LM27341_14 datasheet(23/42 Pages) TI1

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RTJA =

165

oC - TA

PINTERNAL

LM27341, LM27342, LM27341Q, LM27342Q

www.ti.com

SNVS497D – NOVEMBER 2008 – REVISED FEBRUARY 2010

METHOD 2 EXAMPLE

The operating conditions are the same as the previous Efficiency Calculation:

VIN = 12V

VOUT = 3.3V

IOUT = 2A

fSW = 2 MHz

VD1 = 0.5V

RDCR = 20 mΩ

Internal Power Losses are:

PCOND = IOUT

2 x R

DSON x D

= 2

2 x 0.15Ω x 0.314

= 188 mW

(52)

PSW = (VIN x IOUT x fSW x tFALL)

= (12V x 2A x 2 MHz x 10ns)

= 480 mW

(53)

PQ = IQ x VIN

= 1.5 mA x 12V

= 29 mW

(54)

PBOOST = IBOOST x VBOOST

= 7 mA x 4.5V

= 37 mW

(55)

PINTERNAL = PCOND + PSW + PQ + PBOOST = 733 mW

(56)

The junction temperature can now be estimated as:

TJ = (RθJC x PINTERNAL) + TC

(57)

Texas

Instruments

MSOP-PowerPad

evaluation

board

was

used

determine

the

TJ of the

LM27341/LM27342. The four layer PCB is constructed using FR4 with 2oz copper traces. There is a ground

plane on the internal layer directly beneath the device, and a ground plane on the bottom layer. The ground

plane is accessed by fourteen 10 mil vias. The board measures 2in x 2in (50.8mm x 50.8mm). It was placed in a

container with no airflow. The case temperature measured on this LM27342MY Demo Board was 48.7°C.

Therefore,

TJ = (9.5 °C/W x 733 mW) + 48.7 °C

(58)

TJ = 55.66 °C

(59)

To keep the Junction temperature below 125 °C for this layout, the ambient temperature must stay below 94.33

°C.

TA_MAX = TJ_MAX - TJ +TA

(60)

TA_MAX = 125 °C - 55.66 °C + 25 °C

(61)

TA_MAX = 94.33 °C

(62)

Method 3:

The third method can also give a very accurate estimate of silicon junction temperature. The first step is to

determine RθJA of the application. The LM27341/LM27342 has over-temperature protection circuitry. When the

silicon temperature reaches 165 °C, the device stops switching. The protection circuitry has a hysteresis of 15

°C. Once the silicon temperature has decreased to approximately 150 °C, the device will start to switch again.

Knowing this, the RθJA for any PCB can be characterized during the early stages of the design by raising the

ambient temperature in the given application until the circuit enters thermal shutdown. If the SW-pin is monitored,

it will be obvious when the internal NFET stops switching indicating a junction temperature of 165 °C. We can

calculate the internal power dissipation from the above methods. All that is needed for calculation is the estimate

of RDSON at 165 °C. This can be extracted from the graph of RDSON vs. Temperature. The value is approximately

0.267 ohms. With this, the junction temperature, and the ambient temperature RθJA can be determined.

(63)

Once this is determined, the maximum ambient temperature allowed for a desired junction temperature can be

found.

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