Datasheet

LM20123
SNVS524E –OCTOBER 2007–REVISED MARCH 2013 www.ti.com
THERMAL CONSIDERATIONS
The thermal characteristics of the LM20123 are specified using the parameter θ
JA
, which relates the junction
temperature to the ambient temperature. Although the value of θ
JA
is dependent on many variables, it still can be
used to approximate the operating junction temperature of the device.
To obtain an estimate of the device junction temperature, one may use the following relationship:
T
J
= P
D
θ
JA
+ T
A
(13)
and
P
D
= P
IN
x (1 - Efficiency) - 1.1 x I
OUT
2 x DCR
where
• T
J
is the junction temperature in °C
• P
IN
is the input power in Watts (P
IN
= V
IN
x I
IN
)
• θ
JA
is the junction to ambient thermal resistance for the LM20123
• T
A
is the ambient temperature in °C
• I
OUT
is the output load current
• DCR is the inductor series resistance (14)
It is important to always keep the operating junction temperature (T
J
) below 125°C for reliable operation. If the
junction temperature exceeds 160°C the device will cycle in and out of thermal shutdown. If thermal shutdown
occurs it is a sign of inadequate heatsinking or excessive power dissipation in the device.
Figure 32., shown below, provides a better approximation of the θ
JA
for a given PCB copper area. The PCB
heatsink area consists of 2oz. copper located on the bottom layer of the PCB directly under the HTSSOP
exposed pad. The bottom copper area is connected to the HTSSOP exposed pad by means of a 4 x 4 array of
12 mil thermal vias.
Figure 32. Thermal Resistance vs PCB Area
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