
Analog Integrated Circuit Device Data
Freescale Semiconductor
14
33291L
FUNCTIONAL DESCRIPTION
FUNCTIONAL PIN DESCRIPTION
SERIAL OUTPUT (SO)
The serial output (SO) pin is the tri-stateable output from
the Shift register. The SO pin remains in a high impedance
state until the
CS
pin goes to a logic low state. The SO data
reports the drain status, either high or low relative to the
previous command word. The SO pin changes state on the
rising edge of SCLK and reads out on the falling edge of
SCLK. When an output is OFF and not faulted, the
corresponding SO data-bit is a high state. When an output is
ON, and there is no fault, the corresponding data-bit on the
SO pin will be a low logic state. The SI/SO shifting of data
follows a first-in-first-out (FIFO) protocol with both input and
output words transferring the MSB first. Referring to
Figure 17
, the DO
bit is the MSB corresponding to Output 7
relative to the previous command word. The SO pin is not
affected by the status of the
RST
pin.
RESET (RST)
The 33291L Reset (
RST
) pin is active low. It is used to
clear the SPI Shift register. In doing so, all output switches
are set at OFF. With the device in a system with an MCU,
upon initial system power-up, the MCU holds the
RST
pin of
the device in a logic low state, ensuring all outputs to be OFF
until both the VDD and VPWR pin voltages are adequate for
predictable operation. After the 33291L is reset, the MCU is
ready to assert system control with all output switches initially
OFF.
If the VPWR pin of the 33291L experiences a low voltage,
following normal operation, the MCU should pull the
RST
pin
low to shut down the outputs and clear the input data register.
The
RST
pin is active low and has an internal pull-down
incorporated to ensure operational predictability should the
external pull-down of the MCU open circuit. The internal pull-
down is only 25
μ
A, affording safe and easy interfacing to the
MCU. The
RST
pin of the 33291L should be pulled to a logic
low state for a duration of at least 250 ns to ensure reliable a
reset.
A simple power ON reset delay of the system can be
programmed through the use of an RC network comprised of
a shunt capacitor from the
RST
pin to Ground and a resistor
to V
DD
, illustrated in
Figure 16
. Care should be exercised
ensuring proper discharge of the capacitor. Careful attention
eliminates adverse delay of the
RST
and damage of the MCU
if it pulls the Reset line low, thereby accomplishing
initialization for turn ON delay. It may be easier to incorporate
delay into the software program and use a parallel port pin of
the MCU to control the 33291L
RST
pin.
Figure 16. Power ON Reset
SHORT FAULT PROTECT DISABLE (SFPD)
The Short Fault Protect Disable (SFPD) pin is used to
prevent the outputs from latching-off due to an overcurrent
condition. This feature provides control of incandescent lamp
loads where in-rush currents exceed the device’s analog
current limits. Essentially the SFPD pin determines whether
the 33291L output(s) will instantly shut down upon sensing an
output short or remain ON in a current limiting mode of
operation until the output short is removed or thermal
shutdown is reached. If the SFPD pin is tied to V
DD
= 5.0 V
the 33291L output(s) will remain ON in a current limited mode
of operation upon encountering a load short to supply or
overcurrent condition. When the SFPD pin is grounded, a
short circuit will immediately shut down only the output
affected. Other outputs not having a fault condition will
operate normally. The short circuit operation is addressed in
more detail later.
+
V
DD
RDLY
C
DLY
Reset
20
μ
A
Reset
MCU
33291