LT3667
24
3667fb
For more information www.linear.com/LT3667
APPLICATIONS INFORMATION
External Programmable Current Limit, Enable
Each EN/ILIM pin (EN2/ILIM2 and EN3/ILIM3) is the col-
lector of
a PNP which mirrors the corresponding LDO’s
output at a ratio of 1:799 (see Block Diagram). The EN2/
ILIM2
and EN3/ILIM3 pins are also the inputs to preci
-
sion current limit amplifiers. If an output load increases
to the point where it causes the corresponding current
limit amplifier input voltage to reach 0.4V, the current
limit amplifier takes control of output regulation so that
its input clamps at 0.4V, regardless of the output voltage.
The current limit threshold (I
LIMIT
) of an LDO is set by
attaching a resistor (R
IMAX
) from the corresponding EN/
ILIM pin to ground:
R
IMAX
=
I
–80Ω
In order to maintain stability, each EN/ILIM pin requires
a 47nF capacitor from that pin to ground.
In cases where the input to output voltage differential
exceeds 10V, foldback current limit will lower the inter
-
nal current level limit, possibly causing it to preempt the
external
programmable current limit. See the Internal
Current Limit vs Input/Output Differential graph in the
Typical Performance Characteristics section.
If an external current limit is not needed, the correspond
-
ing EN
/ILIM pin must be connected to ground, in which
case no capacitor is required.
Each
LDO can be individually shut down by pulling its EN/
ILIM pin above 1.2V (1V typical). Note that in this case this
pin will draw up to 500µA in certain operating conditions
until the LDO is shut down, which the circuit driving this
pin must be able to deliver. When an EN/ILIM pin is only
used to enable/disable an LDO, no capacitor is required
on this pin.
Overload Recovery
Each LDO of the LT3667 has a safe operating area pro
-
tection, which
decreases current limit as input-to-output
voltage increases, and keeps the power transistor inside
a safe operating region for all
values of input-to-output
voltage.
Each LDO provides some output current at all
values of input-to-output voltage up to the device break
-
down. When power is first applied to an LDO, the input
voltage
rises and the output follows the input; allowing the
regulator to start-up into very heavy loads. During start-up,
as the input voltage is rising, the input-to-output voltage
differential is small, allowing the regulator to supply large
output currents. With a high input voltage, a problem can
occur wherein the removal of an output short will not allow
the output to recover. The problem occurs with a heavy
output load when the input voltage is high and the output
voltage is low. Common situations are: immediately after
the removal of a short-circuit or if an LDO is enabled via
its EN/ILIM pin after the input voltage is already turned
on. In such cases, the regulator would have to operate its
power device outside its safe operating are (high voltage
and high current) in order to bring up the output voltage.
Since this is prevented by the safe operating area protec
-
tion, the output gets stuck at a low voltage. Essentially,
the
load line for such a load intersects the output current
curve at two points, resulting in two stable output operating
points for the regulator. With this double intersection, the
input power supply needs to be cycled down to zero and
brought up again to make the output recover.
Protection Features
The LT3667 LDO’s protect against reverse-input volt
-
ages, reverse
-output
voltages and reverse output-to-input
voltages. Current limit protection and thermal overload
protection protect the LDOs against current overload
conditions at their outputs. For normal operation, do not
exceed the maximum operating junction temperature. The
LT3667 IN2 and IN3 (QFN only) pins withstand reverse
voltages of 45V. The device limits current flow to less than
300μA (typically less than 10μA) and no negative voltages
appear at OUT2 or OUT3. The LDOs incur no damage if
their outputs are pulled below ground. If an input is left
open circuit or grounded, the corresponding output can be
pulled below ground by 45V. No current flows through the
pass transistor from the output. However, current flows in
(but is limited by) the corresponding resistor divider that
sets the output voltage. Current flows from the bottom
resistor
in the divider and from the FB2/FB3 pin’s internal
clamp through the top resistor in the divider to the external
circuitry pulling OUT2/OUT3 below ground. If the input
is powered by a voltage source, the output sources cur
-
