MAX6954
The bit is cleared if the switch has not been detected
as pressed by the key-scanning circuit during the last
test. Reading a pressed register does not clear that
register or clear the IRQ output.
Display Test Register
The display test register (Table 36) operates in two
modes: normal and display test. Display test mode
turns all LEDs on (including DPs) by overriding, but not
altering, all controls and digit registers (including the
shutdown register), except for the digit-type register
and the GPIO configuration register. The duty cycle,
while in display test mode, is 7/16 (see the
Choosing
Supply Voltage to Minimize Power Dissipation
section).
Selecting External Components R
SET
and
C
SET
to Set Oscillator Frequency and
Peak Segment Current
The RC oscillator uses an external resistor, R
SET
, and
an external capacitor, C
SET
, to set the frequency, f
OSC
.
The allowed range of f
OSC
is 1MHz to 8MHz. R
SET
also
sets the peak segment current. The recommended val-
ues of R
SET
and C
SET
set the oscillator to 4MHz, which
makes the blink frequencies selectable between 0.5Hz
and 1Hz. The recommended value of R
SET
also sets
the peak current to 40mA, which makes the segment
current adjustable from 2.5mA to 37.5mA in 2.5mA
steps.
I
SEG
= K
L
/ R
SET
mA
f
OSC
= K
F
/ (R
SET
x C
SET
) MHz
where:
K
L
= 2240
K
F
= 5376
R
SET
= external resistor in kΩ
C
SET
= external capacitor in pF
C
STRAY
= stray capacitance from OSC pin to GND in
pF, typically 2pF
The recommended value of R
SET
is 56kΩ and the rec-
ommended value of C
SET
is 22pF.
The recommended value or R
SET
is the minimum
allowed value, since it sets the display driver to the
maximum allowed peak segment current. R
SET
can be
set to a higher value to set the segment current to a
lower peak value where desired. The user must also
ensure that the peak current specifications of the LEDs
connected to the driver are not exceeded.
The effective value of R
SET
includes not only the actual
external capacitor used, but also the stray capacitance
from OSC to GND. This capacitance is usually in the
1pF to 5pF range, depending on the layout used.
Applications Information
Driving Bicolor LEDs
Bicolor digits group a red and a green die together for
each display element, so that the element can be lit red
or green (or orange), depending on which die (or both)
is lit. The MAX6954 allows each segment’s current to
be set individually from the 1/16th (minimum current
and LED intensity) to 15/16th (maximum current and
LED intensity), as well as off (zero current). Thus, a
bicolor (red-green) segment pair can be set to 256
color/intensity combinations.
Choosing Supply Voltage to Minimize
Power Dissipation
The MAX6954 drives a peak current of 40mA into LEDs
with a 2.2V forward-voltage drop when operated from a
supply voltage of at least 3.0V. The minimum voltage
drop across the internal LED drivers is therefore (3.0V -
2.2V) = 0.8V. If a higher supply voltage is used, the dri-
ver absorbs a higher voltage, and the driver’s power
dissipation increases accordingly. However, if the LEDs
used have a higher forward voltage drop than 2.2V, the
supply voltage must be raised accordingly to ensure
that the driver always has at least 0.8V of headroom.
The voltage drop across the drivers with a nominal 5V
supply (5.0V - 2.2V) = 2.8V is nearly 3 times the drop
across the drivers with a nominal 3.3V supply (3.3V -
2.2V) = 1.1V. In most systems, consumption is an
important design criterion, and the MAX6954 should be
operated from the system’s 3.3V nominal supply. In
other designs, the lowest supply voltage may be 5V.
The issue now is to ensure the dissipation limit for the
MAX6954 is not exceeded. This can be achieved by
inserting a series resistor in the supply to the MAX6954,
ensuring that the supply decoupling capacitors are still
on the MAX6954 side of the resistor. For example, con-
sider the requirement that the minimum supply voltage
to a MAX6954 must be 3.0V, and the input supply
range is 5V ±5%. Maximum supply current is 35mA +
(40mA x 17) = 715mA. Minimum input supply voltage is
4.75V. Maximum series resistor value is (4.75V -
3.0V)/0.715A = 2.44Ω. We choose 2.2Ω ±5%. Worst-
case resistor dissipation is at maximum toleranced
resistance, i.e., (0.715A) 2 x (2.2Ω x 1.05) = 1.18W. The
maximum MAX6954 supply voltage is at maximum
input supply voltage and minimum toleranced resis-
tance, i.e., 5.25V - (0.715A x 2.2Ω x 0.95) = 3.76V.
Low-Voltage Operation
The MAX6954 works over the 2.7V to 5.5V supply
range. The minimum useful supply voltage is deter-
mined by the forward voltage drop of the LEDs at the
4-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
18 ______________________________________________________________________________________
