MC33293A

13

MOTOROLA ANALOG IC DEVICE DATA

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:

(11)

1/2 Lload (Iload)2 >> 1/2 CO (VOoff(th))2

toff false fault = [(CO ∆V) / Iload] + toff

where: Iload = Vload / [Rload + RDS(on)]

∆V = VOoff(th) – [Iload RDS(on)]

toff = toff(dly) + tf

(7)

(8)

(9)

(10)

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:

[Vload – VOC(limit)]

IDS(limit)

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Rload(detect) =