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Revision 1.21

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AS1109

Datasheet - Application In form atio n

9.2 Cascading Devices

To cascade multiple AS1109 devices, pin SDO must be connected to pin SDI of the next AS1109 (see Figure 23). At each rising edge of CLK the

LSB of the shift register will be written into the shift register SDI of the next AS1109 in the chain. Data at the SDI pin is clocked in at the rising

edge of the CLK pulse and is clocked out at the SDO pin 8.5 clock cycles later at the falling edge of the CLK pulse.

Note:

When n*AS1109 devices are in one chain, n*8 clock pulses are needed to latch-in the input data.

Figure 23. Cascading AS1109 Devices

9.3 Constant Current

In LED display applications, the AS1109 provides virtually no current variations from channel-to-channel and from AS1109-to-AS1109. This is

mostly due to 2 factors:



While IOUT

≥ 50mA, the maximum current skew is less than ±2% between channels and less than ±2% between AS1109 devices.



In the saturation region, the characteristics curve of the output stage is flat (see Figure 5 on page 6). Thus, the output current can be kept

constant regardless of the variations of LED forward voltages (VF).

9.4 Adjusting Output Current

The AS1109 scales up the reference current (IREF) set by external resistor (REXT) to sink a current (IOUT) at each output port. As shown in

Figure 3 on page 6 the output current in the saturation region is extremely flat so that it is possible to define it as target current (IOUT TARGET).

IOUT TARGET can be calculated by:

VREXT = 1.253V

(EQ 1)

IREF = VREXT/REXT (if the other end of REXT is connected to ground)

(EQ 2)

IOUT TARGET = IREF*15 = (1.253V/REXT)*15

(EQ 3)

Where:

REXT is the resistance of the external resistor connected to pin REXT.

VREXT is the voltage on pin REXT.

The magnitude of current (as a function of REXT) is around 100mA at 186

Ω, 50.52mA at 372Ω and 25.26mA at 744Ω. Figure 3 on page 6

shows the relationship curve between the IOUT TARGET of each channel and the corresponding external resistor (REXT).

AS1109 #n-1

SDI

SDO

CLK

LD

OEN

SDI

CLK

LD

OEN

AS1109 #1

SDI

SDO

CLK

LD

OEN

AS1109 #2

SDI

SDO

CLK

LD

OEN