HCMS-2913 datasheet(14/16 Pages) AVAGO

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Appendix A. Thermal Considerations

The display IC has a maximum junction temperature of

150°C. The IC junction temperature can be calculated

with Equation 1 below.

A typical value for RqJA is 100°C/W. This value is typical

for a display mounted in a socket and covered with a

plastic filter. The socket is soldered to a .062 in. thick PCB

with .020 inch wide, one ounce copper traces.

PD can be calculated as Equation 2 below.

Figure 4 shows how to derate the power of one IC versus

ambient temperature. Operation at high ambient tem-

peratures may require the power per IC to be reduced.

The power consumption can be reduced by changing

either the N, IPIXEL, Osc cyc or VLED. Changing VLOGIC has

very little impact on the power consumption.

Appendix B. Electrical Considerations

Equation 1:

TJMAX = TA + PD * RqJA

Where:

TJMAX = maximum IC junction temperature

TA = ambient temperature surrounding the display

RqJA = thermal resistance from the IC junction to ambient

PD = power dissipated by the IC

Equation 2:

PD = (N * IPIXEL * Duty Factor * VLED) + ILOGIC * VLOGIC

Where:

PD = total power dissipation

N = number of pixels on (maximum 4 char * 5 * 7 = 140)

IPIXEL = peak pixel current.

Duty Factor = 1/8 * Osccyc/64

Osc cyc = number of ON oscillator cycles per row

ILOGIC = IC logic current

VLOGIC = logic supply voltage

Equation 3:

IPEAK = M * 20 * IPIXEL

Where:

IPEAK = maximum instantaneous peak current for the display

M = number of ICs in the system

20 = maximum number of LEDs on per IC

IPIXEL = peak current for one LED

Equation 4:

ILED(AVG) = N * IPIXEL * 1/8 * (oscillator cycles)/64

(see Variable Definitions above)

Figure 4.

TA – AMBIENT TEMPERATURE – °C

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.8

0.9

1.0

1.1

1.2

1.3

= 100

°C/W

J-A

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