MAX6974_07 Datasheet, PDF (21/23 Pages) Maxim Integrated Products – 24-Output PWM LED Drivers for Message Boards

24-Output PWM LED Drivers
for Message Boards
Use the same length interface signal paths, whether
differential or CMOS, to ensure a uniform propagation
delay for each signal.
Power-Supply Considerations
The MAX6974/MAX6975 operate with a power-supply
voltage of 3.0V to 3.6V. Bypass the V_DD power supply
to GND with a 0.1µF ceramic capacitor as close as
possible to the device pins. If the LED supply is shared
with the V_DD supply, adequately decouple the V_DD
supply with bulk capacitance to ensure that the fast-
rising, high-current LED drive currents do not cause
transient dips in V_DD.
Driving LEDs from a Supply Higher than 7V
An external npn transistor in a cascode configuration
extends the output drive voltage above 7V. The external
pass transistor’s emitter clamps to a V_BE below its
base, which is connected to the MAX6974/MAX6975’s
supply voltage. An optional emitter resistor reduces the
voltage drop across the MAX6974/MAX6975’s output
transistor and effectively takes the dissipation off the
device into the resistor. The external transistor’s collector
current is equal to its emitter current (less a small base
current), and the MAX6974/MAX6975 accurately
control the emitter current with a constant current sink
driver structure.
Example of using an external npn transistor:
V_DD = 3.3V ±5%, I_OUT = 30mA, external pass transistor
V_BE = 0.7V to 1V at 30mA emitter current.
For best output current accuracy, design V_O to be at
least 1.2V:
R1_(MAX) = (3.15 - 1 - 1.2) / 0.030 = 31.7Ω, so choose
R1 = 30Ω.
+3.3V
+3.3V
+24V
V_DD
MAX6974
Q1
MAX6975
R1
R0
R1
R2
R3
R4
R5
R6
GND ^R7
30mA
Figure 13. External Cascode npn Transistor
Typical Operating Circuit
SYSTEM
CLK
DATA
LOAD
MAX6974
CLKI
CLKO
DINI
DINO
LOADI
LOADO
8 RGB LEDs
R0/G0/B0
R1/G1/B1
R2/G2/B2
R3/G3/B3
R4/G4/B4
R5/G5/B5
R6/G6/B6
R7/G7/B7
MAX6974
CLKI
CLKO
DINI
DINO
LOADI
LOADO
R0/G0/B0
R1/G1/B1
R2/G2/B2
R3/G3/B3
R4/G4/B4
R5/G5/B5
R6/G6/B6
R7/G7/B7
PROCESS: BiCMOS
Chip Information
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