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LT3065 Series

3065fc

For more information www.linear.com/LT3065

applicaTions inForMaTion

current limit amplifier. Connecting a resistor (RIMAX) from

IMAX to GND sets the current limit threshold. If the output

load increases to a level such that the IMAX pin voltage

reaches 0.6V, the current limit amplifier takes control

and regulates the IMAX voltage to 0.6V, regardless of the

output voltage. Calculate the required RIMAX value for a

given current limit from the following formula:

RIMAX = 500 •

0.6V

ILIMIT

In cases where the IN to OUT differential voltage exceeds

10V, current limit foldback lowers the internal current

limit level, possibly causing it to override the external

programmable current limit. See the Internal Current

Limit vs VIN – VOUT graph in the Typical Performance

Characteristics section.

The IMAX pin requires a 22nF decoupling capacitor. If the

external programmable current limit is not used, connect

the IMAX pin directly to GND. LT3065 power dissipation

increases the IMAX threshold at a rate of approximately

0.5 percent per watt.

PWRGD Pin Operation

ThePWRGDpinisanopen-drainhighvoltageNMOSdigital

outputcapableofsinking50µA.ThePWRGDpinde-asserts

and becomes high impedance if the output rises above

90% of its nominal value. If the output falls below 88.4%

of its nominal value for more than 25μs, the PWRGD pin

asserts low. The PWRGD comparator has 1.6% hysteresis

and 25μs of deglitching. The PWRGD comparator has a

dedicated reference that does not soft-start if a capacitor

is used on the REF/BYP pin.

The use of a feed-forward capacitor, CFF, as shown in

Figure 4, can result in the ADJ pin being pulled artificially

high during startup transients, which causes the PWRGD

flag to assert early. To avoid this problem, ensure that

the REF/BYP capacitor is significantly larger than the

feed-forward capacitor, causing REF/BYP time constant

to dominate over the time constant of the resistor divider

network.

Operation in Dropout

Some degradation of the IMAX current mirror accuracy

occurs for output currents less than 50mA when operat-

ing in dropout.

Overload Recovery

Like many IC power regulators, the LT3065 has safe oper-

ating area protection. The safe area protection 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. The LT3065 pro-

vides some output current at all values of input-to-output

voltage up to the device’s Absolute Maximum Rating.

When power is first applied, 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. Other regulators, such as the LT1083/LT1084/

LT1085familyandLT1764Aalsoexhibitthisphenomenon,

so it is not unique to the LT3065. 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 the shutdown pin

is pulled high after the input voltage is already turned on.

The load line intersects the output current curve at two

points. If this happens, there are two stable output operat-

ing points for the regulator. With this double intersection,

the input power supply needs to be cycled down to zero

and back up again to recover the output.

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