HCMS-2913 datasheet(14/16 Pages) AVAGO

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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 temperatures

may require the power per IC to

be reduced. The power con-

sumption can be reduced by

changing either the N, IPIXEL,

Osc cyc or VLED. Changing

VLOGIC has very little impact on

the power consumption.

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.

Equation 1:

JMAX = TA + PD * RqJA

Where:

JMAX = maximum IC junction temperature

A = ambient temperature surrounding the display

JA = thermal resistance from the IC junction to ambient

D = power dissipated by the IC

Equation 2:

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

Where:

D = total power dissipation

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

PIXEL = peak pixel current.

Duty Factor = 1/8 * Osccyc/64

Osc cyc = number of ON oscillator cycles per row

LOGIC = IC logic current

LOGIC = logic supply voltage

Equation 3:

PEAK = M * 20 * IPIXEL

Where:

PEAK = maximum instantaneous peak current for the display

M= number of ICs in the system

20 = maximum number of LEDs on per IC

PIXEL = peak current for one LED

Equation 4:

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

(see Variable Definitions above)

Appendix B. Electrical

Considerations

Current Calculations

The peak and average display

current requirements have a

significant impact on power

supply selection. The maximum

peak current is calculated with

Equation 3 below.

The average current required by

the display can be calculated

with Equation 4 below.

The power supply has to be able

to supply IPEAK transients and

supply ILED(AVG) continuously.

The range on VLED allows noise

on this supply without signifi-

cantly changing the display

brightness.

VLOGIC and VLED Considerations

The display uses two indepen-

dent electrical systems. One

system is used to power the

display’s logic and the other to

power the display’s LEDs. These

two systems keep the logic

supply clean.

Separate electrical systems

allow the voltage applied to VLED

and VLOGIC to be varied

independently. Thus, VLED can

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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