Datasheet

LM5112
www.ti.com SNVS234B –SEPTEMBER 2004–REVISED APRIL 2006
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.
Therefore, 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)
In addition to the above gate charge power dissipation, - transient power is dissipated in the driver during output
transitions. When either output of the LM5112 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.
Characterization of the LM5112 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 12V V
GATE
supply produce a 12 mW typical quiescent power.
Therefore the total power dissipation
P
D
= 0.118 + 0.008 + 0.012 = 0.138W. (3)
We know that the junction temperature is given by
T
J
= P
D
x θ
JA
+ T
A
(4)
Or the rise in temperature is given by
T
RISE
= T
J
− T
A
= P
D
x θ
JA
(5)
For WSON-6 package, the integrated circuit die is attached to leadframe die pad which is soldered directly to the
printed circuit board. This substantially decreases the junction to ambient thermal resistance (θ
JA
). By providing
suitable means of heat dispersion from the IC to the ambient through exposed copper pad, which can readily
dissipate heat to the surroundings, θ
JA
as low as 40°C / Watt is achievable with the package. The resulting Trise
for the driver example above is thereby reduced to just 5.5 degrees.
Therefore T
RISE
is equal to
T
RISE
= 0.138 x 40 = 5.5°C (6)
For MSOP-PowerPAD θ
JA
is typically 60°C/W.
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