LED DRIVER POWER CONSUMPTION
LM2750 LED Drive Application
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 (PIN) is
simply the product of input voltage (VIN) and input current
(IIN):
(Continued)
PWM BRIGHTNESS/DIMMING CONTROL
Brightness of the LEDs can be adjusted in an application by
driving the SD pin of the LM2750 with a PWM signal. When
the PWM signal is high, the LM2750 is ON, and current flows
through the LEDs, as described in the previous section. A
low PWM signal turns the part and the LEDs OFF. The
perceived brightness of the LEDs is proportional to ON
current of the LEDs and the duty cycle (D) of the PWM signal
(the percentage of time the LEDs are ON).
PIN = VIN x IIN
LM2750 input current is equal to twice the output current
(IOUT), plus the supply current of the part (nominally 5mA):
IIN = (2xIOUT) + 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.
To achieve good brightness/dimming control with this circuit,
proper selection of the PWM frequency is required. The
PWM frequency (FPWM) should be set higher than 100Hz to
avoid visible flickering of the LED light. An upper bound on
this frequency is also needed to accomodate the turn-on
time of the LM2750 (TON = 0.5ms typ.). This maximum
recommended PWM frequency is similarly dependent on the
minimum duty cycle (DMIN) of the application. The following
equation puts bounds on the reommended PWM frequency
range:
In the previous section, LED Driver Efficiency was defined
as:
ELED = PLED/PIN = (NxVLEDxILED) / {VIN x [(2xIOUT) + 5mA]}
The equation above can be simplified by recognizing the
following:
<
<
100Hz FPWM DMIN ÷ TON
>>
Choosing a PWM frequency within these limits will result in
fairly linear control of the time-averaged LED current over
the full duty-cycle adjustment range. For most applications, a
PWM frequency between 100Hz and 500Hz is recom-
mended. A PWM frequency up to 1kHz may be acceptable in
some designs.
2 x IOUT
5mA (high output current applications)
N x ILED = IOUT
Simplification yields:
ELED = VLED / VIN
This is in direct contrast to the previous assertion that
showed that power consumption was completely indepen-
dent 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 effic-
tiveness of LED drivers.
LED DRIVER POWER EFFICIENCY
Efficiency of an LED driver (ELED) is typically defined as the
power consumed by the LEDs (PLED) divided by the power
consumed at the input of the circuit. Input power consump-
tion of the LM2750 was explained and defined in the previ-
ous section titled: Power Efficiency and Power Dissipa-
tion. Assuming LED forward voltages and currents match
reasonably well, LED power consumption is the product of
the number of LEDs in the circuit (N), the LED forward
voltage (VLED), and the LED forward current (ILED):
One final note: efficiency of an LED drive solution should not
be confused with an efficiency calculation for a standard
power converter (EP).
EP = POUT / PIN = (VOUTx IOUT) / (VIN x IIN
)
PLED = N x VLED x ILED
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.
ELED = PLED / PIN = (NxVLEDxILED) / {VIN x [(2xIOUT) + 5mA]}
Figure 7 is an efficiency curve for a typical LM2750 LED-
drive application.
20035127
FIGURE 7. LM2750 LED Drive Efficiency. 6 LEDs, ILED
20mA each, VLED = 4.0V
=
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