MIC4100_11 Datasheet, PDF (14/18 Pages) MIC GROUP RECTIFIERS – 100V Half Bridge MOSFET Drivers

Micrel, Inc.
MIC4100/1
E _dirver = Qg × V_gs
and
P_driver = Qg ×V_gs × fs
where
E_driver is the energy dissipated per switching cycle
P_driver is the power dissipated by switching the MOSFET on and off
Qg is the total gate charge at Vgs
Vgs is the gate to source voltage on the MOSFET
fs is the switching frequency of the gate drive circuit
The power dissipated inside the MIC4100/1 is equal to the
ratio of Ron & Roff to the external resistive losses in Rg
and Rg_fet. Letting Ron =Roff, the power dissipated in the
MIC4100 due to driving the external MOSFET is:
Pdiss _drive
=
P_driver
Ron +
Ron
Rg + Rg _
fet
Supply Current Power Dissipation
Power is dissipated in the MIC4100 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. The supply
current is proportional to operating frequency and the Vdd
and Vhb voltages. The typical characteristic graphs show
how supply current varies with switching frequency and
supply voltage.
The power dissipated by the MIC4100 due to supply
current is
Pdiss_{sup ply} = Vdd × Idd + Vhb × Ihb
Total power dissipation and Thermal Considerations
Total power dissipation in the MIC4100 or MIC4101 is
equal to the power dissipation caused by driving the
external MOSFETs, the supply current and the internal
bootstrap diode.
Pdiss_total = Pdiss_{sup ply} + Pdiss _drive + Pdiode_total
The die temperature may be calculated once the total
power dissipation is known.
T_J = T_A + Pdiss_total ×θ _JA
where :
T_A is the maximum ambient temperature
T_J is the junction temperature (°C)
Pdiss_total is the power dissipation of the MIC4100/1
θ_JC is the thermal resistance from junction to ambient air (°C/W)
Propagation Delay and Delay Matching and other
Timing Considerations
Propagation delay and signal timing is an important
consideration in a high performance power supply. The
MIC4100 is designed not only to minimize propagation
delay but to minimize the mismatch in delay between the
high-side and low-side drivers.
Fast propagation delay between the input and output drive
waveform is desirable. It improves overcurrent protection
by decreasing the response time between the control
signal and the MOSFET gate drive. Minimizing
propagation delay also minimizes phase shift errors in
power supplies with wide bandwidth control loops.
Many power supply topologies use two switching
MOSFETs operating 180º out of phase from each other.
These MOSFETs must not be on at the same time or a
short circuit will occur, causing high peak currents and
higher power dissipation in the MOSFETs. The MIC4100
and MIC4101 output gate drivers are not designed with
anti-shoot-through protection circuitry. The output drives
signals simply follow the inputs. The power supply design
must include timing delays (dead-time) between the input
signals to prevent shoot-through. The MIC4100 &
MIC4101 drivers specify delay matching between the two
drivers to help improve power supply performance by
reducing the amount of dead-time required between the
input signals.
Care must be taken to insure the input signal pulse width
is greater than the minimum specified pulse width. An
input signal that is less than the minimum pulse width may
result in no output pulse or an output pulse whose width is
significantly less than the input.
The maximum duty cycle (ratio of high side on-time to
switching period) is controlled by the minimum pulse width
of the low side and by the time required for the C_B
capacitor to charge during the off-time. Adequate time
must be allowed for the C_B capacitor to charge up before
the high-side driver is turned on.
Decoupling and Bootstrap Capacitor Selection
Decoupling capacitors are required for both the low side
(Vdd) and high side (HB) supply pins. These capacitors
supply the charge necessary to drive the external
MOSFETs as well as minimize the voltage ripple on these
pins. The capacitor from HB to HS serves double duty by
providing decoupling for the high-side circuitry as well as
providing current to the high-side circuit while the high-side
external MOSFET is on. Ceramic capacitors are
recommended because of their low impedance and small
size. Z5U type ceramic capacitor dielectrics are not
recommended due to the large change in capacitance over
temperature and voltage. A minimum value of 0.1uf is
required for each of the capacitors, regardless of the
MOSFETs being driven. Larger MOSFETs may require
larger capacitance values for proper operation. The
voltage rating of the capacitors depends on the supply
voltage, ambient temperature and the voltage derating
used for reliability. 25V rated X5R or X7R ceramic
March 2006
14
M9999-031506