STLED325 datasheet(35/62 Pages) STMICROELECTRONICS

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STLED325

Electrical ratings

Doc ID 17576 Rev 1

35/62

4.3

Power consumption estimation

Each port of the STLED325 can sink a maximum current of 40 mA into an LED with a 3.4 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 - 3.4 = 1.6 V. The STLED325 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 STLED325 dissipates (5.0 V-3.4 V) x 320 mA = 512 mW when driving 8 of

these 3.4 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 STLED325 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 in-crease 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.

Equation 1 Efficiency = Vdiode x Idiode / VCC x ICC

As an example, consider LED with forward voltage of VF = 2.4 V, 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.

Note that the above analysis is for a typical condition. If the VF is higher and the supply

voltage is lower than 5V, then it is recommended to operate the LED at a lower current than

40mA in order to have enough headroom for the digit and segment drivers so as not to affect

the brightness and matching.

Table 13.

Voltage drop estimation with RGB LED

LED

Typical forward

voltage

Typical

current

Typical supply

voltage

Digit driver

drop

External

Segment

driver

drop

Red

2.2 V

40 mA

5 V

1 V

2.2 V

1.8

Green

2.5 V

40 mA

5 V

1 V

2.5 V

1.5

Blue

40 mA

3 V

Table 14.

Capacitance (TA = 25°C, f = 1 MHz)

Symbol

Parameter

Test conditions

Min

Typ

Max

Unit

CIN

Input capacitance

(all digital pins)

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