MIC4604 datasheet(10/18 Pages) MICREL

10 / 18 page

Micrel, Inc.

MIC4604

June 25, 2013

Revision 1.0

dissipation due to reverse leakage is typically much less

than 1mW and can be ignored.

Reverse recovery time is the time required for the injected

minority carriers to be swept away from the depletion

region during turn-off of the diode. Power dissipation due

to reverse recovery can be calculated by computing the

average reverse current due to reverse recovery charge

times the reverse voltage across the diode. The average

reverse current and power dissipation due to reverse

recovery can be estimated by:

REV

)

AVE

(

RRM

)

AVE

(

Pdiode

Eq. 3

Where:

IRRM = peak reverse recovery current

trr = reverse recovery time

The total diode power dissipation is:

fwd

total

Pdiode

Eq. 4

An optional external bootstrap diode may be used instead

of the internal diode (Figure 5). An external diode may be

useful if high gate charge MOSFETs are being driven and

the power dissipation of the internal diode is contributing to

excessive die temperatures. The voltage drop of the

external diode must be less than the internal diode for this

option to work. The reverse voltage across the diode will

be equal to the input voltage minus the VDD supply

voltage. The above equations can be used to calculate

power dissipation in the external diode; however, if the

external diode has significant reverse leakage current, the

power dissipated in that diode due to reverse leakage can

be calculated as:

)

(

Pdiode

REV

−

Eq. 5

Where:

IR = reverse current flow at VREV and TJ

VREV = diode reverse voltage

D = duty cycle = tON × fS

The on-time is the time the high-side switch is conducting.

In most topologies, the diode is reverse biased during the

switching cycle off-time.

Figure 5. Optional Bootstrap Diode

Gate Driver Power Dissipation

Power dissipation in the output driver stage is mainly

caused by charging and discharging the gate to source

and gate to drain capacitance of the external MOSFET.

Figure 6 shows a simplified equivalent circuit of the

MIC4604 driving an external MOSFET.

Figure 6. MIC4604 Driving an External MOSFET

Dissipation during the External MOSFET Turn-On

Energy from capacitor CB is used to charge up the input

capacitance of the MOSFET (Cgd and Cgs). The energy

delivered to the MOSFET is dissipated in the three

resistive components, Ron, Rg and Rg_fet. Ron is the on

resistance of the upper driver MOSFET in the MIC4604.

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