LM2735XMFSLASHNOPB datasheet(46/59 Pages) TI

46 / 59 page

Dissipatio

- CASE

and

Dissipatio

- A

LM2735, LM2735-Q1

SNVS485G – JUNE 2007 – REVISED AUGUST 2015

www.ti.com

Total Power Losses are:

Table 4. Power Loss Tabulation

PARAMETER

VALUE

PARAMETER

VALUE

VIN

5 V

VOUT

12 V

IOUT

500 mA

POUT

6 W

0.4 V

PDIODE

236 mW

FSW

1.6 MHz

TRISE

6 nS

PSWR

80 mW

TFALL

5 nS

PSWF

70 mW

4 mA

20 mW

RDSon

250 m

Ω

PCOND

305 mW

RDCR

75 m

Ω

PIND

145 mW

0.623

86%

PLOSS

856 mW

PINTERNAL = PCOND + PSW = 475 mW

(59)

10.3.5 Calculating RθJA and RΨJC

(60)

Now the internal power dissipation is known, and the junction temperature is attempted to be kept at or below

125°C. The next step is to calculate the value for RθJA and/or RψJC. This is actually very simple to accomplish,

and necessary if marginality is a possibility in regards to thermals or determining what package option is correct.

The LM2735 has a thermal shutdown comparator. When the silicon reaches a temperature of 160°C, the device

shuts down until the temperature reduces to 150°C. Knowing this, the RθJA or the RψJC of a specific application

can be calculated. Because the junction-to-top case thermal impedance is much lower than the thermal

impedance of junction to ambient air, the error in calculating RψJC is lower than for RθJA . However, a small

thermocouple will need to be attached onto the top case of the LM2735 to obtain the RψJC value.

Knowing the temperature of the silicon when the device shuts down allows three of the four variables to be

known. Once the thermal impedance is calculated, work backwards with the junction temperature set to 125°C to

determine what maximum ambient air temperature keeps the silicon below the 125°C temperature.

10.3.5.1 Procedure

Place the application into a thermal chamber. Sufficient power will need to be dissipated in the device so that a

good thermal impedance value may be obtained.

Raise the ambient air temperature until the device goes into thermal shutdown. Record the temperatures of the

ambient air and/or the top case temperature of the LM2735. Calculate the thermal impedances.

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