VHIGH

Q2

VGATE

RG

Q1

VTRIG

CIN

15

LM5111

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SNVS300H – JULY 2004 – REVISED SEPTEMBER 2016

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Thermal Considerations (continued)

Figure 16. Driver Output Stage and Load

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 coulombs. 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 × QG × FSW

where

•

FSW = switching frequency of the MOSFET

(2)

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

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

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

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

(3)

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 0.216 W.

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.

Characterization of the LM5111 provides accurate estimates of the transient and quiescent power dissipation

components. At 300-kHz switching frequency and 30-nC load used in the example, the transient power will be 8

mW. The 1-mA nominal quiescent current and 12-V VGATE supply produce a 12-mW typical quiescent power.

Therefore the total power dissipation

PD = 0.216 + 0.008 + 0.012 = 0.236 W.

(4)

We know that the junction temperature is given by

TJ = PD × θJA + TA

(5)