LT3085 datasheet(14/28 Pages) LINER

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LT3085

3085fb

APPLICATIONS INFORMATION

The problem occurs with a heavy output load when the

input voltage is high and the output voltage is low. Com-

mon situations are immediately after the removal of a

short circuit. The load line for such a load may intersect

the output current curve at two points. If this happens,

there are two stable operating points for the regulator.

With this double intersection, the input power supply may

need to be cycled down to zero and brought up again to

make the output recover.

Load Regulation

Because the LT3085 is a ﬂoating device (there is no ground

pin on the part, all quiescent and drive current is delivered

to the load), it is not possible to provide true remote load

sensing. Load regulation will be limited by the resistance

of the connections between the regulator and the load.

The data sheet speciﬁcation for load regulation is Kelvin

sensed at the pins of the package. Negative side sensing

is a true Kelvin connection, with the bottom of the voltage

setting resistor returned to the negative side of the load

(see Figure 7). Connected as shown, system load regulation

will be the sum of the LT3085 load regulation and the

parasitic line resistance multiplied by the output current.

It is important to keep the positive connection between

the regulator and load as short as possible and use large

wire or PC board traces.

Internal Parasitic Diodes and Protection Diodes

In normal operation, the LT3085 does not require protection

diodes. Older three-terminal regulators require protection

diodes between the VOUT pin and the input pin or between

the ADJ pin and the VOUT pin to prevent die overstress.

Figure 7. Connections for Best Load Regulation

–

LT3085

VCONTROL

OUT

3085 F07

SET RSET

PARASITIC

RESISTANCE

LOAD

On the LT3085, internal resistors and diodes limit current

paths on the SET pin. Even with bypass capacitors on the

SET pin, no protection diode is needed to ensure device

safety under short-circuit conditions. The SET pin handles

±10V (either transient or DC) with respect to OUT without

any device degradation.

Internal parasitic diodes exist between OUT and the two

inputs. Negative input voltages are transferred to the output

and may damage sensitive loads. Reverse-biasing either

input to OUT will turn on these parasitic diodes and allow

current ﬂow. This current ﬂow will bias internal nodes

of the LT3085 to levels that possibly cause errors when

suddenly returning to normal operating conditions and

expecting the device to start and operate. Prediction of

results of a bias fault is impossible, immediate return to

normal operating conditions can be just as difﬁcult after

a bias fault. Sufﬁce it to say that extra wait time, power

cycling, or protection diodes may be needed to allow the

LT3085 to return to a normal operating mode as quickly

as possible.

Protection diodes are not otherwise needed between

the OUT pin and IN pin. The internal diodes can handle

microsecond surge currents of up to 50A. Even with

large output capacitors, obtaining surge currents of those

magnitudes is difﬁcult in normal operation. Only with large

output capacitors, such as 1000μF to 5000μF, and with

IN instantaneously shorted to ground will damage occur.

A crowbar circuit at IN is capable of generating those

levels of currents, and then protection diodes from OUT

to IN are recommended. Normal power supply cycling or

system “hot plugging and unplugging” does not do any

damage.

A protection diode between OUT and VCONTROL is usually

not needed. The internal parasitic diode on VCONTROL of

the LT3085 handles microsecond surge currents of 1A to

10A. Again, this only occurs when using crowbar circuits

with large value output capacitors. Since the VCONTROL

pin is usually a low current supply, this is unlikely. Still,

a protection diode is recommended if VCONTROL can be

instantaneously shorted to ground. Normal power supply

cycling or system “hot plugging and unplugging” does

not do any damage.

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