MIC4606 datasheet(26/40 Pages) MICROCHIP

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MIC4606

DS20005604D-page 26

 2017-2019 Microchip Technology Inc.

The power dissipated in the driver equals the ratio of

RON and ROFF to the external resistive losses in RG

and RG_FET. Letting RON = ROFF, the power dissipated

in the driver due to driving the external MOSFET is:

EQUATION 7-9:

There are four MOSFETs driven by the MIC4606. The

power dissipation for each of the drivers must be calcu-

lated and summed to obtain the total driver diode power

dissipation for the package.

In some cases, the high-side FET of one phase may be

pulsed at a frequency, fS, while the low-side FET of the

other phase is kept continuously on. Since the

MOSFET gate is capacitive, there is no driver power if

the FET is not switched. The operation of each of the

four drivers must be considered to accurately calculate

power dissipation.

7.7

Supply Current Power Dissipation

Power is dissipated in the input and control sections of

the MIC4606, even if there is no external load. Current

is still drawn from the VDD and HB pins for the internal

circuitry, the level-shifting circuitry and shoot-through

current in the output drivers. The VDD and HB currents

are proportional to the 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 MIC4606 due to supply

current is:

EQUATION 7-10:

Values for IDD and IHB are found in the Electrical Char-

acteristics

tables and the Typical Performance

Curves

graphs.

7.8

Total Power Dissipation and

Thermal Considerations

Total power dissipation in the MIC4606 is equal to the

power dissipation caused by driving the external

MOSFETs, the supply currents and the internal

bootstrap diodes.

EQUATION 7-11:

The die temperature can be calculated after the total

power dissipation is known.

EQUATION 7-12:

7.9

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.

7.10

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 0.1 µF is required for CB

(HB to HS capacitors) and 1 µF for the VDD capacitor,

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 applica-

tions. 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.

Pdissdriver

PDRIVER

RON

RON RG RG_FET

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



Pdisssupply

VDD IDD VHB IHB



Pdisstotal

Pdisssupply Pdissdrive PDIODE

TA PDISStotal





Where:

TA = Maximum Ambient Temperature

TJ = Junction Temperature

PDISStotal = Total Power Dissipation

θJA = Thermal Resistance from Junction

to Ambient Air

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