segment drivers at full current. On a 3.3V supply, a
MAX6956 dissipates (3.3V - 2.4V)
672mA = 0.6W
when driving 28 of these 2.4V forward-voltage drop
LEDs at full current. This dissipation is within the ratings
of the 36-pin SSOP package with an ambient tempera-
ture up to +98°C. If a higher supply voltage is used or
the LEDs used have a lower forward-voltage drop than
2.4V, the MAX6956 absorbs a higher voltage, and the
MAX6956’s power dissipation increases.
If the application requires high drive current and high
supply voltage, consider adding a series resistor to
each LED to drop excessive drive voltage off-chip. For
example, consider the requirement that the MAX6956
must drive LEDs with a 2.0V to 2.4V specified forward-
voltage drop, from an input supply range is 5V ±5%
with a maximum LED current of 20mA. Minimum input
supply voltage is 4.75V. Maximum LED series resistor
value is (4.75V - 2.4V - 0.6V)/0.020A = 87.5Ω. We
choose 82Ω ±2%. Worst-case resistor dissipation is at
maximum toleranced resistance, i.e., (0.020A)
2
(82Ω
1.02) = 34mW. The maximum MAX6956 dissipation
per LED is at maximum input supply voltage, minimum
toleranced resistance, minimum toleranced LED for-
ward-voltage drop, i.e., 0.020 x (5.25V - 2.0V - (0.020A
82Ω x 0.98)) = 32.86mW. Worst-case MAX6956 dissi-
pation is 920mW driving all 28 LEDs at 20mA full cur-
rent at once, which meets the 941mW dissipation
ratings of the 36-pin SSOP package.
Low-Voltage Operation
The MAX6956 operates down to 2V supply voltage
(although the sourcing and sinking currents are not guar-
anteed), providing that the MAX6956 is powered up ini-
tially to at least 2.5V to trigger the device’s internal reset.
Serial Interface Latency
When a MAX6956 register is written through the I
2
C inter-
face, the register is updated on the rising edge of SCL
during the data byte’s acknowledge bit (Figure 5). The
delay from the rising edge of SCL to the internal register
being updated can range from 50ns to 350ns.
PC Board Layout Considerations
Ensure that all of the MAX6956 GND connections are
used. A ground plane is not necessary, but may be useful
to reduce supply impedance if the MAX6956 outputs are
to be heavily loaded. Keep the track length from the ISET
pin to the R
ISET
resistor as short as possible, and take the
GND end of the resistor either to the ground plane or
directly to the GND pins.
Power-Supply Considerations
The MAX6956 operates with power-supply voltages of
2.5V to 5.5V. Bypass the power supply to GND with a
0.047µF capacitor as close to the device as possible.
Add a 1µF capacitor if the MAX6956 is far away from
the board’s input bulk decoupling capacitor.
MAX6956
2-Wire-Interfaced, 2.5V to 5.5V, 20-Port or
28-Port LED Display Driver and I/O Expander
______________________________________________________________________________________ 21
Table 15. Transition Detection Mask Register
