MC33293A Datasheet, PDF (13/16 Pages) Motorola, Inc – QUAD LOW SIDE SWITCH

MC33293A
The purpose for the tos(off) delay is to prevent false fault
reporting experienced with capacitance type loads. The load
capacitance causes the rise in output voltage to lag even
after the load has been turned OFF. The normal voltage lag
caused by load capacitance could be misinterpreted as an
open-load if it weren’t for the built-in delay. This delay, or
masking, is accomplished with four separate timers that reset
independent of each other when the corresponding input is
switched from a high to a low logic state. Internal logic
prevents an output-off open-load Fault from being reported at
the Fault pin when any input is high. An output-off open-load
Fault will be reported at the Fault Status pin after an open
load occurs, all inputs not corresponding to the faulted output
are low and a time in excess of tos(off) is exceeded after
switching OFF the input corresponding to the faulted output.
An important note that bears repeating is that an output-off
open-load Fault will not be reported at the Fault Status pin
unless all input pins are at a logic low state (Figure 13). This
is a Fault Status interrogation feature. It helps in
distinguishing between an output-on open-load Fault and an
output-on over-current Fault. (Fault Status interrogation is
explained in greater detail in a later section).
The time the output voltage takes to reach VOoff(th) after
being turned OFF is toff _{false fault.} It is a function of the load
resistance (Rload); load inductance (Lload); load current
(Iload); output-on resistance (RDS(on)), output capacitance
(CO); load supply voltage (Vload); and the turn OFF time (toff).
The value of toff is comprised of the Vin high-to-low
propagation delay time (toff(dly)), and the output voltage fall
time (tf).
For the case when:
1/2 Lload (Iload)2 >> 1/2 CO (VOoff(th))2
(7)
toff _{false fault} = [(CO ∆V) / Iload] + toff
(8)
where: Iload = Vload / [Rload + RDS(on)]
(9)
∆V = VOoff(th) – [Iload RDS(on)]
(10)
toff = toff(dly) + tf
(11)
Using Equation 7 for the transient case,
when: Vload = 14 V
RDS(on) = 0.3 Ω
Lload = 10 mH
Rload = 14 Ω
CO = 0.001 µF
VOoff(th) = 3.7 V
an Output-Off open-load Fault will be detected but not
reported after initial turn OFF for a duration of 3.5 ns + toff.
From Equation 7, the energy stored in the load inductor will
be 4.8 mJ. This is much greater than the 68 nJ needed to
charge the output capacitance. This allows the use of
Equation 8 in determining the false output-off open-load Fault
duration following turn OFF because it assures that the
output capacitance will be charged by the energy stored in
the load inductance.
Over-Current Fault
An over-current (short circuit or current limit) Fault is the
detection and reporting of any output over-current condition.
An over-current condition is defined as a condition where
load current exceeds the internal current limit value (typically
4.0 A). An over-current condition activates the current limit
circuit. This circuit then sends an analog signal to the gate
control circuit, lowering the voltage on the output transistor’s
gate. Lowering the gate voltage forces the output transistor to
transition from the resistive (fully enhanced) mode of
operation to the current limit (between fully enhanced and
fully OFF) mode.
The actual detection of an over-current condition does not
occur at the initial onset of current limit. The onset of current
limit causes the voltage on the affected output to increase.
The actual Over-Current detection occurs when the output
voltage increases and exceeds the over-current Detect
Voltage Threshold (VOC(limit), typically 3.7 V), while the
corresponding input signal is in a logic high state.
After detection, the reporting of an over-current Fault at
the Fault Status output is delayed by a time equal to the
over-current Sense Time (toc), see Figures 5 and 6. This
delay time is typically 55 µs. If the over-current condition no
longer exists after the over-current Sense Time has passed,
then no fault is reported. The purpose of the Fault reporting
delay is to blank any false faults that might be reported due to
high inrush current loads such as incandescent lamps. If the
over-current condition still exists after the delay time has
passed, then a fault will be reported at the Fault Status output
and the affected output is turned OFF.
The Over-Current Sense Time is accomplished internally
with four separate timers that reset and start independent of
each other whenever a corresponding output is turned ON,
either due to the corresponding input turning ON or the
completion of the over-current Refresh Time (tref) explained
in the next paragraph, (see Figures 5 and 6). An over-current
Fault will be reported at the Fault Status output when an
over-current condition is detected and a lapse time in excess
of toc is exceeded after turning ON the affected output.
At the same time the over-current Fault is reported, a
single internal over-current refresh timer resets, causing any
over-current outputs to be turned OFF for a duration of tref,
typically 3.6 ms. After a time tref, the faulted output(s) will be
turned ON again to check if the over-current condition still
exists. If the over-current condition still exists, the output(s)
will be turned OFF again after a time toc. This periodic retry
continues turning ON and OFF over-current loads at a duty
cycle of toc /(toc + tref) with a period of toc + tref until either the
input is turned OFF or the over-current condition is removed.
Any subsequent over-current conditions will reset and restart
the tref timer.
Detection of an over-current condition coincides with, but
does not occur until after the onset of current limit. This
allows a specific but small current limit range to go
undetected. The factors that determine the value of load
resistance causing an over-current condition to be detected
are: the Output-Load Current Limit [IDS(limit)]; load voltage
(Vload); and the Over-Current Detect Threshold Voltage
[VOC(limit)] as shown below:
Rload(detect)
=
[Vload – VOC(limit)]
IDS(limit)
(12)
MOTOROLA ANALOG IC DEVICE DATA
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