LM3916 datasheet(13/27 Pages) NSC

13 / 27 page

Application Hints (Continued)

mA to flow from pin 7 into the divider which means that the

LED current will be at least 10 mA. R1 will typically be be-

tween 1 k

Ω and5kΩ. To trim the reference voltage, vary R2.

The current in

Figure 11 shows how to add a LED intensity

control which can vary LED current from 5 mA to 28 mA.

Choosing V

REF = 5V lowers the current drawn by the ladder,

increasing the intensity adjustment range. The reference ad-

justment has some effect on LED intensity but the reverse is

not true.

Multiple Drivers

Figure 12 shows how to obtain a common reference trim and

intensity control for two drivers. The two ICs may be con-

nected in cascade or may be handling separate channels for

stereo. This technique can be extended for larger numbers

of drivers by varying the values of R1, R2 and R3. Because

the LM3915 has a greater ladder resistance, R5 was picked

less than R7 in such a way as to provide equal reference

load currents. The ICs’ internal references track within

100 mV so that worst case error from chip to chip is only 0.2

dB for V

REF = 5V.

The scheme in

Figure 13 is useful when the reference and

LED intensity must be adjusted independently over a wide

range. The R

HI voltage can be adjusted from 1.2V to 10V

with no effect on LED current. Since the internal divider here

does not load down the reference, minimum LED current is

much lower. At the minimum recommended reference load

of 80 µA, LED current is about 0.8 mA. The resistor values

shown give a LED current range from 1.5 mA to 25 mA.

At the low end of the intensity adjustment, the voltage drop

across the 510

Ω current-sharing resistors is so small that

chip to chip variation in reference voltage may yield a visible

variation in LED intensity. The optional approach shown of

connecting the bottom end of the intensity control pot to a

negative supply overcomes this problem by allowing a larger

voltage drop across the (larger) current-sharing resistors.

DS007971-17

FIGURE 10. Design Equations for Fixed LED Intensity

DS007971-18

5mA

≤ ILED ≤ 28 mA @ VREF = 5V

FIGURE 11. Varying LED Intensity

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