2987

8-CHANNEL

SOURCE DRIVER

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

APPLICATIONS INFORMATION

AND CIRCUIT DESCRIPTION

As with all power integrated circuits, the UDN2987A

and UDN2987LW have a maximum allowable output

current rating. The 500 mA rating does not imply that

operation at that value is permitted or even obtainable.

The channel output current trip point is specified as -370

mA, minimum; therefore, attempted operation at current

levels greater than -370 mA may cause a fault indication

and channel shutdown. The device is tested at a maxi-

mum of -350 mA and that is the recommended maximum

output current per driver. It provides protection for current

overloads or shorted loads up to 35 V.

All outputs are enabled by pulling the OE/R input high.

When OE/R is low or allowed to float (internal pull-down),

all outputs are inhibited and the latches are reset. Note

that the RESET pulse duration (OE/R low) should be

at least 1

µs. This will ensure safe operation under

attempted RESET conditions with a shorted load. The

latches are also reset during power up, regardless of the

state of the OE/R input.

The load current causes a small voltage drop across

the internal low-value sense resistor. This voltage is

compared to the voltage drop across a reference resistor

with a constant current. The two resistors are matched to

eliminate errors due to manufacturing tolerances or

temperature effects. Each channel includes a comparator

will set the affected latch and shut down only that channel.

All other channels will continue to operate normally. The

latch includes a 1

µs delay (t

triggering due to crossover currents generated when

switching inductive loads. For an abrupt short circuit, the

delay and output switching times will allow a brief,

permissable current in excess of the trip current before the

output driver is turned OFF.

A common thermal shutdown disables all outputs

if the chip temperature exceeds +165

°C. At thermal

shutdown, all latches are reset. The outputs are disabled

until the chip cools down to about +150

°C (thermal

hysteresis).

A common open-collector FAULT output is used to

indicate any channel over-current condition or chip

thermal shutdown.

ALLOWABLE OUTPUT CURRENT

AS A FUNCTION OF DUTY CYCLE

(UDN2987A shown, multiply by 78% for UDN2987LW)

At +25

°C

At +50

°C