LM2794TLX datasheet(14/16 Pages) NSC

14 / 16 page

Application Information (Continued)

E=(P

LED ÷PIN)

Efficiency, as defined here, is in part dependent on LED

voltage. Variation in LED voltage does not affect power

consumed by the circuit and typically does not relate to the

brightness of the LED. For an advanced analysis, it is rec-

ommended that power consumed by the circuit (V

IN xIIN)be

evaluated rather than power efficiency. Figure 7 shows the

power consumption of the LM2794/5 Typical Application Cir-

cuit.

POWER DISSIPATION

The power dissipation (P

DISSIPATION) and junction tempera-

ture (T

J) can be approximated with the equations below. PIN

is the power generated by the 1.5x charge pump, P

LED is the

power consumed by the LEDs, P

POUT is the power provided

through the P

OUT pin, TAis the ambient temperature, and

is the junction-to-ambient thermal resistance for the micro

SMD-14 package. V

IN is the input voltage to the LM2794/5,

DX is the LED forward voltage, IDX is the programmed LED

current, and I

POUT is the current drawn through POUT.

DISSIPATION =PIN -PLED −PPOUT

= [1.5xV

INx(4IDX +IPOUT)]−(VDXx4IDX) − (1.5xVINxIPOUT)

J =TA +(PDISSIPATION x

JA)

The junction temperature rating takes precedence over the

ambient temperature rating. The LM2794/5 may be operated

outside the ambient temperature rating, so long as the junc-

tion temperature of the device does not exceed the maxi-

mum operating rating of 100˚C. The maximum ambient tem-

perature rating must be derated in applications where high

power dissipation and/or poor thermal resistance causes the

junction temperature to exceed 100˚C.

MICRO SMD MOUNTING

The LM2794/5 is a 14-bump micro SMD with a bump size of

300 micron diameter. The micro SMD package requires

specific mounting techniques detailed in National Semicon-

ductor Application Note (AN -1112). NSMD (non-solder mask

defined) layout pattern is recommended over the SMD (sol-

der mask defined) since the NSMD requires larger solder

mask openings over the pad size as opposed to the SMD.

This reduces stress on the PCB and prevents possible

cracking at the solder joint. For best results during assembly,

alignment ordinals on the PC board should be used to

facilitate placement of the micro SMD device. Micro SMD is

a wafer level chip size package, which means the dimen-

sions of the package are equal to the die size. As such, the

micro SMD package lacks the plastic encapsulation charac-

teristics of the larger devices and is sensitive to direct expo-

sure to light sources such as infrared, halogen, and sun light.

The wavelengths of these light sources may cause unpre-

dictable operation.

20028537

FIGURE 6. Efficiency vs V

4 LEDs, V

LED = 3.6V, ILED = 15mA

20028538

FIGURE 7. P

IN vs VIN

4 LEDs, 2.5

≤ V

≤ 3.9V, I

DX = 15mA

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