HCMS-2913 datasheet(14/16 Pages) HP

14 / 16 page

D 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 consumption

can be reduced by changing

either the N, I

PIXEL, Osc cyc or

LED. 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 R

JA 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 * RθJA

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 I

PEAK transients and

supply I

LED(AVG) continuously.

The range on V

LED allows noise on

this supply without significantly

changing the display brightness.

LOGIC and VLED Considerations

The display uses two independent

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 V

LED and

LOGIC to be varied independently.

Thus, V

LED can vary from 0 to 5.5

V without affecting either the Dot

or the Control Registers. V

LED can

– 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

= 100°C/W

J-A

1.3

Figure 4.

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