LM2750 LED Drive Application

The LM2750 is an excellent part for driving white and blue

LEDs for display backlighting and other general-purpose

lighting functions. The circuits of Figure 5 and Figure 6 show

LED driver circuits for the LM2750-5.0 and the LM2750-ADJ,

respectively. Simply placing a resistor (R) in series with each

LED sets the current through the LEDs:

I

In the equation above, I

a particular LED, and V

at the given current. As can be seen in the equation above,

LED current will vary with changes in LED forward voltage

(V

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 6, con-

necting the feedback pin to the primary LED-resistor junction

(LED1-R1) regulates the current through that LED. The volt-

age 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

I

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 ex-

ample: 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.

20035124

FIGURE 5. LM2750-5.0 LED Drive Application Circuit

20035125

FIGURE 6. LM2750-ADJ LED Drive Application Circuit

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