MIC4103 datasheet(13/18 Pages) MICREL

13 / 18 page

Micrel

MIC4103/4104

November 2010

M9999-110910-B

Application Information

The average reverse current and power dissipation due to

reverse recovery can be estimated by:

Time

Recovery

Reverse

Current

Recovery

Reverse

Peak

where

RRM

)

(

)

(

REV

AVE

RRM

AVE

Pdiode

The total diode power dissipation is:

fwd

total

Pdiode

An optional external bootstrap diode may be used instead

of the internal diode (Figure 6). 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.

A 100V Schottky diode will work for most 72V input

telecom applications. 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:

supply

power

the

frequency

switching

Cycle

Duty

Voltage

Reverse

Diode

and

flow

current

Reverse

where

)

(

REV

−

REV

Pdiode

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

In most power supply topologies, the diode is reverse

biased during the switching cycle off-time.

Vdd

Level

shift

Vss

Vin

Figure 6. 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 7 shows a simplified equivalent circuit of the

MIC4103 driving an external MOSFET.

External

FET

Vdd

Rg_fet

Ron

Roff

Cgd

Cgs

Figure 7. MIC4103 Driving an External MOSFET

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