33879 datasheet(18/23 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33879

FUNCTIONAL DESCRIPTION

DEVICE OPERATION

declare the load open in the OFF state when the output drain-

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.

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.

SHORTED LOAD FAULT

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.

There are two safety circuits progressively in operation

during load short conditions that provide system protection:

1. The device’s output current is monitored in an analog

fashion using SENSEFET approach and current

limited.

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.

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.

OUTPUT VOLTAGE CLAMP

Each output of the 33879 incorporates an internal voltage

clamp to provide fast turn-off and transient protection of each

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).

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.

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.

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.

Current

Area (IA)

Clamp Energy

(EJ = IA x VCL)

Clamp Energy

(EJ = IA x VCL)

Drain Voltage

Source Voltage

Time

BAT

Drain-to-Source Clamp

Voltage (VCL = 45 V)

Drain Current

(ID = 0.3 A)

GND

Drain-to-Source ON

Voltage (VDS(ON))

Drain-to-Source ON

Voltage (VDS(ON))

Current

Area (IA)

Source Current

(IS = 0.3 A)

Source Clamp Voltage

(VCL = -15 V)

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