CONSTANT-CURRENT DRIVERS OUTPUT CURRENT mA/BIT CURRENT-CONTR

The Datasheet Archive - 100 Million Datasheets from 7500 Manufacturers.

CONSTANT-CURRENT DRIVERS OUTPUT CURRENT mA/BIT CURRENT-CONTR

Datasheet Thumbnail

Top Searches for this datasheet

Introduction LEDs current-driven devices that require current limiting when driven from voltage source. most applications, desirable drive LEDs with constant-current source. current source used regulate current through regardless power supply (voltage) variations changes forward voltage drops (VF) between LEDs. Allegro MicroSystems' A6275, A6276, A6277 optimized display-driving applications, lending cost effectiveness compactness solution. family features both 8-bit 16bit versions available standard plastic SOIC packages. Constant Current Allegro's A6275/76/77 offer designer ability configure displays with virtually variation brightness across display variation current between outputs). With typical output-tooutput current variation only ±1.5% (between outputs single device), noticeable variation intensity eliminated. devices also allow user magnitude constant current LEDs. Once set, current will remain constant regardless voltage variation, supply voltage variation, other circuit parameters that could otherwise affect current. output current controlled current mirror (15.25:1), bandgap regulator (1.23 external current-control resistor (REXT) shown figure A6275 A6276 provide maximum output, A6277 provide maximum 150mA output, making them suitable various large display applications such scoreboards gaming equipment.
CONSTANT-CURRENT DRIVERS
OUTPUT CURRENT mA/BIT
CURRENT-CONTROL RESISTANCE, REXT OHMS
Dwg. GP-061-1
Figure Output current REXT
output current amperes output) also calculated from equation: 15.25 1.23/REXT 18.76/REXT. 8-Bit 16-Bit Versions Allegro's family drivers includes A6275/77 (8-bit) A6276 (16-bit), which offer designer flexibility select device best suited particular application. example, 8bit versions will find their into indicators bar-graph displays while 16-bit version will employed systems using 7-segment display elements. devices cascaded additional drive lines when applications require more than bits.
Constant-Current Drivers Applications Information
Serial Input further reduce device's terminal-count board-space requirements, serial input utilized allowing data entry with only three terminals direct interfacing with microprocessor-based systems: SERIAL DATA CLOCK, LATCH ENABLE. These inputs drive CMOS shift register latches. With logic supply, data rates 20MHz possible, allowing wide range microprocessor products perform data-input function. device's SERIAL DATA enables designer cascade devices applications requiring more than bits. With multiple devices, REXT trimmed provide current matching between devices. applications requiring interdigit blanking, output drivers disabled with OUTPUT ENABLE input high (internal pullup). OUTPUT ENABLE input also used operate device duty cycle below 100%, allowing LEDs operated high peak currents, power dissipation reduction desirable features some applications. Under-Voltage Lockout feature traditionally found display drivers, internal under-voltage lockout (UVLO) disables driver's outputs event logic supply voltage dropping below minimum acceptable level, preventing display erroneous information, necessary function some critical applications.
Thermal Considerations maximum allowable package power dissipation (PD(max)) determined package's thermal resistance (RJA), operating ambient temperature (TA), allowable junction temperature (TJ). relationship between these parameters PD(max) TA)RJA where 55°C/W (A6277xA), 60°C/W (A6275xA A6276xA), 70°C/W (A6277xLW), 75°C/W (A6276xLW), 80°C/W (A6275xLW). maximum operating ambient temperature must also include factors such heating from other components, circulation, etc. strict rules exist regarding maximum junction temperature except that absolute maximum allowable 150°C. Typically should design maximum continuous junction temperature 100°C 130°C taking into consideration that every 10°C rise junction temperature approximately halves expected life device every 10°C decrease junction temperature doubles expected life device. actual package power dissipation (PD(act)) output drivers' power dissipation logic power dissipation. relationship between these parameters PD(act) IDD) where duty cycle, difference between supply voltage (VLED) forward voltage (VF), number device outputs 16). When calculating power dissipation, total number available device outputs usually used worst-case situation (i.e., displaying 8s).
Northeast Cutoff, 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 Copyright 2001 Allegro MicroSystems, Inc.
Constant-Current Drivers Applications Information
circuit design, equations combined [dc([VLED IDD)] RJA+ simplified RJA) Note that quantities contributing PD(act) (dc, VDD, IDD) EXCEPT VCE, generally defined total system requirements. Thermal Design Example design example, A6276xA (24-pin plastic package) 16-bit driver operated ambient temperature 70°C. design uses green LEDs, which have forward voltage (VF) 2.0V when operated 15mA 1250 5.0V supply provides power both A6276 LEDs. From data sheet, that 50°C/W, junction temperature under these conditions calculated (from equations [1([5 0.015 0.020)] [(0.72 0.1)W 50°C/W] +70°C 111°C With junction temperature 111°C, device well within operating region. Conversely, same equations used calculate maximum allowable current under given conditions. minimize voltage drop across driver output thus reduce device power dissipation, desirable external voltage dropping (see figure Selection this value depends particular application value current selected. cases where combination supply voltage (VLED) voltage drop (VF) results voltage across output driver (VCE), external voltage dropping might required. A6275/ 76/77 drivers most effective when operated with
