Protected Quad Driver
with Fault Detection and Sleep Mode
A2557
CIRCUIT DESCRIPTION AND APPLICATION
The A2557 low-current quad power drivers provide protected
output driver functions, combined with a fault diagnostic scheme,
plus an automatic low-current Sleep-Mode function. These de-
vices 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.
some independence between the output channels, i.e., one chan-
nel 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
H
ENABLE OUTN FAULT
H
H
L
L
H
H
H
H
H
H
H
Normal Load
L
A logic low at the ENABLE input causes all outputs to be
switched off regardless of the state of the IN terminals. In addi-
tion, 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
Sleep Mode
All L
X
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
R = Linear drive, current limited.
connection at the K terminal.
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.
Incandescent lamp driver
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 fila-
ment is at minimum resistance and would normally allow a 10x
to 12x in-rush current.
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 detect-
ing 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.
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).
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 maximum rated
voltage to the lamp.
To keep the device within its safe operating area (SOA) this out-
put current limit is further reduced:
•
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 temperature).
As each channel has its own thermal limit circuitry this provides
Allegro MicroSystems, Inc.
115 Northeast Cutoff
7
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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