LM3916MWC National Semiconductor Corporation, LM3916MWC Datasheet - Page 8

• Manufacturer Part Number: LM3916MWC
• Description: Dot/bar Display Driver
• Manufacturer: National Semiconductor Corporation

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Mode Pin Functional Description
DOT OR BAR MODE SELECTION
The voltage at pin 9 is sensed by comparator C1, nominally
referenced to (V
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
open circuit) for dot mode. In most applications, pin 9 is ei-
ther open (dot mode) or tied to V
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
1 .
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
(Continued)
+
for bar mode and more than 200 mV below V
+
−100 mV). The chip is in bar mode when
FIGURE 2. Cascading Drivers in Dot Mode with Pin 1 of Driver # 2 Unused
+
FIGURE 1. Cascading LM3914/15/16 Series in Dot Mode
(bar mode). In bar mode,
+
(or
8
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
V
enced 600 mV below V
which shuts off output transistor Q2, extinguishing LED # 10.
V
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
through two series-connected diodes as shown in Figure 2 .
Shunting the diodes with a 1k resistor provides a path for
driver leakage current.
LED
LED
. This condition is sensed by comparator C2, refer-
is sensed via the 20k resistor connected to pin 11. The
LED
. This forces the output of C2 low,
DS007971-8
DS007971-9
LED