STLED316S_08 datasheet(15/33 Pages) STMICROELECTRONICS

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STLED316S

Recommended operating conditions

15/33

5.2

Power consumption estimation

Each port of the STLED316S can sink a maximum current of 40 mA into an LED with a

4.4 V forward voltage drop when operated from a supply voltage of 5.0 V. The minimum

voltage drop across the internal LED drivers is thus 5.0 - 4.4 = 0.6 V. The STLED316S can

sink 8 x 40 = 320 mA when all outputs are operating as LED segment drivers at full current.

On a 5.0 V supply, a STLED316S dissipates (5.0 V - 4.4 V) x 320 mA = 192 mW when

driving 8 of these 4.4 V forward voltage drop LEDs at full current. If the application requires

high drive current, consider adding a series resistor to each LED to drop excessive drive

voltage off-chip.

If the forward voltage of the LED is lesser than 4.4 V (say 2.4 V), then the maximum power

dissipation of STLED316S when all segments are turned on will be ( 5 - 2.4) V x

320 mA = 832 mW. To lower the power dissipation, consider adding a small series resistor

in the supply. Another alternative is to increase the value of the RSET to lower the current of

the LEDs from 40 mA to say 30 or 20 mA.

The efficiency will be the power consumption in the LEDs divided by the input power

consumed.

Efficiency = Vdiode x Idiode / VCC x ICC

As an example, consider LED with forward voltage of VF = 2.4V, Ipeak = 40 mA,

VCC (max) = 5.5 V, N = number of segments = 8 (max), D = duty cycle = 15/16,

Power dissipation, PD (max) = 5 mA x 5.5 V + (5.5 - 2.4) V x (15/16) x 40 mA x 8 = 27.5 + 780

= 807.5 mW. To lower this value, add a series resistor with the supply.

5.3

Capacitance

5.4

Power supply characteristics

Table 7.

Capacitance (TA =25°C, f=1MHz)

Symbol

Parameter

Test conditions

Min

Typ

Max

Unit

CIN

Input capacitance

Table 8.

Power supply characteristics (TA = -40 to +85 °C)

Symbol

Parameter

Test conditions

Min.

Typ.

Max.

Unit

ISTBY

Quiescent power supply

current

VCC = 5.5 V,

All inputs = VCC or GND

1000

µA

ICC

Operating power supply

current

(display ON)

All segments ON, all digits

scanned, intensity set to full,

internal oscillator, no display

load connected

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