Typical operating circuit chip information – Rainbow Electronics MAX6975 User Manual

Page 21

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MAX6974/MAX6975

24-Output PWM LED Drivers

for Message Boards

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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

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

supply, adequately decouple the V

supply with bulk capacitance to ensure that the fast-
rising, high-current LED drive currents do not cause
transient dips in V

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

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:

= 3.3V ±5%, I

OUT

= 30mA, external pass transistor

= 0.7V to 1V at 30mA emitter current.

For best output current accuracy, design V

to be at

least 1.2V:

(MAX)

= (3.15 - 1 - 1.2) / 0.030 = 31.7

Ω, so choose

R1 = 30

Ω.

MAX6974
MAX6975

30mA

+3.3V

+24V

GND

Figure 13. External Cascode npn Transistor

MAX6974

SYSTEM

CLK

DATA

LOAD

CLKO

DINO

LOADO

CLKI

DINI

LOADI

R0/G0/B0

8 RGB LEDs

R1/G1/B1

R2/G2/B2

R3/G3/B3

R4/G4/B4

R5/G5/B5

R6/G6/B6

R7/G7/B7