Application Note 2
Micrel
Application Note 2
MIC4807 Display Dimmer
by Mitchell Lee
Abstract
The MIC4807 is an 8 channel, addressable low side driver
and is guaranteed to deliver 100mA minimum at up to 80V
per channel. This note discusses the operation of the
MIC4807 and shows how it can be used as a display driver
with dimming for incandescent indicators.
Incandescent Lamp Characteristics
Owing to their superior light output, incandescent lamps are
preferred over other display devices for use in bright envi-
ronments. Unfortunately, incandescent lamps have a num-
ber of characteristics that make them difficult to work with in
practical applications. For example, lamps do not lend
themselves to multiplexing. It is technically possible to
multiplex lamps by a higher-than-rated supply voltage in
conjunction with PWM techniques to control filament power
dissipation.
A major pitfall of multiplexing is reliability. If the multiplex
circuit fails to advance for any reason (power-up phenom-
enon, slow or stuck oscillator, etc.) the lamps will burn out
instantly. In addition, the switched current increases pro-
portionally with the supply voltage, necessitating larger
switches.
Since multiplexing is impractical, each lamp must have its
own dedicated driver. This adds circuit overhead not only
in the number of drivers, but also in terms of communicating
with the drivers.
The brightness of an incandescent lamp is an asset in
brightly illuminated environments, but what happens at
night? Under contrasting conditions of low ambient light
levels, the bright display can temporarily blind persons
viewing it. Examples of environments with wide-ranging
light levels include the cockpit of an airplane, or the operator鈥檚
cab on farm or construction machinery. A dimming feature
is highly desirable for any incandescent display.
Introduction
The MIC4807 contains 8 low side drivers that are controlled
by addressable latches (see Figure 1). Open-drain,
N-channel MOSFETs of approximately 5.1鈩?鈥渙n鈥?resis-
tance are used as output devices. The MOSFETs are
designed for operation to 80V.
Each output is controlled by its own addressable latch; the
latches are selected by a 3-bit parallel address (A
in
, B
in
, and
C
in
). A 鈥?鈥?at the data input turns the corresponding
MOSFET on.
Power ICs demand protection from excessive current and
dissipation, and to this end the MIC4807 includes short-
circuit current limiting and thermal shutdown. In fact, the
chip can withstand a dead short to 80V without damage.
The output limits at typically 200mA, and the chip is guaran-
teed to deliver 100mA minimum over temperature. While
current limiting provides short-term protection from load
faults, thermal shutdown protects against sustained fault
conditions by shutting off all outputs when the die tempera-
ture exceeds 150擄C. Current limiting and thermal shutdown
are indispensable, yet they are sorely lacking in many other
functionally similar ICs where the implementation of protec-
tion circuits is left as an exercise for the user.
OE
V
DD
12
Thermal
Shutdown
4
17 HVOUT
0
18 HVOUT
1
1 HVOUT
2
2 HVOUT
3
8 HVOUT
4
9 HVOUT
5
10 HVOUT
6
11 HVOUT
7
A
in
16
Addressing
B
in
15
C
in
14
Address
Decoder
Latches
Driver
鈥?/div>
鈥?/div>
鈥?/div>
鈥?/div>
鈥?/div>
鈥?/div>
Driver
Current
Limit
Ground 5
6
CS
13
7
Data-in Clear
Figure 1. MIC4807 Block Diagram
7-70
1997
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