Datasheet

LM2750
www.ti.com SNVS180L –APRIL 2002–REVISED MAY 2013
I
LED
= (V
OUT
- V
LED
) / R
In the equation above, I
LED
is the current that flows through a particular LED, and V
LED
is the forward voltage of
the LED at the given current. As can be seen in the equation above, LED current will vary with changes in LED
forward voltage (V
LED
). Mismatch of LED currents will result in brightness mismatch from one LED to the next.
The feedback pin of the LM2750-ADJ can be utilized to help better control brightness levels and negate the
effects of LED forward voltage variation. As shown in Figure 18, connecting the feedback pin to the primary LED-
resistor junction (LED1-R1) regulates the current through that LED. The voltage across the primary resistor (R1)
is the feedback pin voltage (1.23V typ.), and the current through the LED is the current through that resistor.
Current through all other LEDs (LEDx) will not be regulated, however, and will vary with LED forward voltage
variations. When using the LM2750-ADJ in current-mode, LED currents can be calculated with the equations
below:
I
LED1
= 1.23V / R1
I
LEDx
= (1.23V + V
LED1
- V
LEDx
) / Rx
The current-mode configuration does not improve brightness matching from one LED to another in a single
circuit, but will keep currents similar from one circuit to the next. For example: if there is forward voltage
mismatch from LED1 to LED2 on a single board, the current-mode LM2750-ADJ solution provides no benefit. But
if the forward voltage of LED1 on one board is different than the forward voltage of LED1 on another board, the
currents through LED1 in both phones will match. THis helps keep LED currents fairly consistent from one
product to the next, adn helps to offset lot-to-lot variation of LED forward voltage characteristics.
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).
To achieve good brightness/dimming control with this circuit, proper selection of the PWM frequency is required.
The PWM frequency (F
PWM
) should be set higher than 100Hz to avoid visible flickering of the LED light. An upper
bound on this frequency is also needed to accommodate the turn-on time of the LM2750 (T
ON
= 0.5ms typ.). This
maximum recommended PWM frequency is similarly dependent on the minimum duty cycle (D
MIN
) of the
application. The following equation puts bounds on the reommended PWM frequency range:
100Hz < F
PWM
< D
MIN
÷ T
ON
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
recommended. A PWM frequency up to 1kHz may be acceptable in some designs.
LED DRIVER POWER EFFICIENCY
Efficiency of an LED driver (E
LED
) is typically defined as the power consumed by the LEDs (P
LED
) divided by the
power consumed at the input of the circuit. Input power consumption of the LM2750 was explained and defined
in the previous section titled: POWER EFFICIENCY AND POWER DISSIPATION. 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 (V
LED
), and the LED forward current (I
LED
):
P
LED
= N × V
LED
× I
LED
E
LED
= P
LED
/ P
IN
= (N×V
LED
×I
LED
) / {V
IN
× [(2×I
OUT
) + 5mA]}
Figure 19 is an efficiency curve for a typical LM2750 LED-drive application.
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