33879 Datasheet, PDF (16/24 Pages) Freescale Semiconductor, Inc – Configurable Octal Serial Switch with Open Load Detect Current Disable

FUNCTIONAL DESCRIPTION
MCU INTERFACE DESCRIPTION
S1– S8 PINS
The S1 to S8 pins are the source outputs of the 33879. For
high-side drive configurations, the source pins are connected
directly to the load. In low-side drive configurations, the
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 (R_DS(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.
MC68HCxx
Microcontroller
33879
MOSI
DI
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
register is then transferred to the output driver.
SCLK
Parallel Port
33879
MC68HCxx
CS
MISO
Microcontroller
with
DO DI
SPI Interface
33879
CS SCLK
DO
DI
33879
CS SCLK
DO
DI
Shift Register
16 Bits
MISO
Shift Register
DO
16 Bits
Receive
Buffer
Parallel
Ports
SCLK
CS
To
Logic
Figure 15. SPI Interface with Microcontroller
8 Outputs
MOSI
8 Outputs
8 Outputs
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.
33879
16
Analog Integrated Circuit Device Data
Freescale Semiconductor