WS2801DI データシートの表示（PDF） - Unspecified

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WS2801DI

3-Channel Constant Current LED Driver With Programmable PWM Outputs

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Preliminary WS2801

3-Channel Constant Current

LED Driver With Programmable

PWM Output

This mode can be used to drive the LED strings under high input voltage (VIN>6V), the highest application voltage is determined

by the external NPN power switch. The current flows through the collector of the NPN transistor:

β

I = I β＋1 NPNC

XOUT

--------------------------------------------------------------- (4)

The current flow through the base is:

I = I β1＋1 NPNB

XOUT

--------------------------------------------------------------- (5)

During normal operation, to avoid too much power dissipating on WS2801, the voltage at R/G/BOUT should be set around 1.2

volts; the value of RB can be derived by the following equation:

RB

= V DD−VXOUT

I NPNB

---------------------------------------------------------------------- (6)

Gray Scale Data Word Format and Timing

CLK pin keeps low more than 500uS will make the WS2801 internal status register reset, and at this moment, the gray scale data

in the data shift registers will be latched. Now WS2801 is ready for gray scale data transport. When the WS2801 receives total 24

clock rising edges, the WS2801 enters relay mode, which relays the input signal got from CKI and SDI with very low distortion

(100pS typical) and very low delay time ( 8 ns typical). The data format and the timing diagram can be seen in Fig 11.

The gray scale data frame is comprised of 3 gray data words, each data word is composed of 8 gray scale data bits, total 24 bits.

The 8bit gray data can easily realize the linear 256 LED gray levels control.

During data transfer, the first bit is the data frame signal, then is the MSB of the ROUT channel, the last one transferred into the

data shift register is the LSB of the BOUT channel.

The duty cycle of the LED driving current can be calculated as:

D XOUT

=

L[7]* 27

+

L[6]* 26

+ L[5]* 25

+ L[4]* 24 +

256

L[3]* 23

+ L[2]* 22

+

L[1]* 21

+ L[0] ----- (7)

Where L[x]=”0” or “1”, which is R、G、B[x], x=0,1,2……7 in the gray scale data. Gray scale data

0000 0000 is for the minimum 0 duty ratio, and 1111 1111 is for the maximum 255/256 duty ratio.

WS-Preliminay-V0.3

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