LM3916 datasheet(8/27 Pages) NSC

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Mode Pin Functional Description

(Continued)

DOT OR BAR MODE SELECTION

The voltage at pin 9 is sensed by comparator C1, nominally

referenced to (V

+ −100 mV). The chip is in bar mode when

pin 9 is above this level; otherwise it’s in dot mode. The com-

parator is designed so that pin 9 can be left open circuit for

dot mode.

Taking into account comparator gain and variation in the

100 mV reference level, pin 9 should be no more than 20 mV

below V

+ for bar mode and more than 200 mV below V+ (or

open circuit) for dot mode. In most applications, pin 9 is ei-

ther open (dot mode) or tied to V

+ (bar mode). In bar mode,

pin 9 should be connected directly to pin 3. Large currents

drawn from the power supply (LED current, for example)

should not share this path so that large IR drops are avoided.

DOT MODE CARRY

In order for display to make sense when multiple drivers are

cascaded in dot mode, special circuitry has been included to

shut off LED #10 of the first device when LED #1 of the sec-

ond device comes on. The connection for cascading in dot

mode has already been described and is depicted in

Figure

As long as the input signal voltage is below the threshold of

the second driver, LED #11 is off. Pin 9 of driver #1 thus sees

effectively an open circuit so the chip is in dot mode. As soon

as the input voltage reaches the threshold of LED #11, pin 9

of driver #1 is pulled an LED drop (1.5V or more) below

LED. This condition is sensed by comparator C2, refer-

enced 600 mV below V

LED. This forces the output of C2 low,

which shuts off output transistor Q2, extinguishing LED #10.

LED is sensed via the 20k resistor connected to pin 11. The

very small current (less than 100 µA) that is diverted from

LED #9 does not noticeably affect its intensity.

An auxiliary current source at pin 1 keeps at least 100 µA

flowing through LED #11 even if the input voltage rises high

enough to extinguish the LED. This ensures that pin 9 of

driver #1 is held low enough to force LED #10 off when

any

higher LED is illuminated. While 100 µA does not normally

produce significant LED illumination, it may be noticeable

when using high-efficiency LEDs in a dark environment. If

this is bothersome, the simple cure is to shunt LED #11 (and

LED #1) with a 10k resistor. The 1V 1R drop is more than the

900 mV worst case required to hold off LED #10 yet small

enough that LED #11 does not conduct significantly.

In some circuits a number of outputs on the higher device

are not used. Examples include the high resolution VU meter

and the expanded range VU meter circuits (see Typical Ap-

plications). To provide the proper carry sense voltage in dot

mode, the LEDs of the higher driver IC are tied to V

LED

through two series-connected diodes as shown in

Figure 2.

Shunting the diodes with a 1k resistor provides a path for

driver leakage current.

DS007971-8

FIGURE 1. Cascading LM3914/15/16 Series in Dot Mode

DS007971-9

FIGURE 2. Cascading Drivers in Dot Mode with Pin 1 of Driver #2 Unused

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