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6277

8-BIT SERIAL-INPUT, CONSTANT CURRENT LATCHED LED DRIVER

Allegro MicroSystems 

6277

8-BIT SERIAL-INPUT,

CONSTANT-CURRENT

LATCHED LED DRIVER

Applications Information

The load current per bit (IO) is set by the external resistor

(REXT) as shown in the figure below.

140

VCE = 0.7 V

120

100

80

60

40

20

0

100

200 300

500 700 1 k

2k 3k

5k

CURRENT-CONTROL RESISTANCE, REXT IN OHMS

Dwg. GP-061-1

0.7 V per diode) for a group of drivers. If the available

voltage source will cause unacceptable dissipation and

series resistors or diode(s) are undesirable, a regulator

such as the Sanken Series SAI or Series SI can be used to

provide supply voltages as low as 3.3 V.

For reference, typical LED forward voltages are:

Blue

3.0 – 4.0 V

Green

1.8 – 2.2 V

Yellow

2.0 – 2.1 V

Amber

1.9 – 2.65 V

Red

1.6 – 2.25 V

Infrared

1.2 – 1.5 V

Pattern Layout. This device has separate logic-ground

and power-ground terminals. If ground pattern layout

contains large common-mode resistance, and the voltage

between the system ground and the LATCH ENABLE or

CLOCK terminals exceeds 2.5 V (because of switching

noise), these devices may not operate correctly.

Package Power Dissipation (PD). The maximum allow-

able package power dissipation is determined as

PD(max) = (150 - TA)/RθJA.

The actual package power dissipation is

PD(act) = dc(VCE • IO • 8) + (VDD • IDD).

When the load supply voltage is greater than 3 V to 5 V,

considering the package power dissipating limits of these

devices, or if PD(act) > PD(max), an external voltage

reducer (VDROP) should be used.

Load Supply Voltage (VLED). These devices are de-

signed to operate with driver voltage drops (VCE) of 0.4 V

to 0.7 V with LED forward voltages (VF) of 1.2 V to

4.0 V. If higher voltages are dropped across the driver,

package power dissipation will be increased significantly.

To minimize package power dissipation, it is recom-

mended to use the lowest possible load supply voltage or

to set any series dropping voltage (VDROP) as

VDROP = VLED - VF - VCE

with VDROP = Io • RDROP for a single driver, or a Zener

diode (VZ), or a series string of diodes (approximately

VLED

V DROP

VF

V CE

Dwg. EP-064

115 Northeast Cutoff, Box 15036

10

Worcester, Massachusetts 01615-0036 (508) 853-5000

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