Q2

Q1

LM5111

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SNVS300F – JULY 2004 – REVISED MAY 2011

Thermal Performance

INTRODUCTION

components in worst case operating conditions. The junction temperature is estimated based on the power

dissipated in the IC and the junction to ambient thermal resistance

and environment. The

design and the operating environment.

DRIVE POWER REQUIREMENT CALCULATIONS IN LM5111

The LM5111 dual low side MOSFET driver is capable of sourcing/sinking 3A/5A peak currents for short intervals

to drive a MOSFET without exceeding package power dissipation limits. High peak currents are required to

switch the MOSFET gate very quickly for operation at high frequencies.

The schematic above shows a conceptual diagram of the LM5111 output and MOSFET load. Q1 and Q2 are the

capacitance of the MOSFET. The gate resistance Rg is usually very small and losses in it can be neglected. The

Assuming negligible gate resistance, the total power dissipated in the MOSFET driver due to gate charge is

approximated by

where

•

(1)

The power dissipation in the driver due to charging and discharging of MOSFET gate capacitances at switching

(2)

If both channels of the LM5111 are operating at equal frequency with equivalent loads, the total losses will be

twice as this value which is 0.216W.

In addition to the above gate charge power dissipation, - transient power is dissipated in the driver during output

interval of time through the output totem-pole N and P channel MOSFETs. The final component of power

dissipation in the driver is the power associated with the quiescent bias current consumed by the driver input

stage and Under-voltage lockout sections.

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