33879 datasheet(15/23 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33879

FUNCTIONAL DESCRIPTION

MCU INTERFACE DESCRIPTION

source is connected to ground. All outputs may be configured

individually as desired. When high side drive is used, the

33879 will limit the negative inductive transient to negative

20 V.

EXPOSED PAD PIN

Device will perform as specified with the Exposed Pad un-

terminated (floating) however, it is recommended that the

Exposed Pad be terminated to pin 1 (GND) and system

ground.

MCU INTERFACE DESCRIPTION

INTRODUCTION

The 33879 is an 8-output hardware-configurable power

switch with 16-bit serial control. A simplified internal block

diagram of the 33879 is shown in Figure 2 on page 3.

The 33879 device uses high-efficiency up-drain power

DMOS output transistors exhibiting low drain-to-source ON

resistance (RDS(ON) = 0.75 Ω at 25°C typical) and dense

CMOS control logic. All outputs have independent voltage

clamps to provide fast inductive turn-off and transient

protection.

In operation, the 33879 functions as an 8-output serial

switch serving as a MCU bus expander and buffer with fault

management and fault reporting features. In doing so, the

device directly relieves the MCU of the fault management

functions. This device directly interfaces to an MCU using a

SPI for control and diagnostic readout. Figure 15 illustrates

the basic SPI configuration between an MCU and one 33879.

Figure 15. SPI Interface with Microcontroller

All inputs are compatible with 5.0 V and 3.3 V CMOS logic

levels and incorporate positive logic. When a SPI bit is

programmed to a logic [0], the corresponding output will be

OFF. Conversely, when a SPI bit is programmed to logic [1]

the output being controlled will be ON. Diagnostics are

treated in a similar manner. Outputs with a fault will feed back

(via DO) a logic [1] to the microcontroller, while normal

operating outputs will provide a logic [0].

Figure 16 illustrates the daisy chain configuration using

the 33879. Data from the MCU is clocked daisy chain through

each device while the CS bit is commanded low by the MCU.

During each clock cycle, output status from the daisy chain is

transferred to the MCU via the Master In Slave Out (MISO)

line. On rising edge of CS, command data stored in the input

Figure 16. 33879 SPI System Daisy Chain

Multiple 33879 devices can be controlled in a parallel input

fashion using the SPI. Figure 17 illustrates the control of

24 loads using three dedicated parallel MCU ports for chip

select.

Figure 17. Parallel Input SPI Control

Receive

Buffer

Parallel

Ports

Logic

33879

MC68HCxx

Microcontroller

SCLK

MISO

MOSI

Shift Register

16 Bits

MC68HCxx

Microcontroller

with

SPI Interface

8 Outputs

MISO

MOSI

Parallel Port

SCLK

33879

SCLK

Parallel

Ports

MOSI

MISO

SCLK

8 Outputs

MC68HCxx

Microcontroller

with

SPI Interface

33879

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