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LT3667

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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 (ILIMIT) of an LDO is set by

attaching a resistor (RIMAX) from the corresponding EN/

ILIM pin to ground:

RIMAX =

799 •0.4V

ILIM

– 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

regulatortostart-upintoveryheavyloads.Duringstart-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

curveattwopoints,resultingintwostableoutputoperating

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-outputvoltagesandreverseoutput-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

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