2557
PROTECTED QUAD DRIVER
WITH FAULT DETECTION
& SLEEP MODE
CIRCUIT DESCRIPTION AND APPLICATION
The A2557 low-current quad power drivers provide the
same protected output driver function as (and are pin compat-
ible with) the UDx2543/49/59 devices, combined with a fault
diagnostic scheme similar to the UDx2547, plus an automatic
low-current Sleep-Mode function. These devices monitor their
outputs for fault (open or shorted) conditions. For each channel
the input and output levels are compared. If these are different
from the expected levels then a fault condition is flagged by
pulling the common FAULT output low.
•
if the power dissipation in the output device increases the
local junction temperature above 165°C (nominal), so as to limit
the power dissipation (and hence the local junction tempera-
ture). As each channel has its own thermal limit circuitry this
provides some independence between the output channels, i.e.,
one channel can be operating in thermally reduced current limit,
while the others can provide full drive capability.
•
as a function of the output voltage. Full current limit of
500 mA (nominal) is available up to approximately VO = 8 V;
above this the limit is reduced linearly to about 350 mA at VO =
32 V. This helps to improve SOA by immediately reducing the
peak power pulse into a shorted load at high VO.
Status
INN ENABLE OUTN
FAULT
H
L
H
H
L
H
H
H
Normal Load
A logic low at the ENABLE input causes all outputs to be
switched off regardless of the state of the IN terminals. In
addition, the device is put into a low quiescent current ‘sleep’
mode, reducing ICC below 100 µA. If ENABLE is taken high
and any of the inputs go high, the circuit will ‘auto-wake-up’.
However, if the device is enabled, but all inputs stay low, then
the circuit remains in ‘sleep’ mode.
X
All L
L
X
H
H
H
H
Sleep Mode
Over-Current or
Short to Supply
H
H
R
L
Open Load or
Short to Ground
L
H
H
L
L
L
Thermal Fault
H
H
All outputs have internal flyback diodes, with a common-
cathode connection at the K terminal.
R = Linear drive, current limited.
Incandescent lamp driver
The FAULT output is operational only if ENABLE is high.
The output state is detected by monitoring the OUTn terminal
using a comparator whose threshold is typically 2.5 V. In order
to detect open-circuit outputs, a 30 µA current sink pulls the
output below the comparator threshold. To ensure correct fault
operation, a minimum load of approximately 1 mA is required.
The fault function is disabled when in ‘sleep’ mode, i.e.,
FAULT goes high and the 30 µA output sinks are turned off.
The FAULT output is a switched current sink of typically
60 µA.
High incandescent lamp turn-on (in-rush currents) can
contribute to poor lamp reliability and destroy semiconductor
lamp drivers. When an incandescent lamp is initially turned on,
the cold filament is at minimum resistance and would normally
allow a 10x to 12x in-rush current.
Warming (parallel) or current-limiting (series) resistors
protect both driver and lamp but use significant power either
when the lamp is off or when the lamp is on, respectively.
Lamps with steady-state current ratings up to 300 mA can be
driven without the need for warming or current-limiting
resistors, if lamp turn-on time is not a concern (10s of ms).
Each channel consists of a TTL/CMOS-compatible logic
input gated with a common ENABLE input. A logic high at the
input will provide drive to turn on the output npn switch. Each
output has a current-limit circuit that limits the output current
by detecting the voltage drop across a low-value internal
resistor in the emitter of the output switch. If this drop reaches
a threshold, then the base drive to the output switch is reduced
to maintain constant current in the output.
With these drivers, during turn-on, the high in-rush current
is sensed by the internal sense resistor, drive current to the
output stage is reduced, and the output operates in a linear mode
with the load current limited to approximately 500 mA. During
lamp warmup, the filament resistance increases to its maximum
value, the output driver goes into saturation and applies maxi-
mum rated voltage to the lamp.
To keep the device within its safe operating area (SOA) this
output current limit is further reduced
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