MAX6975 Maxim Integrated Products, MAX6975 Datasheet - Page 21

• Manufacturer Part Number: MAX6975
• Description: (MAX6974 / MAX6975) 24-Output PWM LED Drivers
• Manufacturer: Maxim Integrated Products

Use the same length interface signal paths, whether
differential or CMOS, to ensure a uniform propagation
delay for each signal.
The MAX6974/MAX6975 operate with a power-supply
voltage of 3.0V to 3.6V. Bypass the V
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
supply with bulk capacitance to ensure that the fast-
rising, high-current LED drive currents do not cause
transient dips in V
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
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.
PROCESS: BiCMOS
Driving LEDs from a Supply Higher than 7V
DD
supply, adequately decouple the V
Power-Supply Considerations
DD
______________________________________________________________________________________
.
SYSTEM
Chip Information
DATA
LOAD
CLK
8 RGB LEDs
DD
power supply
BE
below its
CLKI
DINI
LOADI
R0/G0/B0
R1/G1/B1
R2/G2/B2
R3/G3/B3
R4/G4/B4
R5/G5/B5
R6/G6/B6
R7/G7/B7
24-Output PWM LED Drivers
MAX6974
DD
LOADO
CLKO
DINO
Example of using an external npn transistor:
For best output current accuracy, design V
least 1.2V:
V
Figure 13. External Cascode npn Transistor
DD
R1
(MAX)
= 3.3V ±5%, I
MAX6974
MAX6975
+3.3V
GND
V
V
for Message Boards
DD
BE
= (3.15 - 1 - 1.2) / 0.030 = 31.7Ω, so choose
= 0.7V to 1V at 30mA emitter current.
R0
R1
R2
R3
R4
R5
R6
R7
CLKI
DINI
LOADI
R0/G0/B0
R1/G1/B1
R2/G2/B2
R3/G3/B3
R4/G4/B4
R5/G5/B5
R6/G6/B6
R7/G7/B7
+3.3V
Typical Operating Circuit
30mA
MAX6974
OUT
R1 = 30Ω.
= 30mA, external pass transistor
LOADO
CLKO
DINO
R1
Q1
O
to be at
+24V
21