LM3915 datasheet(9/25 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

comparator 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

either 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 the display to make sense when multiple

LM3915s are cascaded in dot mode, special circuitry has

been included to shut off LED #10 of the first device when

LED #1 of the second device comes on. The connection for

cascading in dot mode has already been described and is

depicted below.

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

the second LM3915, LED #11 is off. Pin 9 of LM3915 #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 LM3915 #1 is pulled an LED drop (1.5V or more)

below V

LED. This condition is sensed by comparator C2,

referenced 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

LM3915 #1 is held low enough to force LED #10 off when

any higher LED is illuminated. While 100 µA does not nor-

mally produce significant LED illumination, it may be notice-

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

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

with a 10k resistor. The 1V IR 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.

OTHER DEVICE CHARACTERISTICS

The LM3915 is relatively low-powered itself, and since any

number of LEDs can be powered from about 3V, it is a very

efficient display driver. Typical standby supply current (all

LEDs OFF) is 1.6 mA. However, any reference loading adds

4 times that current drain to the V

+ (pin 3) supply input. For

example, an LM3916 witha1mA reference pin load (1.3k)

would supply almost 10 mA to every LED while drawing only

10 mA from its V

+ pin supply. At full-scale, the IC is typically

drawing less than 10% of the current supplied to the display.

The display driver does not have built-in hysteresis so that

the display does not jump instantly from one LED to the next.

Under rapidly changing signal conditions, this cuts down

high frequency noise and often an annoying flicker. An “over-

lap” is built in so that at no time are all segments completely

off in the dot mode. Generally 1 LED fades in while the other

fades out over a mV or more of range. The change may be

much more rapid between LED #10 of one device and LED

#1 of a second device “chained” to the first.

Application Hints

The most difficult problem occurs when large LED currents

are being drawn, especially in bar graph mode. These cur-

rents flowing out of the ground pin cause voltage drops in

external wiring, and thus errors and oscillations. Bringing the

return wires from signal sources, reference ground and bot-

tom of the resistor string to a single point very near pin 2 is

the best solution.

Long wires from V

LED to LED anode common can cause

oscillations. Depending on the severity of the problem

0.05 µF to 2.2 µF decoupling capacitors from LED anode

common to pin 2 will damp the circuit. If LED anode line

wiring is inaccessible, often similar decoupling from pin 1 to

pin 2 will be sufficient.

If LED turn ON seems slow (bar mode) or several LEDs light

(dot mode), oscillation or excessive noise is usually the

problem. In cases where proper wiring and bypassing fail to

stop oscillations, V

+ voltage at pin 3 is usually below sug-

gested limits. Expanded scale meter applications may have

one or both ends of the internal voltage divider terminated at

relatively high value resistors. These high-impedance ends

should be bypassed to pin 2 with at least a 0.001 µF capaci-

tor, or up to 0.1 µF in noisy environments.

Cascading LM3915s in Dot Mode

00510408

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