stled325 STMicroelectronics, stled325 Datasheet - Page 35

• Manufacturer Part Number: stled325
• Description: I?c Interfaced, Advanced Led Controller/driver With Keyscan, Standby Power Management And Real Time Clock Rtc
• Manufacturer: STMicroelectronics

STLED325
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 / V
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.
Table 13.
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 14.
Green
Symbol
LED
Blue
Red
C
IN
Typical forward
Voltage drop estimation with RGB LED
Capacitance (T
voltage
2.2 V
2.5 V
3 V
Input capacitance
VF
(all digital pins)
Parameter
A
Doc ID 17576 Rev 1
= 25°C, f = 1 MHz)
Typical
current
40 mA
40 mA
40 mA
Test conditions
Typical supply
CC
voltage
x I
5 V
5 V
5 V
CC
Digit driver
drop
Min
1 V
1 V
1 V
Typ
External
2.2 V
2.5 V
3 V
Electrical ratings
VF
Max
15
Segment
driver
drop
1 V
1.8
1.5
Unit
35/62
pF