Datasheet

I
SINK
(MAX) :=
T
J
(MAX) - T
A
T
JA
· R
DS
(ON)
LM5110
SNVS255A –MAY 2004–REVISED MAY 2004 www.ti.com
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
As an 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.
If both channels of the LM5110 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 LM5110 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 LM5110 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
= 0.216 + 0.008 + 0.012 = 0.236W.
We know that the junction temperature is given by
T
J
= P
D
x θ
JA
+ T
A
Or the rise in temperature is given by
T
RISE
= T
J
− T
A
= P
D
x θ
JA
For SOIC-8 package θ
JA
is estimated as 170°C/W for the conditions of natural convection.
Therefore T
RISE
is equal to
T
RISE
= 0.236 x 170 = 40.1°C
For WSON-10 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
). θ
JA
as low
as 40°C/W is achievable with the WSON10 package. The resulting T
RISE
for the dual driver example above is
thereby reduced to just 9.5 degrees.
CONTINUOUS CURRENT RATING OF LM5110
The LM5110 can deliver pulsed source/sink currents of 3A and 5A to capacitive loads. In applications requiring
continuous load current (resistive or inductive loads), package power dissipation, limits the LM5110 current
capability far below the 5A sink/3A source capability. Rated continuous current can be estimated both when
sourcing current to or sinking current from the load. For example when sinking, the maximum sink current can be
calculated as
where
• R
DS
(on) is the on resistance of lower MOSFET in the output stage of LM5110
Consider T
J
(max) of 125°C and θ
JA
of 170°C/W for an SO-8 package under the condition of natural convection
and no air flow. If the ambient temperature (T
A
) is 60°C, and the R
DS
(on) of the LM5110 output at T
J
(max) is
2.5Ω, this equation yields I
SINK
(max) of 391mA which is much smaller than 5A peak pulsed currents.
Similarly, the maximum continuous source current can be calculated as
10 Submit Documentation Feedback Copyright © 2004, Texas Instruments Incorporated
Product Folder Links: LM5110