FUNCTIONAL DESCRIPTION
DEVICE OPERATION
declare the load open in the OFF state when the output drain-
output. Each clamp independently limits the drain-to-source
voltage to 45 V for low side drive configurations and -20 V for
high side drive configurations. The total energy clamped (EJ)
can be calculated by multiplying the current area under the
current curve (IA) times the clamp voltage (VCL) (see
Figure 18).
to-source voltage is less than VOUT(FLT-TH)
.
This device has an internal 80 μA current source
connected from drain to source of the output MOSFET. The
current source may be programmed on or off via SPI. The
Power-ON Reset state for the current source is “off” and must
be enabled via SPI. To achieve low Sleep mode quiescent
currents, the Open Load Detection Current source of each
driver is switched off when VDD or EN is removed.
Characterization of the output clamps, using a single pulse
non-repetitive method at 0.35 A, indicates the maximum
energy per output to be 50 mJ at 150°C junction temperature.
During output switching, especially with capacitive loads,
a false output OFF open load fault may be triggered. To
prevent this false fault from being reported, an internal fault
filter of 100 μs to 300 μs is incorporated. A false fault
reporting is a function of the load impedance, RDS(ON), COUT
of the MOSFET, as well as the supply voltage, VPWR. The
rising edge of CS triggers the built-in fault delay timer. The
timer will time out before the fault comparator is enabled and
the fault is detected. Once the condition causing the open
load fault is removed, the device will resume normal
operation. The open load fault, however, will be latched in the
output DO register for the MCU to read.
Drain-to-Source Clamp
Voltage (VCL = 45 V)
Drain Voltage
Clamp Energy
Drain Current
(ID = 0.3 A)
(E = I x V )
J
A
CL
Drain-to-Source ON
Voltage (V
)
Current
Area (I
DS(ON)
)
A
GND
Drain-to-Source ON
Voltage (V
Time
BAT
Time
SHORTED LOAD FAULT
)
DS(ON)
VS
A shorted load (over-current) fault can be caused by any
output being shorted directly to supply, or an output
experiencing a current greater than the current limit.
GND
Current
Area (I )
A
There are two safety circuits progressively in operation
during load short conditions that provide system protection:
Clamp Energy
(E = I x V )
CL
J
A
1. The device’s output current is monitored in an analog
fashion using SENSEFET approach and current
limited.
Source Current
(I = 0.3 A)
S
Source Clamp Voltage
Source Voltage
2. The device’s output thermal limit is sensed and when
attained causes only the specific faulted output to shut
down. The output will remain off until cooled. The
device will then reassert the output automatically. The
cycle will continue until fault is removed or the
command bit instructs the output off. Shorted load
faults will be reported properly through the SPI
regardless of Open Load Detection Current enable
bits.
(V
= -15 V)
CL
Figure 18. Output Voltage Clamping
SPI CONFIGURATIONS
The SPI configuration on the 33879 device is consistent
with other devices in the Octal Serial Switch (OSS) family.
This device may be used in serial SPI or parallel SPI with the
33298 and 33291. Different SPI configurations may be
provided. For more information, contact Freescale Analog
Products Division or the local Freescale representative.
UNDER-VOLTAGE SHUTDOWN
An under-voltage condition on VDD or VPWR will result in
the shutdown of all outputs. The VDD under-voltage threshold
is between 0.8 and 3.0 V. VPWR under-voltage threshold is
between 3.0 and 5.0 V. When the supplies fall below their
respective thresholds, all outputs are turned OFF. As both
supplies returns to normal levels, internal logic is reset and
the device resumes normal operation.
REVERSE BATTERY
The 33879 has been designed with reverse battery
protection on the VPWR pin.
All outputs consist of a power MOSFET with an integral
substrate diode. During the reverse battery condition, current
will flow through the load via the substrate diode. Under this
circumstance, relays may energize and lamps will turn on.
Where load reverse battery protection is desired, a reverse
battery blocking diode must be placed in series with the load.
OUTPUT VOLTAGE CLAMP
Each output of the 33879 incorporates an internal voltage
clamp to provide fast turn-off and transient protection of each
33879
Analog Integrated Circuit Device Data
Freescale Semiconductor
18
FREESCALE [ Freescale ]
