peak current I
SEG
, plus the 0.8V headroom required by
the driver output stages. The MAX6954 correctly regu-
lates I
SEG
with a supply voltage above this minimum
voltage. If the supply drops below this minimum volt-
age, the driver output stages may brown out, and be
unable to regulate the current correctly. As the supply
voltage drops further, the LED segment drive current
becomes effectively limited by the output driver's on-
resistance, and the LED drive current drops. The char-
acteristics of each individual LED in a display digit are
well matched, so the result is that the display intensity
dims uniformly as supply voltage drops out of regula-
tion and beyond.
Computing Power Dissipation
The upper limit for power dissipation (P
D
) for the
MAX6954 is determined from the following equation:
P
D
= (V+ x 35mA) + (V+ - V
LED
) (DUTY x I
SEG
x N)
where:
V+ = supply voltage
DUTY = duty cycle set by intensity register
N = number of segments driven (worst case is 17)
V
LED
= LED forward voltage at I
SEG
I
SEG
= segment current set by R
SET
P
D
= Power dissipation, in mW if currents are in mA
Dissipation example:
I
SEG
= 30mA, N = 17, DUTY = 15/16,
V
LED
= 2.4V at 30mA, V+ = 3.6V
P
D
= 3.6V (35mA) + (3.6V - 2.4V)(15/16 x
30mA x 17) = 0.700W
Thus, for a 36-pin SSOP package (T
JA
= 1 / 0.0118 =
+85°C/W from Operating Ratings), the maximum
allowed ambient temperature T
A
is given by:
T
J(MAX)
= T
A
+ (P
D
x T
JA
) = +150°C
= T
A
+ (0.700 x +85°C/W)
So T
A
= +90.5°C. Thus, the part can be operated safely
at a maximum package temperature of +85°C.
Power Supplies
The MAX6954 operates from a single 2.7V to 5.5V
power supply. Bypass the power supply to GND with a
0.1µF capacitor as close to the device as possible. Add
a 47µF capacitor if the MAX6954 is not close to the
board's input bulk decoupling capacitor.
Terminating the Serial Interface
The MAX6954 uses fixed voltage thresholds of 0.6V
and 1.8V for the 4-wire interface inputs. These fixed
thresholds allow the MAX6954 to be controlled by a
host operating from a lower supply voltage than the
MAX6954; for example, 2.5V. The fixed thresholds also
reduce the logic input noise margin when operating the
MAX6954 from a higher supply voltage, such as 5V. At
higher supply voltages, it may be necessary to fit termi-
nation components to the CLK, DIN, and CS inputs to
avoid signal reflections that the MAX6954 could
respond to as multiple transitions. Suitable termination
components can be either a 33pF capacitor or 4.7kΩ
resistor fitted from each of the CLK, DIN, and CS inputs
to GND.
MAX6954
4-Wire Interfaced, 2.7V to 5.5V LED Display
Driver with I/O Expander and Key Scan
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