MIC4600 Datasheet, PDF (13/26 Pages) Microchip Technology – 28V Half-Bridge MOSFET Driver

6.0 APPLICATION INFORMATION
6.1 Adjustable Dead Time
Dead-time control prevents shoot-through current from
flowing through the external power MOSFETs during
switching transitions. The delay allows enough time for
the high-side driver to turn off before the low-side driver
turns on. It also prevents the high-side driver from
turning on before the low-side driver has turned off.
The dead-time between the high and low-side
MOSFETs can be adjusted with a resistor on the
DELAY pin. The dead-time can be approximated with
the formula in Equation 6-1. See the Typical
Performance Curves section for a more precise
determination of R_DELAY vs. T_DEAD.
EQUATION 6-1:
T _DEAD = 12  10^–9 + R_DELAY  0.9  10^–10
Where:
T_DEAD is the break-before-make delay between the
highside and low-side gate drive signals
R_DELAY is the DELAY pin resistance in kΩ.
6.2 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.
6.3 Single Input Operation
Both outputs can be controlled from a single input
signal by pulling the LSI input high to VDD and applying
the input signal to the HSI pin. In this configuration, the
dead-time between the DH and DL transitions is set by
the resistor value connected to the DELAY pin.
When the HSI pin goes from a low to a high, the DL pin
goes low and the DH pin goes high after the dead time
delay. When the HSI pin changes from a high to a low,
the DH pin goes low. After the delay time, the DL pin
goes high.
MIC4600
6.4 Bootstrap Diode and Capacitor
The gate drive voltage of the high-side driver equals
the V_DD voltage minus the voltage drop across the
bootstrap diode. A Schottky diode is recommended
due to the lower forward voltage drop.
Power dissipation in the bootstrap diode can be
calculated using the following equations. The average
current drawn by repeated charging of the high-side
MOSFET is calculated by Equation 6-2:
EQUATION 6-2:
I_{FAVE} = Q_gate  f _S
where;
Q_gate = total gate charge at V_DD
f_s = gate drive switching frequency
The average power dissipated by the forward voltage
drop of the diode equals:
EQUATION 6-3:
Pdiode _fwd = I_{FAVE}  V _F
where; V_F = diode forward voltage drop
The value of V_F should be taken at the peak current
through the diode; however, this current is difficult to
calculate because of differences in source
impedances. The peak current can either be measured
or the value of V_F at the average current can be used,
which will yield a good approximation of diode power
dissipation.
The voltage on the bootstrap capacitor drops each time
it delivers charge to turn on the MOSFET. The voltage
drop depends on the gate charge required by the
MOSFET. Most MOSFET specifications specify gate
charge versus V_GS voltage. Based on this information
and a suggested capacitor voltage drop of less than
0.1V, the minimum value of bootstrap capacitance is:
EQUATION 6-4:
C_B

-^Q----^G----^A---^T---^E--
V _BST
Where:
Q_GATE = Total gate charge at V_DD
V_BST = Voltage drop at the BST pin
 2016 Microchip Technology Inc.
DS20005584A-page 13