MIC4600 datasheet(13/26 Pages) MICROCHIP

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DS20005584A-page 13

MIC4600

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 RDELAY vs. TDEAD.

EQUATION 6-1:

Where:

TDEAD is the break-before-make delay between the

highside and low-side gate drive signals

RDELAY 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 CB

capacitor to charge during the off-time. Adequate time

must be allowed for the CB 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.

6.4

Bootstrap Diode and Capacitor

The gate drive voltage of the high-side driver equals

the VDD 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:

where;

Qgate = total gate charge at VDD

fs = gate drive switching frequency

The average power dissipated by the forward voltage

drop of the diode equals:

EQUATION 6-3:

where; VF = diode forward voltage drop

The value of VF should be taken at the peak current

through the diode; however, this current is difficult to

calculate

because

differences

source

impedances. The peak current can either be measured

or the value of VF 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 VGS 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:

Where:

QGATE = Total gate charge at VDD

VBST = Voltage drop at the BST pin

DEAD

–

DELAY

0.9

–



FAVE



gate



Pdiode

fwd

FAVE





GATE

V

BST

-----------------



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