MIC4607A datasheet(22/41 Pages) MICROCHIP

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MIC4607A

DS-20006957A-page 22

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6.0

APPLICATION INFORMATION

6.1

Adaptive Dead-Time

For each phase, it is important that both MOSFETs of

the same phase branch 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 dissipation in the

MOSFETs, voltage spikes and ringing. The high switch-

ing 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 varia-

tions, 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 6-1 shows

an equivalent circuit of the high-side gate drive.

FIGURE 6-1:

MIC4607A Driving an

External MOSFET.

The internal gate resistance (RG_FET) and any external

damping resistor (RG) and HS pin resistor (RHS), iso-

late 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.

The MIC4607A uses a combination of active sensing

and passive delay to ensure that both MOSFETs are

not on at the same time. Figure 6-2 illustrates how the

adaptive dead-time circuitry works.

FIGURE 6-2:

Adaptive Dead-Time Logic

Diagram.

A transition of HI to low and LI to high is caused by a

high level on the xPWM pin for the MIC4607A-2. This

causes the xLO pin to go low. The MIC4607A monitors

the xLO pin voltage and prevents the xHO pin from

turning on until the voltage on the xLO pin reaches the

VLOOFF threshold. After a short delay, the MIC4607A

drives the xHO pin high. Monitoring the xLO voltage

eliminates any excessive delay due to the MOSFET

driver’s turn-off time and the short delay accounts for

the MOSFET turn-off delay as well as letting the xLO

pin voltage settle out. If an external resistor is used

between the xLO output and the MOSFET gate, it must

be made small enough to prevent excessive voltage

drop across the resistor during turn-off. Figure 6-3 illus-

trates using a diode (DLS) and resistor (RLS2) in parallel

with the gate resistor to prevent a large voltage drop

between the xLO pin and MOSFET gate voltages

during turn-off.

FIGURE 6-3:

Low-Side Drive Gate

Resistor Configuration.

A low on the xPWM pin causes HI to go high and LO to

go low. This causes the xHO pin to go low after a short

delay (tHOOFF). Before the xLO pin can go high, the

voltage on the switching node (xHS pin) must have

dropped to 2.2V. Monitoring the switch voltage instead

of the xHO pin voltage eliminates timing variations and

012

013

014

567589ÿ

MIC4607A

Level

Shift

VDD

VSS

LS1

LS2

ALI

AHI

AHB

AHO

AHS

ALO

VIN

AST

LOW-SIDE

SENSE

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