MAX6954_09 MAXIM [Maxim Integrated Products], MAX6954_09 Datasheet - Page 19

• Manufacturer Part Number: MAX6954_09
• Description: 4-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key Scan
• Manufacturer: MAXIM [Maxim Integrated Products]

peak current I
the driver output stages. The MAX6954 correctly regu-
lates I
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.
The upper limit for power dissipation (P
MAX6954 is determined from the following equation:
where:
V+ = supply voltage
DUTY = duty cycle set by intensity register
N = number of segments driven (worst case is 17)
V
I
P
Dissipation example:
SEG
LED
D
P
= Power dissipation, in mW if currents are in mA
D
= segment current set by R
= LED forward voltage at I
= (V+ x 35mA) + (V+ - V
SEG
V
I
SEG
LED
P
D
4-Wire Interfaced, 2.7V to 5.5V LED Display
with a supply voltage above this minimum
= 3.6V (35mA) + (3.6V - 2.4V)(15/16 x
= 30mA, N = 17, DUTY = 15/16,
= 2.4V at 30mA, V+ = 3.6V
SEG
30mA x 17) = 0.700W
Computing Power Dissipation
, plus the 0.8V headroom required by
______________________________________________________________________________________
Driver with I/O Expander and Key Scan
LED
SEG
SET
) (DUTY x I
SEG
D
) for the
x N)
Thus, for a 36-pin SSOP package (T
+85°C/W from Operating Ratings), the maximum
allowed ambient temperature T
So T
at a maximum package temperature of +85°C.
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.
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.
T
A
J(MAX)
= +90.5°C. Thus, the part can be operated safely
= T
= T
Terminating the Serial Interface
A
A
+ (P
+ (0.700 x +85°C/W)
D
x T
JA
) = +150°C
A
is given by:
Power Supplies
JA
= 1 / 0.0118 =
19