Preliminary

WS2803

18-Channel Constant Current LED Driver

With Programmable PWM Outputs

5/8

VREXT=1.25V. To set the output current at 20mA, a resistor with 1.25Kohm resistance should be connected from REXT to

GND. The relationship between IOUT and REXT is shown in the following figure.

Package Power Dissipation (PD)

The maximum allowable package power dissipation is determined as PD(max)=(Tj-Ta)/Rth(j-a), when all 18 output

channels are turned on simultaneously, the actual package power dissipation is:

)

×

×

×

(

+

)

×

(

=

)

(

18

DS

OUT

DD

DD

D

V

Duty

I

V

I

act

P

Therefore, to keep PD(act)≤PD(MAX), the maximum output current as a function of duty cycle is:

18

/

/

/

)

×

(

=

)

(

Duty

V

V

I

R

T

T

I

DS

CC

CC

a

j

th

a

j

OUT

The maximum power dissipation PD(MAX)=(Tj-Ta)/Rth(j-a), decreases as the ambient temperature increases.

Load Supply Voltage (VLED)

WS2803 is designed to operate VDS ranging from 0.4 to 1.0V, considering the package power dissipating limits. VDS

maybe high enough to ensure PD(ACT)>PD(MAX), when VLED=24V and VDS=VLED-VF, in which VLED is the load

supply voltage. In case it is recommended to use the lowest possible supply voltage or to set an external voltage reducer,

VDROP, to dissipate the excessive power.

A voltage reducer lets VDS=(VLED-VF)-VDROP.

Resistors or zener diode can be used in the applications as shown in the following figures.

Fig 3 power reducer of WS2803

2-wire Control Mode

When MS pin is connected to VCC, WS2803 operates in 2-wire control mode, at this mode, only 2 wires are needed to

transmit the grayscale data. It is a low cost solution. Only 2 ports of MCU are needed to control over thousand WS2803

chips.At this mode, CKI pin keeps LOW more than 600us, the internal logic control circuitry will be reset. Once reset, the

internal circuitry wait for the positive pulse on the CKI pin, at this state, the signal on SDI pin is shifted in to the internal data

registers, once total 144 pulses are detected, WS2803 will latch the signal from the shift registers, and the WS2803

operates as a signal relay, at this state, the CKO and SDO are enabled. If the CKI keeps LOW for more than 600us, all the

internal data control circuit will back to the original states, wait for next data transmi

WS-Preliminary-V0.1

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