33291 datasheet(19/26 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33291

FUNCTIONAL DESCRIPTION

An Output OFF Open Load Fault is indicates when the

output voltage is less than the Output Threshold Voltage

(VTHRES) of 0.6 to 0.8 x VDD. Since the 33291 outputs function

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 VDS voltage of the MOSFET is less than the

threshold voltage and the OFF state VDS voltage is greater

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 Figure 20 for an understanding of the Short

Circuit Detect circuit.) With VDD = 5.0 V, an OFF state output

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.

The 33291 has an internal pull-down current source of

µA, illustrated in Figure 19, page 18, between the

MOSFET drain and ground. This current source prevents the

output from floating up to VPWR if there is an open load or

internal wire bond failure. The internal comparator compares

the drain voltage with a reference voltage, VTHRES (0.6 to 0.8

x VDD). If the output voltage is less than this reference

voltage, the 33291 will declare the condition to be an open

load fault.

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 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 (RL,CL,LL), RDS(ON), and COUT of the

MOSFET as well as the supply voltage (VPWR). The rising

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.

SHORTED LOAD FAULT

A short load, or overcurrent fault can be caused by any

output being shorted directly to supply, or an output

experiencing a current greater than the current limit.

There are three safety circuits progressively in operation

during load short conditions providing system protection.

They are as follows:

1. The output current of the device is monitored in an

analog fashion using a SENSEFET approach and

current limited.

2. The output current of the device is sensed by

monitoring the MOSFET drain voltage.

3. The output thermal limit of the device is sensed and

when attained causes only the specific faulted output

to be latched OFF, allowing all remaining outputs to

operate normally.

All three protection mechanisms are incorporated in their

output, affording robust independent output operation.

The analog current limit circuit is always active and

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 VDS voltage will exceed the VTHRES voltage and

the overcurrent comparator will be tripped as shown in

Figure 20.

Figure 20. Short Circuit Detect and

Analog Current Limiting Circuit

The status of SFPD determines whether the 33291 will

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.

Grounding the SFPD pin will enable the short fault

protection shutdown circuitry. Consider a load short (output

short to supply) occurring on an output before, during, and

after output turn ON. When the CS signal rises to the high

logic state, the corresponding output is turned ON and a

delay timer is activated. The duration of the delay timer is

µs to 250 µs. If the short circuit takes place before the

output is turned ON, the delay experienced is the entire 70

µs

to 250

µs followed by shutdown. If the short occurs during the

delay time, the shutdown still occurs after the delay time has

elapsed. However, if the short circuit occurs after the delay

time, shutdown is immediate (within 20

µs after sensing). The

purpose of the delay timer is to prevent false faults from being

reported when switching capacitive loads.

If the SFPD pin is at 5.0 V (or VDD), an output will not be

disabled when an overcurrent is detected. The specific output

will, within 5.0

µs to 10 µs of encountering the short circuit, go

into an analog current limited mode. This feature is especially

VThres

2.5 to 3.5 V

–

High = Fault

Output

VPWR

33291

–

MOSFET ON

Digital

Analog

Vref

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