MIC4605 Datasheet, PDF (19/25 Pages) Micrel Semiconductor – 85V Half-Bridge MOSFET Drivers

Micrel, Inc.
The power dissipated inside the MIC4605 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 MIC4605 due to driving the external MOSFET is
illustrated in Equation 8:
P_DISSDRIVER
=
P_DRIVER
R _ON
R _ON
+ R_G + R_{G _ FET}
Eq. 8
Supply Current Power Dissipation
Power is dissipated in the MIC4605 even if nothing is
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 MIC4605 due to supply
current is illustrated in Equation 9:
P_DISSSUPPLY = VDD × IDD + VHB × IHB
Eq. 9
Total Power Dissipation and Thermal Considerations
Total power dissipation in the MIC4605 is equal to the
power dissipation caused by driving the external
MOSFETs, the supply current and the internal bootstrap
diode, as in Equation 10:
P_DISSTOTAL = P_DISSSUPPLY + P_DISSDRIVE + P_DIODETOTAL
Eq. 10
MIC4605
The die temperature can be calculated after the total
power dissipation is known, as in Equation 11:
T_J = T_A + P_DISSTOTAL × θ_JA
Eq. 11
Where:
T_A = Maximum ambient temperature
T_J = Junction temperature (°C)
P_DISSTOTAL = Power dissipation of the MIC4605
θ_JA = Thermal resistance from junction to ambient air
Other Timing Considerations
Make sure 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 and also minimize the voltage ripple on these
pins. The capacitor from HB to HS has two functions: it
provides decoupling for the high-side circuitry and also
provides 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 because of the large change in
capacitance over temperature and voltage. A minimum
value of 0.1µF 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 capacitors are recommended for most
applications. The minimum capacitance value should be
increased if low voltage capacitors are used because
even good quality dielectric capacitors, such as X5R, will
lose 40% to 70% of their capacitance value at the rated
voltage.
November 11, 2013
19
Revision 1.0