AAT2491 Datasheet, PDF (10/12 Pages) Skyworks Solutions Inc. – Dual N-Channel HV Cascode-Clamp, Lateral TrenchDMOS Array

DATA SHEET
AAT2491
Dual N-Channel HV Cascode-Clamp, Lateral TrenchDMOS Array
In high-voltage LED backlighting applications, the low
side TrenchDMOS devices of the AAT2491 are normally
used as part of a linear control and feedback circuit to
regulate the current in each series string of LEDs accord-
ing to the gate bias supplied by an LED driver integrated
circuit (such as the AAT2430A-1 or the AAT2405). Since
the source connections of the low side TrenchDMOS
devices are separate from ground, the device is compat-
ible with current sensing using either discrete sense
resistors or I-Precise™ current monitoring and gate drive
available in Skyworks' products, for improved accuracy.
The high-side cascode clamp TrenchDMOS devices are
used to protect an LED driver IC from the high voltages
present in HV LED backlighting systems. Such high volt-
ages, generally ranging from 50V up to 150V, are needed
for forward biasing LED strings having many series con-
nected LEDs. During operation, most of this high voltage
is dropped across the conducting LEDs and not across
the silicon transistors driving the LEDs. When the LEDs
are not conducting (or conducting low currents) or when
one or more LEDs become shorted, a disproportionate
amount of the voltage is impressed on the driver devic-
es. Without the cascode-clamp, the current sink transis-
tors may be permanently damaged.
Using cascode clamping, i.e. where a high voltage
MOSFET is operated as a voltage follower, the maximum
voltage impressed on any current sink device is safely
limited. In such an application, each cascode clamp
TrenchDMOS device has its gate biased to a fixed volt-
age V_G(CLAMP) with its drain connected to the LED string
and its source prewired in series with the current sink
device. As the drain voltage rises in normal operation,
the high-side transistor’s source voltage V_S follows until
the source voltage reaches a potential approximately
one V_T (one threshold) below the fixed gate bias volt-
age, or V_G(CLAMP) – V_T. Above that potential, further drain
voltage increases will not drive the source voltage any
higher. For example if the gate is biased to 12V, the
source can be driven to a potential no higher than 10V,
thereby clamping the drain voltage of the low-side cur-
rent sink to within safe operation range. It is required to
put a 1nF bypass capacitor connected from CG pin to
ground.
The AAT2491 includes a series string of four P-N junction
diodes for monitoring die temperature. To facilitate sim-
ple, over-temperature monitoring, the diode string
should be forward biased by a constant current or a
resistor to approximately 300μA. The voltage of the con-
ducting diode string can easily be monitored with a com-
parator to determine if an over-temperature condition
has occurred, specifically where the diode voltage drops
below a pre-specified value. A characterization curve
comparing the diode string forward voltage to tempera-
ture is included for reference.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
10
201934A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 7, 2012