LM5111 datasheet(9/19 Pages) TI

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VHIGH

VGATE

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LM5111

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SNVS300F – JULY 2004 – REVISED MAY 2011

Thermal Performance

INTRODUCTION

The primary goal of thermal management is to maintain the integrated circuit (IC) junction temperature (TJ) below

a specified maximum operating temperature to ensure reliability. It is essential to estimate the maximum TJ of IC

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

θJA for the IC package in the application board

and environment. The

θJA is not a given constant for the package and depends on the printed circuit board

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

switches within the gate driver. RG is the gate resistance of the external MOSFET, and CIN is the equivalent gate

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

equivalent gate capacitance is a difficult parameter to measure since it is the combination of CGS (gate to source

capacitance) and CGD (gate to drain capacitance). Both of these MOSFET capacitances are not constants and

vary with the gate and drain voltage. The better way of quantifying gate capacitance is the total gate charge QG

in coloumbs. QG combines the charge required by CGS and CGD for a given gate drive voltage VGATE.

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

approximated by

PDRIVER = VGATE x QG x FSW

where

•

FSW = switching frequency of the MOSFET

(1)

For example, consider the MOSFET MTD6N15 whose gate charge specified as 30 nC for VGATE = 12V.

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

frequency of 300 kHz and VGATE of 12V is equal to

PDRIVER = 12V x 30 nC x 300 kHz = 0.108W.

(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

transitions. When either output of the LM5111 changes state, current will flow from VCC to VEE for a very brief

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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