2987
8-CHANNEL
SOURCE DRIVER
ALLOWABLE OUTPUT CURRENT
AS A FUNCTION OF DUTY CYCLE
(UDN2987A shown, multiply by 78% for UDN2987LW)
At +25°C
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
and its own latch. An over-current fault (V
SENSE
> V
REF
)
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
d
) to prevent unwanted
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.
At +50°C
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
