Datasheet

LT3042
22
3042fb
For more information www.linear.com/LT3042
APPLICATIONS INFORMATION
The maximum junction temperature equals the maxi-
mum ambient temperature plus the maximum junction
temperature rise above ambient:
T
JMAX
= 85°C + 20.7°C = 105.7°C
Overload Recovery
Like many IC power regulators, the LT3042 incorporates
safe-operating-area (SOA) protection. The SOA protection
activates at input-to-output differential voltages greater
than 12V. The SOA protection decreases the current limit
as the input-to-output differential increases and keeps
the power transistor inside a safe operating region for
all values of input-to-output voltages up to the LT3042’s
absolute maximum ratings. The LT3042 provides some
level of output current for all values of input-to-output dif
-
ferentials. Refer to the Current Limit curves in the Typical
Per
formance Characteristics section. When power is first
applied and input voltage rises, the output follows the input
and keeps the input-to-output differential low to allow the
regulator to supply large output current and start-up into
high current loads.
Due to current limit foldback, however, at high input volt
-
ages a problem can occur if the output voltage is low and
the load
current is high. Such situations occur after the
removal of a short-circuit or if the EN/UV pin is pulled high
after the input voltage has already turned ON. The load line
in such cases intersects the output current profile at two
points. The regulator now has two stable operating points.
With this double intersection, the input power supply may
need to be cycled down to zero and brought back up again
to make the output recover. Other linear regulators with
foldback current limit protection (such as the LT1965 and
LT1963A, etc.) also exhibit this phenomenon, so it is not
unique to the LT3042.
Protection Features
The LT3042 incorporates several protection features for
battery-powered applications. Precision current limit and
thermal overload protection protect the LT3042 against
overload and fault conditions at the device’s output. For
normal operation, do not allow the junction temperature
to exceed 125°C (E-, I-grade) or 150°C (H-, MP-grade).
To protect the LT3042’s low noise error amplifier, the SET-
to-OUTS protection clamp limits the maximum voltage
between SET and OUTS with a maximum DC current of
20mA through the clamp. So for applications where SET
is actively driven by a voltage source, the voltage source
must be current limited to 20mA or less. Moreover, to limit
the transient current flowing through these clamps during
a transient fault condition, limit the maximum value of the
SET pin capacitor (C
SET
) to 22µF.
The LT3042 also incorporates reverse input protection
whereby the IN pin withstands reverse voltages of up to
–20V without causing any input current flow and without
developing negative voltages at the OUT pin. The regulator
protects both itself and the load against batteries that are
plugged-in backwards.
In circuits where a backup battery is required, several
different input/output conditions can occur. The output
voltage may be held up while the input is either pulled to
GND, pulled to some intermediate voltage, or left open-
circuit. In all of these cases, the reverse current protection
circuitry prevents current flow from output to the input.
Nonetheless, due to the OUTS-to-SET clamp, unless the
SET pin is floating, current can flow to GND through the
SET pin resistor as well as up to 15mA to GND through
the output overshoot recovery circuitry. This current flow
through the output overshoot recovery circuitry can be
significantly reduced by placing a Schottky diode between
OUTS and SET pins, with its anode at the OUTS pin.
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