VLED
DROP
Dwg. EP-064
Figure External Voltage Dropping Reduce Package Power Dissipation
0.4V 0.7V. available voltage source will cause unacceptable dissipation series resistors diode(s) undesirable, regulator such Sanken Series Series used provide supply voltages some applications desirable required operate LEDs duty cycle below 100% with higher peak current. Duty-cycle control achieved OUTPUT ENABLE terminal. Outputs enabled when this input pulled low. Two-Digit Application Figure shows two-digit application using 16-bit A6276 7-segment (plus decimal point) LEDs. such application, serial data SERIAL DATA terminal, along with CLOCK LATCH ENABLE signals. Additional digits driven cascading (SERIAL DATA connected SERIAL DATA next) drivers with CLOCK inputs tied together LATCH ENABLE inputs tied together.
www.allegromicro.com
Constant-Current Drivers Applications Information
7-SEGMENT DISPLAY WITH DECIMAL POINT, COMMON-ANODE
7-SEGMENT DISPLAY WITH DECIMAL POINT, COMMON-ANODE
DIGIT DRIVER UCN5891 DRIVERS SHOWN)
7-SEGMENT DISPLAY WITH DECIMAL POINT, COMMON-ANODE
7-SEGMENT DISPLAY WITH DECIMAL POINT, COMMON-ANODE
SEGMENT DRIVER A6276 DRIVERS SHOWN)
OTHER DIGITS
Dwg. EP-065-2
Figure Typical Two-Digit Application
SEGMENT DRIVER A6275 DRIVERS SHOWN)
Dwg. EP-065-1
Multiplexing Typical Application Multiplexing popular solution driving many digits. With segments digits parallel, only desired digit enabled (turned time. array digits then scanned sequentially enable each individual digit. Multiplexed displays must typically operated current times nominal operating current (where number digits), obtain sufficient brightness. Figure shows typical eight-digit, 7-segment application employing A6275 with output sink drivers controlling segments source-driving UCN5891A/LW digit control. Information display drivers provided microprocessor microcontroller that provides SERIAL DATA CLOCK, LATCH ENABLE signals. such configuration, necessary disable display while source driver switches from digit next. This technique called "interdigit blanking" necessary prevent partial illumination (ghosting) segments intended off. This phenomena results from source driver requiring
Figure Typical Eight-Digit Multiplexed Application
more time turn than sink driver takes turn Blanking will delay sink driver turn will allow source driver turn completely. This performed with OUTPUT ENABLE function. More than eight digits, more than segments, will require additional source sink drivers cascaded configuration. REXT Selection A6275/76/77 constant-current-output devices. output-current-controlling resistor (REXT) value determined from either figure equation should noted that capacitance between REXT terminal logic supply terminal should minimized during design prevent unintended feedback possible oscillation.
Northeast Cutoff, 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000
Constant-Current Drivers Applications Information
Voltage Control Output Current some applications, desirable control output current with variable-voltage source. Alternatively, voltage source used drive several A6275/76/77 drivers same output currents. either configuration, microcontroller provide digital information digital-to-analog (D/A) converter, which will provide analog voltage A6275/76/77 driver series with REXT (figure With multiple devices, REXT trimmed provide current matching between devices. determined equation (1), maximum output current series resistor REXT. output current then decreases control voltage (Vcontrol) approaches internal reference voltage 1.23V. 15.25(1.23 Vcontrol)/REXT that Vcontrol determined REXT, Vcontrol 1.23 zero. Special care must taken minimize capacitance from REXT terminal. addition above considerations, necessary that converter capable sinking maximum mirrored load current each driver: IEXT IO/15.25 REXT less than required (>75 driver output), then small inductor series with resistor usually advisable.
REXT CONVERTER
DRIVER(S) A6275/76/77
REGULATOR
FROM
Dwg. EP-073
Figure Voltage Control Output Current
Pattern Layout A6275 A6276 have common logicground power-ground terminal; A6277 separate logic-ground power-ground terminals (REXT must returned logic ground). ground-pattern layout contains large common-mode resistance, voltage between system ground LATCH ENABLE CLOCK terminals exceeds (because switching noise), these devices operate correctly.
www.allegromicro.com
Constant-Current Drivers Applications Information
DISPLAY DRIVERS
ORDER OUTPUT CURRENT, NUMBER DRIVERS
Application Output Number Current (mA)* Drivers -120 -120 -120 -350 -350 -350 -350 1250 1500 1800 Common Cathode Digit Drive Segment Drive Common Anode Digit Drive Segment Drive Part Number 6275 6276 2595 6B259 6B273 6B595 5832 5833 2585 2985 5895 6277 6259 6273 6595 2596 2003 2004 2580 2588 2987 2981 2982 5891 5821 2803 2597 2068 2065 2540
Current maximum test condition. Negative current defined coming (sourcing) output. Complete part number includes additional characters indicate operating temperature range package style. Serial input, latched parallel outputs. User-selected, constant-current outputs. multiplexed displays, segment current should times steady-state current, where number digits. additional devices drivers, complete Allegro sink drivers source drivers listings.
Northeast Cutoff, 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000

CONSTANT-CURRENT DRIVERS OUTPUT CURRENT mA/BIT CURRENT-CONTR

Top Searches for this datasheet

Other recent searches