LM20145

SNVS530E – OCTOBER 2007 – REVISED MARCH 2013

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THERMAL CONSIDERATIONS

The thermal characteristics of the LM20145 are specified using the parameter

θJA, which relates the junction

temperature to the ambient temperature. Although the value of

θJA is dependant 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:

TJ = PDθJA + TA

(14)

and

PD = PIN x (1 - Efficiency) - 1.1 x IOUT2 x DC

where

•

TJ is the junction temperature in °C

•

PIN is the input power in Watts (PIN = VIN x IIN)

•

θJA is the junction to ambient thermal resistance for the LM20145

•

TA is the ambient temperature in °C

•

IOUT is the output load current

•

DCR is the inductor series resistanceR

(15)

It is important to always keep the operating junction temperature (TJ) 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 33, shown below, provides a better approximation of the

θJA 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 33. Thermal Resistance vs PCB Area

20

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