Datasheet
I
SOURCE
(MAX) :=
T
J
(MAX) - T
A
T
JA
· V
DIODE
I
SINK
(MAX) :=
T
J
(MAX) - T
A
T
JA
· R
DS
(ON)
LM5111
SNVS300G –JULY 2004–REVISED MARCH 2013
www.ti.com
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 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. (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 SOIC package, θ
JA
is estimated as 170°C/W for the conditions of natural convection. For VSSOP, θ
JA
is
typically 60°C/W.
Therefore for SOIC T
RISE
is equal to
T
RISE
= 0.236 x 170 = 40.1°C (6)
CONTINUOUS CURRENT RATING OF LM5111
The LM5111 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 LM5111 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 LM5111 (7)
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 LM5111 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
where
• V
DIODE
is the voltage drop across hybrid output stage which varies over temperature and can be assumed to
be about 1.1V at T
J
(max) of 125°C (8)
Assuming the same parameters as above, this equation yields I
SOURCE
(max) of 347mA.
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