11

LT1584/LT1585/LT1587

158457a

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

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 cir-

cuitry 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

adjust pin capacitor at the ripple frequency should be less

than the value of R1 (typically in the range of 100

Ω to

120

Ω) 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 100

Ω and the ripple frequency equals 120Hz, the

adjust pin capacitor should be 22

µF. 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

load current of 10mA. The current out of the adjust pin adds

to the current from R1 and is typically 55

µA. Its output

voltage contribution is small and only needs consideration

when very precise output voltage setting is required.

LT1584

ADJ

IN

R1

R2

OUT

55

µA

LT1585 • F03

C1

10

µF

C2

22

µF

+

+

Figure 3. Basic Adjustable Regulator

Load Regulation

It is not possible to provide true remote load sensing

because the LT1584/LT1585/LT1587 are three-terminal

devices. Load regulation is limited by the resistance of the

wire connecting the regulators to the load. Load regulation

per the data sheet specification is measured at the bottom

of the package.

For fixed voltage devices, negative side sensing is a true

Kelvin connection with the ground pin of the device re-

turned to the negative side of the load. This is illustrated in

Figure 4.

LT1584-3.3

PARASITIC

LINE RESISTANCE

GND

IN

OUT

LT1585 • F04

Figure 4. Connection for Best Load Regulation