OTL-888W FUNCTIONAL DESCRIPTION

The OTL-888W is an IC capable of driving 64 LEDs having a common cathode from a serial digital interface. Both the amplitude of each LED drive current and the on time for each LED can be controlled independently allowing easy merging of LED uniformity compensation and grey scale. The size of IC chip is based on use in a 600 DPI LED printhead which has a central LED line with driver ICs on each side of the LED line.

An independent 1/32 decoder is incorporated to facilitate use of the IC in other applications such as XY-addressable LED arrays where a linescan is being performed.

The IC organization is depicted in Fig. 1. 64 groups of 8 bits each are used for individual amplitude control of each LED current, and 64 groups of 8 bits each are used to control the on time of each LED.

The 8 x 64 bits used for amplitude modulation (AM) and the 8 x 64 bits for pulse width modulation (PWM) are marched into separate 8 x 64 shift registers and latched. The 64 groups of 8 bits each used for AM are fed to 64 D/A converters producing LED currents proportional to the digital inputs. The 64 D/A converters have a common reference voltage or control voltage (VC) allowing common amplitude control of the 64 current sources.

Maximum LED current Imax is 25.5 mA with control voltage Vc = 2.55 volt and amplitude data DA = 255 for REXT = 800.

A higher REXT value may be used to reduce IMAX if desired. An internal resistor RINT = 800 ± 5% is available for applications not requiring the accuracy achievable with a precision external resistor.

Since LED power output does not increase linearly with increase in LED current and since individual LED characteristics may be different, LED uniformity compensation should be done by searching for the desired LED output power. In other words, the 8 bit number in the Amplitude Data register is increased until the desired output power from each LED is the same by measurement. When a search as described is used for equalizing the power output for each LED, the absolute LED value of the maximum current IMAX need not be an accurate absolute value so that the internal resistor RINT can be used without reduced uniformity of LED power output. The 8 bits available for setting the current for each LED gives a resolution of 1/255 (about 0.4% of full scale).

The 8x 64 bits used for (PWM), which are stored in a latch register, are compared against an externally provided 8 bit binary count which serves as a common time base (TB) and runs during the maximum allowed LED on time. The 8 bit timebase is compared against the 8 bit ontime data latched for each LED (DT) and a LED is turned on whenever the timebase is less than the latched on time data (TB < DT).

The 8 bits used for PWM provides individual on time control for each LED and can be used to improve uniformity of energy delivered by each LED or for superimposed grey scale. The increase of LED energy output varies linearly with the LED on time.

OTL-888W CHIP SPECIFICATION

The initial application for the IC is driving a line of LEDs having a 600 DPI resolution. The LED line is made up of 128 element chips each driven by two 64 element drivers, one on each side of the line with LED connections brought out on an alternating basis on a 3.33 mil pitch.

Bonding pads are arranged in 2 staggered rows each having a 6 mil pitch on both sides of the LED line. The pitch reduction from 6.66 to 6.0 mil is needed to arrange the driver ICs in a single row on each side of the LED line. This situation dictates a maximum IC width of 205 mils.

The chip size and bonding pad locations are depicted in Fig. 2.

OTL-888W I/O DESIGNATIONS

Symbol No. of Pads Description
Cd1 - Cd64 64 0 to 25.5 mA constant current source
DAL 8 Amplitude data left
DAR 8 Amplitude data right
CA 1 Amplitude clock
LA 1 Amplitude latch
S 1 Shift direction
DTL 8 On time data left
DTR 8 On time data right
CT 1 On time data clock
LT 1 On time data latch
TB 8 Time base
VC 1 Control voltage
VLED 8 LED supply voltage
Vdd 3 Logic supply voltage
Vss 4 Logic ground
VBB 1 Battery back up
RINT 1 Internal resistor
REXT 1 External resistor
d0-31 32 Decoder outputs
Cd 1 Decoder clock
Dd 1 Decoder data
Ld 1 Decoder latch
Ed 1 Decoder enable
TE 1 Battery fault
Total 165 Bonding pads

OTL-888W CHIP SPECIFICATIONS

All inputs are single ended rather than differential because of the large number of bonding pads.

Logic Supply Vdd = 5V ± 0.5
LED Supply VLED = 4V to 6V
LED Current 25.5 mA max. for each driver

16 mA max. average for 64 drivers

Leakage Current Driver of state leakage is less than 1 µA after switching and less than 0.25 µA 300 ns after switching.
Clock Rate 30 MHz max.
Temperature Range 0 to 85C operating

-55C to +125C storage

OTL-888W INTERFACE TIMING