LM555H883 National Semiconductor, LM555H883 Datasheet

LM555H883

Manufacturer Part Number

LM555H883

Description

CAN-8

Manufacturer

National Semiconductor

Datasheet

1.LM555H883.pdf (4 pages)

Specifications of LM555H883

Date_code

05+

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Techniques for Thermal

Analysis of Switching Power

Supply Designs

To reduce time-to-market and component count, power man-

agement ICs with integrated power transistors such as

National’s new SIMPLE SWITCHER® regulators (LM5576,

LM25576, and others) are often preferred over controllers

with external FETs. However, with the power transistor on-

board, it’s important to do careful thermal analysis of the

power IC to make sure the silicon temperature does not ex-

ceed the maximum allowable junction temperature. Integrat-

ed circuits are rated up to a maximum ‘die’ temperature.

Operation at higher temperatures will put the IC out of spec-

ification and possibly destroy it.

There are three main ways of thermally analyzing a given de-

sign. The following article explains the different approaches,

and discusses the precision of each approach.

The Analytical Approach

The analytical approach is a good way to get a rough estimate

of the die temperature of a given switching regulator. One

approach is to calculate the losses the switching regulator IC

generates. For step down regulators the following formulas

can be used.

There are bias losses which are mainly the ground pin current

times the input voltage:

The power conduction losses are the losses of the built in

transistors while fully turned on and a rough estimation is:

The switching losses are the losses that occur during the

transition times of the internal transistor before and after the

on time and can be estimated by:

Where F is the switching frequency and tLH and tHL are the

transition times from low to high or high to low.

All the individual losses are sometimes difficult to calculate

due to incomplete information regarding parameters such as

the exact rise time, exact Rdson during the on time and other

parasitics which are not easily characterized. Sometimes it is

easier to take the over all efficiency of a given power converter

board and to subtract the losses of the external components

such as the external schottky diode, the inductor, current

flowing through the external resistive divider, and possibly the

capacitors depending on the ESR.

Once we know the losses of the switching regulator IC, the

thermal analysis can be started. The individual datasheets

give the thermal resistance from the junction of the IC to case

(or PCB), which is referred to as θJC. The units are degrees

centigrade per Watt, and knowing the ambient temperature

as well as the dissipated power on the die gives the temper-

ature of the die. The resistance value θJC has a lot to do with

the package the silicon is housed in but it also includes the

switch

cond

= (I

= duty cycle • R

OUT

bias

• V

= I

)/2 • F • (t

• V

dson

• I

300046

OUT

+ t

)

National Semiconductor

Application Note 1566

Frederik Dostal Applications Engineer

February 2007

size of the die, the die attach material, and bond wire type and

number. This is the reason why there is not one θJC per

package type, and why the junction to resistance has to be

thermally measured with each individual newly released IC

product.

The junction to ambient thermal resistance, θJA, depends

greatly on the design of the printed circuit board around the

IC. Generally, datasheets give information about the PCB and

layout situation in which the given thermal resistance is valid.

The precision of the analytical approach depends greatly on

the complexity of the formulas as well as on the precision of

data of components available to the designer. In many cases,

it is more precise to use a practical approach with measure-

ments in the lab rather than mathematical models which lack

accuracy due to many unknowns.

The Simulation Approach

To simplify thermal predictions, National’s WEBENCH® on-

line simulation tool includes a module called WebTHERM®

which offers thermal modeling of many switching regulator

ICs, including National’s new LM557x and LM2557x SIMPLE

SWITCHER regulators. The thermal simulation results are

given in a colorful thermal graph where hotspots can easily

be detected and the temperature of each point on the board

can be found. Heat sinks can be added to improve thermal

dissipation. Also, airflow can be adjusted using fans from dif-

ferent directions. Figure 2 shows a screenshot of a thermal

simulation result with WebTHERM. This approach is very

simple and gives a good idea of how heat dissipates across

a board. It also helps to understand where hotspots exist in

individual designs.

FIGURE 1. Typical Efficiency at 5V V

OUT

vs I

www.national.com

OUT

30004609

and V

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