Datasheet

LM2750
SNVS180L –APRIL 2002–REVISED MAY 2013 www.ti.com
Figure 19. LM2750 LED Drive Efficiency. 6 LEDs, I
LED
= 20mA each, V
LED
= 4.0V
LED DRIVER POWER CONSUMPTION
For battery-powered LED-drive applications, it is strongly recommended that power consumption, rather than
power efficiency, be used as the metric of choice when evaluating power conversion performance. Power
consumed (P
IN
) is simply the product of input voltage (V
IN
) and input current (I
IN
):
P
IN
= V
IN
× I
IN
LM2750 input current is equal to twice the output current (I
OUT
), plus the supply current of the part (nominally
5mA):
I
IN
= (2×I
OUT
) + 5mA
Output voltage and LED voltage do not impact the amount of current consumed by the LM2750 circuit. Thus,
neither factor affects the current draw on a battery. Since output voltage does not impact input current, there is
no power savings with either the LM2750-5.0 or the LM2750-ADJ: both options consume the same amount of
power.
In the previous section, LED Driver Efficiency was defined as:
E
LED
= P
LED
/P
IN
= (N×V
LED
×I
LED
) / {V
IN
× [(2×I
OUT
) + 5mA]}
The equation above can be simplified by recognizing the following:
2 × I
OUT
>> 5mA (high output current applications)
N × I
LED
= I
OUT
Simplification yields:
E
LED
= V
LED
/ V
IN
This is in direct contrast to the previous assertion that showed that power consumption was completely
independent of LED voltage. As is the case here with the LM2750, efficiency is often not a good measure of
power conversion effectiveness of LED driver topologies. This is why it is strongly recommended that power
consumption be studied or measured when comparing the power conversion effectiveness of LED drivers.
One final note: efficiency of an LED drive solution should not be confused with an efficiency calculation for a
standard power converter (E
P
).
E
P
= P
OUT
/ P
IN
= (V
OUT
× I
OUT
) / (V
IN
× I
IN
)
The equation above neglects power losses in the external resistors that set LED currents and is a very poor
metric of LED-drive power conversion performance.
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