AN2846 datasheet(5/35 Pages) STMICROELECTRONICS

5 / 35 page

AN2846

Application guidelines

Doc ID 15130 Rev 1

5/35

The SCLT3-8 has been designed to run with SPI protocol CPHA = 0 and CPOL = 0. The frame

format is 16 bits or 8 bits long according to SPM pin level. When SPM is grounded, 16 bits

are transmitted - 8 input data bits and 8 control bits. When SPM is connected to VDD only

the 8 input data bits are transmitted.

Table 1 defines the significance of the16 bits. Bit 15 is the most significant bit. Detailed SPI

functionality is described in Section 1.4.

1.2

Current-limited inputs

1.2.1

Maximum input current setting

All internal bias currents sources and particularly the input current limiter are defined by the

reference resistor connected to pin REF. A 15 k

Ω resistor will assure a typical input limited

current of 2.35 mA (see Figure 2). The typical limited input current ILIM is given by the

formula:

Figure 2.

Current limiter diagram

The technology used allows a very low current dispersion according to the different

channels (less than 10%). The reference voltage VBG is also compensated over the junction

temperature range from -25 °C to 150 °C enabling a good stability of the limited current (see

Figure 3).

Table 1.

16-bit frame definition

Data bits

Bit15

Bit14

Bit13

Bit12

Bit11

Bit10

Bit09

Bit08

IN8

IN7

IN6

IN5

IN4

IN3

IN2

IN1

Control Bits

Bit07

Bit06

Bit05

Bit04

Bit03

Bit02

Bit01

Bit00

/UVA

/OTA

PC1

PC2

PC3

PC4

()

REF

LIM

cal

with typi

BG =

LED_ON

1 : 20

REF =15k

REF

1 : 1

1.5k

LED

LIM

COMs

BG=1.25V

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