MIC4223_11 Datasheet, PDF (10/20 Pages) MIC GROUP RECTIFIERS – Dual 4A, 4.5V to 18V, 15ns Switch Time, Low-Side MOSFET Drivers with Enable

Micrel, Inc.
MIC4223/MIC4224/MIC4225
Application Information
Power Dissipation Considerations
Power dissipation in the driver can be separated into two
areas:
Output driver stage dissipation
Quiescent current dissipation used to supply the
internal logic and control functions.
Output Driver Stage Power Dissipation
Power dissipation in the driver’s output stage is mainly
caused by charging and discharging the gate to source
and gate to drain capacitance of the external MOSFET.
Figure 3 shows a simplified circuit of the MIC4223 driving
an external MOSFET.
Figure 4. MOSFET Gate Charge vs. V_GS
The energy dissipated during turn-on is calculated as:
E
=
1
2
× C_iss
×V_GS ²
where C_iss is the MOSFET's total gate capacitance
Figure 3. Functional MOSFET/Driver Diagram
Dissipation Caused by Switching the External
MOSFET
Energy from capacitor C_VDD is used to charge up the input
capacitance of the MOSFET (C_GD and C_GS). The energy
delivered to the MOSFET is dissipated in the upper driver
MOSFET and Bipolar impedances. The effective
capacitance of C_GD and C_GS is difficult to calculate since
they vary non-linearly with I_D, V_GS, and V_DS. Fortunately,
most power MOSFET specifications include a typical
graph of total gate charge vs. V_GS. Figure 4 shows a
typical MOSFET gate charge curve. The graph illustrates
that for a gate voltage of 10V, the MOSFET requires about
23.5nC of charge.
but :
Q = C×V
so
E = 1/2 × Q_G ×V_GS
An equivalent amount of energy is dissipated in the
driver’s sink circuit when the MOSFET turns off. The total
energy and power dissipated by the drive components is:
E_DRIVER = Q_G ×V_GS
and
P_DRIVER =Q_G ×V_GS × f_S
Where:
E_DRIVER is the energy dissipated per switching cycle
P_DRIVER is the power dissipated by switching the
MOSFET on and off
Q_G is the total Gate charge at V_GS
V_GS is the MOSFETs Gate to Source voltage
f_S is the switching frequency of the Gate drive
circuit
June 2009
10
M9999-061109-A
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