Datasheet
Applications Information
Driving Bicolor and Tricolor LEDs
Bicolor digits group a red and a green die together for
each display element, so that the element can be lit red,
green (or orange), depending on which die (or both) is lit.
The MAX6957 allows each segment’s current to be set
individually from 1/16th (minimum current and LED inten-
sity) to 16/16th (maximum current and LED intensity), as
well as off (zero current). Thus, a bicolor (red-green) seg-
ment pair can be set to 289 color/intensity combinations.
A discrete or CA tricolor (red-green-yellow or red-green-
blue) segment triad can be set to 4913 color/intensity
combinations.
Power Dissipation Issues
Each MAX6957 port can sink a current of 24mA into an
LED with a 2.4V 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 - 2.4V) = 0.6V. The MAX6957 can sink 28 x 24mA
= 672mA when all outputs are operating as LED segment
drivers at full current. On a 3.3V supply, a MAX6957 dis-
sipates (3.3V - 2.4V) 5 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 MAX6957 absorbs a higher voltage, and the
MAX6957’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 MAX6957
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
5 (82Ω
× 1.02) = 34mW. The maximum MAX6957 dissipation
per LED is at maximum input supply voltage, minimum
toleranced resistance, mini-mum toleranced LED for-
ward-voltage drop, i.e., 0.020 × (5.25V - 2.0V - (0.020A
× 82Ω × 0.98)) = 32.86mW. Worst-case MAX6957 dis-
sipation 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.
Table 11. Even Individual Segment Current Format
LED DRIVE
FRACTION
SEGMENT
CONSTANT
CURRENT WITH
R
ISET
= 39kΩ (mA)
ADDRESS CODE
(HEX)
D7 D6 D5 D4 D3 D2 D1 D0 HEX CODE
1/16 1.5 0x12 to 0x1F 0 0 0 0 0xX0
2/16 3 0x12 to 0x1F 0 0 0 1 0xX1
3/16 4.5 0x12 to 0x1F 0 0 1 0 0xX2
4/16 6 0x12 to 0x1F 0 0 1 1 0xX3
5/16 7.5 0x12 to 0x1F 0 1 0 0 0xX4
6/16 9 0x12 to 0x1F 0 1 0 1 0xX5
7/16 10.5 0x12 to 0x1F 0 1 1 0 0xX6
8/16 12 0x12 to 0x1F See Table 12. 0 1 1 1 0xX7
9/16 13.5 0x12 to 0x1F 1 0 0 0 0xX8
10/16 15 0x12 to 0x1F 1 0 0 1 0xX9
11/16 16.5 0x12 to 0x1F 1 0 1 0 0xXA
12/16 18 0x12 to 0x1F 1 0 1 1 0xXB
13/16 19.5 0x12 to 0x1F 1 1 0 0 0xXC
14/16 21 0x12 to 0x1F 1 1 0 1 0xXD
15/16 22.5 0x12 to 0x1F 1 1 1 0 0xXE
16/16 24 0x12 to 0x1F 1 1 1 1 0xXF
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MAX6957 4-Wire-Interfaced, 2.5V to 5.5V, 20-Port and
28-Port LED Display Driver and I/O Expander