LT1584/LT1585/LT1587
APPLICATIONS INFORMATION
handle microsecond surge currents of 50A to 100A. Even
with large value output capacitors it is difficult to obtain
those values of surge currents in normal operation. Only
with large values of output capacitance, such as 1000µF to
5000µF, and with the input pin instantaneously shorted to
ground can damage occur. A crowbar circuit at the input of
the LT1584/LT1585/LT1587 can generate those levels of
current, and a diode from output to input is then recom-
mended. This is shown in Figure 2. Usually, normal power
supply cycling or system “hot plugging and unplugging”
will not generate current large enough to do any damage.
The adjust pin can be driven on a transient basis
±7V
with
respect to the output, without any device degradation. As
with any IC regulator, exceeding the maximum input-to-
output voltage differential causes the internal transistors to
break down and none of the protection circuitry is then
functional.
D1
1N4002
(OPTIONAL)
V
IN
+
IN
C1
10µF
LT1584-3.3
OUT
GND
+
V
OUT
C2
22µF
D1
1N4002
(OPTIONAL)
LT1584
V
IN
+
IN
C1
10µF
ADJ
OUT
R1
R2
+
+
C
ADJ
Figure 2
Overload Recovery
The LT1584 devices have safe-area protection similar to
the LT1083/LT1084/LT1085. The safe-area protection de-
creases current limit as input-to-output voltage increases.
This behavior keeps the power transistor inside a safe
operating region for all values of input-to-output voltage.
The LT1584 protection circuitry provides some output
current at all values of input-to-output voltage up to the 7V
U
W
U U
maximum supply voltage. When power is first applied, the
input voltage rises and the output voltage follows the input.
The input-to-output voltage remains small and the regula-
tor can supply large output currents. This action permits
the regulator to start-up into very heavy loads.
With higher input voltages, a problem can occur where the
removal of an output short does not permit the output
voltage to recover. This problem is not unique to the
LT1584 devices and is present on the LT1083/LT1084/
LT1085 family and older generation linear regulators. The
problem occurs with a heavy output load, a high input
voltage, and a low output voltage. An example is immedi-
ately after the removal of a short circuit. The load line of
such a load may intersect the output current curve at two
points. If this happens, two stable output operating points
exist for the regulator. With this double intersection, the
power supply may require cycling down to zero and back
up again to make the output recover. This situation does
not occur with the LT1585/LT1587 because no foldback
circuitry is required to provide safe-area protection.
Ripple Rejection
The typical curve for ripple rejection reflects values for the
LT1584/LT1585/LT1587 fixed output voltage parts be-
tween 3.3V and 3.6V. In applications that require improved
ripple rejection, use the adjustable devices. A bypass
capacitor from the adjust pin to ground reduces the output
ripple by the ratio of V
OUT
/1.25V. The impedance of the
adjust pin capacitor at the ripple frequency should be less
than the value of R1 (typically in the range of 100Ω to
120Ω) in the feedback divider network in Figure 2. There-
fore, the value of the required adjust pin capacitor is a
function of the input ripple frequency. For example, if R1
equals 100Ω and the ripple frequency equals 120Hz, the
adjust pin capacitor should be 22µF. At 10kHz, only 0.22µF
is needed.
Output Voltage
The LT1584/LT1585/LT1587 adjustable regulators develop
a 1.25V reference voltage between the output pin and the
adjust pin (see Figure 3). Placing a resistor R1 between
these two terminals causes a constant current to flow
through R1 and down through R2 to set the overall output
voltage. Normally, this current is the specified minimum
V
OUT
C2
22µF
LT1584 • F02
9
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