HCMS-3917 datasheet(19/22 Pages) AVAGO

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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 in Table 3.

The average current required by

the display can be calculated with

Equation 4 in Table 3.

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.

Table 3. Equations.

Equation 1:

TJ MAX = TA + PD * RθJA

Where: TJ MAX = maximum IC junction temperature

TA = ambient temperature surrounding the display

θJA = thermal resistance from the IC junction to ambient

PD = total power dissipation

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)

VLOGIC 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 VLED and

VLOGIC to be varied inde-

pendently. Thus, VLED can vary

from 0 to 5.5 V without affecting

either the Dot or the Control

Registers. VLED can be varied

between 3.1 to 5.5 V without

much noticeable variation in light

output to the human eyes. There

is also no pixel mismatch

observed.

The intensity of the light output

takes a plunge if operated less

than 3.1 V. There is also no pixel

mismatch observed at voltage as

low as 2.6 V. However, operating

below 3.1 V is not recommended.

Dimming the display by pulse

width modulating VLED is also not

recommended.

VLOGIC can vary from 3.0 to 5.5 V

without affecting either the

displayed message or the display

intensity. However, operating

below 3 V may change the timing

and logic levels and may cause

Dot and Control Registers to be

altered. Thus, operation of the

display below 3.0 V is not

recommended.

The logic ground is internally

connected to the LED ground by

a substrate diode. This diode

becomes forward biased and

conducts when the logic ground is

0.4 V greater than the LED

ground. The LED ground and the

logic ground should be connected

to a common ground, which can

withstand the current introduced

by the switching LED drivers.

When separate ground

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