESR of
capacitor C
can be ignored since they are much less than Ron and
Rg_fet.
The effective capacitance of Cgd and Cgs is difficult to
calculate since they vary non-linearly with Id, Vgs, and
Vds. Fortunately, most power MOSFET specifications
include a typical graph of total gate charge vs. Vgs.
Figure 7 shows a typical gate charge curve for an
arbitrary power MOSFET. This chart shows that for a
gate voltage of 10V, the MOSFET requires about 23.5nC
of charge.
components of the gate drive circuit during turn-on is
calculated as:
Micrel, Inc.
November 2006
C
B
E
but
Q
so
E
where
Ciss
Figure 6. MIC4103 Driving an External MOSFET
=
=
=
1/2
C
1
2
is
×
×
the
B
×
Ciss
V
Qg
and the resistance of the connecting etch
The energy dissipated by the resistive
total
Vdd
×
×
V
V
gs
gate
gs
Ron
Roff
2
HS
capacitanc
B
HB
is used to charge up the input
HO
Rg
e
Rg_fet is the gate
Rg_fet
of
Cgd
the
Cgs
External
FET
MOSFET
The
12
The same energy is dissipated by Roff, Rg and Rg_fet
when the driver IC turns the MOSFET off.
The power dissipated inside the MIC4102 equals the
ratio of Ron & Roff to the external resistive losses in Rg
and Rg_fet. The power dissipated in the MIC4102 due
to driving the external MOSFET is:
Supply Current Power Dissipation
Power is dissipated in the MIC4102 even if is there is
nothing being driven. The supply current is drawn by the
bias for the internal circuitry, the level shifting circuitry
and shoot-through current in the output drivers.
supply current is proportional to operating frequency and
the Vdd and Vhb voltages.
graphs show how supply current varies with switching
frequency and supply voltage.
E
where
E
P
Pdiss
and
P
Qg
fs
Vgs
driver
driver
driver
driver
is
is
the
is
drive
the
the
is
is
=
=
switching
Figure 7. Typical Gate Charge vs. V
=
the
total
the
2
1
2
1
P
gate
driver
×
×
power
Qg
energy
Qg
gate
to
×
×
×
Ron
source
V
V
frequency
charge
gs
dissipated
gs
dissipated
+
×
Rg
Ron
fs
voltage
+
at
Rg
of
Vgs
_
during
the
fet
The typical characteristic
during
on
+
gate
P
the
driver
turn
turn
MOSFET
drive
×
-
M9999-112806
-
on
Roff
on
circuit
or
GS
MIC4102
or
+
Rg
turn
Roff
turn
+
Rg
-
The
-
off
off
_
fet