MIC4606 datasheet(21/40 Pages) MICROCHIP

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DS20005604D-page 21

MIC4606

7.0

APPLICATION INFORMATION

7.1

Adaptive Dead Time

The door lock/unlock circuit diagram, shown in

Figure 7-2, is used to illustrate the importance of the

adaptive dead-time feature of the MIC4606. For each

phase, it is important that both MOSFETs are not

conducting at the same time or VIN will be shorted to

ground and current will “shoot through” the MOSFETs.

Excessive shoot-through causes higher power dissipa-

tion in the MOSFETs, voltage spikes and ringing. The

high switching current and voltage ringing generate

conducted and radiated EMI.

Minimizing shoot-through can be done passively,

actively or through a combination of both. Passive

shoot-through protection can be achieved by imple-

menting delays between the high and low gate drivers

to prevent both MOSFETs from being on at the same

time. These delays can be adjusted for different appli-

cations. Although simple, the disadvantage of this

approach is that it requires long delays to account for

process and temperature variations in the MOSFET

and MOSFET driver.

Adaptive dead time monitors voltages on the gate drive

outputs and switch node to determine when to switch

the MOSFETs on and off. This active approach adjusts

the delays to account for some of the variations, but it

too has its disadvantages. High currents and fast

switching voltages in the gate drive and return paths

can cause parasitic ringing to turn the MOSFETs back

on, even while the gate driver output is low. Another

disadvantage is that the driver cannot monitor the gate

voltage inside the MOSFET. Figure 7-1 shows an

equivalent circuit of the high-side gate drive, including

parasitic.

FIGURE 7-1:

MIC4606 Driving an

External MOSFET.

The internal gate resistance (RG_FET) and any external

damping resistor (RG) isolate the MOSFET’s gate from

the driver output. There is a delay between when the

driver output goes low and the MOSFET turns off. This

turn-off delay is usually specified in the MOSFET data

sheet. This delay increases when an external damping

resistor is used.

FIGURE 7-2:

Door Lock/Unlock Circuit.

xHS

xHB

xHO

EXTERNAL

FET

G_FET

OFF

MIC4606

AHB

AHO

AHS

ALO

ALI

AHI

μC

12 VDC

BHB BHO BHS

BLO

BLI

BHI

MIC4606-1

FULL-BRIDGE

DRIVER

1 μF

I/O

COMMUNICATIONS

Unlock

Lock

POWER

1 μF

16V

MIC5283

LDO

12V TO 3.3V

2.2 μF

10V

3.3 VDC

1 μF

2.2 μF

DC MOTOR

12V 140 mA

I/O

S = 20 kHz

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