16-Output PWM LED Drivers

for Message Boards

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21

Power-Supply Considerations

The MAX6972/MAX6973 operate with a power-supply

to GND with a 0.1µF ceramic capacitor as close as

possible to the device pins. If the LED supply is shared

supply with bulk capacitance to ensure that the fast-

rising, high-current LED drive currents do not cause

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

base, which is connected to the MAX6972/MAX6973’s

supply voltage. An optional emitter resistor reduces the

voltage drop across the MAX6972/MAX6973’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 MAX6972/MAX6973 accurately

control the emitter current with a constant current sink

driver structure.

Example of using an external npn transistor:

least 1.2V:

Ω, so choose

R1 = 15

Ω

hence,

= 3.45 - 0.7 - (15 x 0.055) = 1.925V.

MAX6972

MAX6973

R1

Q1

55mA

+3.3V

+3.3V

+24V

Y1

Y2

Y3

Y4

Y5

Y6

Y7

Y0

GND

Figure 13. External Cascode npn Transistor

MAX6972

MAX6972

MAX6972

SYSTEM

CLK

DATA

LOAD

CLKO

DINO

LOADO

CLKI

DINI

LOADI

Y0/Z0

16 RG LEDs

16 BLUE LEDs

Y1/Z1

Y2/Z2

Y3/Z3

Y4/Z4

Y5/B5

Y6/Z6

Y7/Z7

Y0/Z0

Y1/Z1

Y2/Z2

Y3/Z3

Y4/Z4

Y5/B5

Y6/Z6

Y7/Z7

Y0/Z0

Y1/Z1

Y2/Z2

Y3/Z3

Y4/Z4

Y5/B5

Y6/Z6

Y7/Z7

CLKO

DINO

LOADO

CLKI

DINI

LOADI

CLKO

DINO

LOADO

CLKI

DINI

LOADI

Typical Operating Circuit

Chip Information

PROCESS: BiCMOS