mc33291l Freescale Semiconductor, Inc, mc33291l Datasheet - Page 18

• Manufacturer Part Number: mc33291l
• Description: Eight-output Switch With Serial Peripheral Interface I/o
• Manufacturer: Freescale Semiconductor, Inc

output voltage is less than the Output Threshold Voltage
(V
as switches, during normal operation, each MOSFET output
should either be completely turned ON or OFF. By design,
the threshold voltage was selected to be between the ON and
OFF voltage of the MOSFET. During normal operation, the
ON state V
threshold voltage and the OFF state V
than the threshold voltage. This design approach affords
using the same threshold comparator for Output Open Load
Detect in the OFF state and Short Circuit Detect in the ON
state. (See
Circuit Detect circuit.) With V
voltage of less than 3.0 V will be detected as an Output OFF
Open Load Fault while voltages greater than 4.0 V will not be
detected as a fault.
50 µA, illustrated in
MOSFET drain and ground. This current source prevents the
output from floating up to V
internal wire bond failure. The internal comparator compares
the drain voltage with a reference voltage, V
V
the 33291L will declare the condition to be an open load fault.
a false output OFF Open Load Fault may be triggered. To
prevent this false fault from being reported, an internal fault
filter in the range of 25 µs to 100 µs is incorporated. The
duration in which a false fault may be reported is a function of
the load impedance (R
MOSFET as well as the supply voltage (V
edge of CS triggers a built-in fault delay timer which must
time out (25 µs or 100 µs) before the fault comparator is
enabled to detect at faulted threshold. The circuit
automatically returns to normal operation once the condition
causing the Open Load Fault is removed.
18
33291L
FUNCTIONAL DESCRIPTION
DD
Thres
An Output OFF Open Load Fault is indicates when the
The 33291L has an internal pull-down current source of
During output switching, especially with capacitive loads,
). If the output voltage is less than this reference voltage,
) of 0.6 to 0.8 x V
Figure 18. Output OFF Open Load Fault
Low = Fault
DS
Figure 19
voltage of the MOSFET is less than the
+
–
2.5 V to 3.5 V
V
Thres
Figure
for an understanding of the Short
DD
L
,C
33291
. Since the 33291L outputs function
L
MOSFET OFF
18, page
,L
PWR
DD
L
), R
= 5.0 V, an OFF state output
if there is an open load or
DS(ON)
18,
DS
50
, and C
between the
µ
voltage is greater
PWR
A
Thres
). The rising
OUT
R
(0.6 to 0.8 x
L
Output
V
PWR
of the
SHORTED LOAD FAULT
output being shorted directly to supply, or an output
experiencing a current greater than the current limit.
during load short conditions providing system protection.
They are as follows:
output, affording robust independent output operation.
monitors the output drain current. An overcurrent condition
causes the gate control circuitry to reduce the gate-to-source
voltage imposed on the output MOSFET, re-establishing the
load current in compliance with current limit (1.0 A to 3.0 A)
range. The time required for the current limit circuitry to act is
less than 20 µs. Therefore, currents higher than 1.0 A to
3.0 A will never be seen for more than 20 µs (a typical
duration is 10 µs). If the current of an output attempts to
exceed the predetermined limit of 1.0 A to 3.0 A (2.0 A
nominal), the V
the overcurrent comparator will be tripped as shown in
Figure
shut down immediately or continue to operate in an analog
current limited mode until either the short circuit (overcurrent)
condition is removed or thermal shutdown is reached.
1. The output current of the device is monitored in an
2. The output current of the device is sensed by
3. The output thermal limit of the device is sensed and
A short load, or overcurrent fault can be caused by any
There are three safety circuits progressively in operation
All three protection mechanisms are incorporated in their
The analog current limit circuit is always active and
The status of SFPD determines whether the 33291L will
analog fashion using a SENSEFET approach and
current limited.
monitoring the MOSFET drain voltage.
when attained causes only the specific faulted output
to be latched OFF, allowing all remaining outputs to
operate normally.
19.
Analog
Figure 19. Short Circuit Detect and
Analog Current Limiting Circuit
+
DS
–
Digital
High = Fault
voltage will exceed the V
V ref
33291
Analog Integrated Circuit Device Data
2.5 to 3.5 V
+
–
V
Thres
Freescale Semiconductor
MOSFET ON
Thres
voltage and
R
Output
L
V
PWR