Datasheet

V
HIGH
Q2
V
GATE
R
G
Q1
V
TRIG
C
IN
LM5111
www.ti.com SNVS300G –JULY 2004–REVISED MARCH 2013
Thermal Performance
INTRODUCTION
The primary goal of thermal management is to maintain the integrated circuit (IC) junction temperature (T
J
) below
a specified maximum operating temperature to ensure reliability. It is essential to estimate the maximum T
J
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 θ
JA
for the IC package in the application board
and environment. The θ
JA
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.
Figure 14.
The schematic above shows a conceptual diagram of the LM5111 output and MOSFET load. Q1 and Q2 are the
switches within the gate driver. R
G
is the gate resistance of the external MOSFET, and C
IN
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 C
GS
(gate to source
capacitance) and C
GD
(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 Q
G
in coloumbs. Q
G
combines the charge required by C
GS
and C
GD
for a given gate drive voltage V
GATE
.
Assuming negligible gate resistance, the total power dissipated in the MOSFET driver due to gate charge is
approximated by
P
DRIVER
= V
GATE
x Q
G
x F
SW
where
• F
SW
= switching frequency of the MOSFET (1)
For example, consider the MOSFET MTD6N15 whose gate charge specified as 30 nC for V
GATE
= 12V.
The power dissipation in the driver due to charging and discharging of MOSFET gate capacitances at switching
frequency of 300 kHz and V
GATE
of 12V is equal to
P
DRIVER
= 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 V
CC
to V
EE
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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