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LT3086

3086fa

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applicaTions inForMaTion

The master LT3086 is connected exactly the same as a

single regulator where its output current monitor voltage

seen at its ILIM pin is used as the common current tracking

signal.TheslavedevicesconnectthissignaltotheirTRACK

pins to make their output current equal to the master’s.

The TRACK pin has an internal pull-up current that is

typically 15µA at 0.75V. When the TRACK pin is unused,

the pin is pulled up and clamped at 1.25V, disabling the

current tracking amplifier. When the TRACK pin is con-

nected to the master current tracking signal, the TRACK

pinvoltageispulledbelowthe1.2Vthreshold,enablingthe

current tracking amplifier and disabling the slave’s 50µA

reference current, ISET. Disabling the reference current

ensures that the master is the only device controlling the

output voltage. Set the maximum master current tracking

signal to less than 0.8V to prevent external current limit

from triggering prematurely. To prevent the slave current

tracking amplifier from ever being disabled, the slave

TRACK pin must be tied to the master ILIM pin. The master

ILIM pin has an internal 1V clamp that is below the slave

1.2V current tracking amplifier enable threshold.

When multiple slaves are used, a smaller master RMON

resistor should be used to compensate for the pull-up

currents from all the TRACK pins of the slaves. For ex-

ample, a master sourcing 2.1A typically has 0.697V at its

ILIM pin with an RMON resistor of 332Ω. Referring to the

TRACK pin pull-up current curve in the Typical Perfor-

mance Characteristics, with 0.697V on the TRACK pin,

each slave typically adds 15µA to the master’s 2.1mA

IMON output. For an application with 5 slaves connected,

decrease RMON’s value to:

RMON =

0.697V

[2.1mA

+ 5•15µA

(

)]

= 325Ω

The closest 1% resistor value equals 324Ω.

All slave regulators must have their SET pins connected

to the master SET pin. The TRACK amplifier operates by

adjusting the slave internal reference voltage slightly as

a function of the difference in master and slave current

monitor voltages. This has a strong effect on the slave

output current, which forces the slave output current to

match the master.

Mismatch between master and slave internal reference

voltages and current monitor outputs, offset in the slave

TRACK amplifier and TRACK pin pull-up currents all

contribute to output current sharing error. In the case of

negative offset, a slave runs less current than the master.

At very light loads, negative offset enables the slave output

overshoot pull-down circuit, forcing the master to supply

current to keep the output voltage within regulation. As a

result, quiescent current may increase for very light loads

in the master/slave configuration.

In some applications, multiple regulators may be spaced

some distance apart to optimize heat distribution. That

makes the use of low resistance traces important to con-

necteachregulatortothelocalgroundsystemandtoavoid

ground loops created by load currents. Ground currents

can be as high as 30mA at 1.5A and 50mA at 2.1A, for

each regulator. Limiting differential ground pin voltages to

less than 10mV minimizes tracking errors. Ground trace

resistance between master and slaves should be less than

10mV/30mA = 0.33Ω at 1.5A load, and 10mV/50mA =

0.2Ω for 2.1A load.

Output Capacitance

The LT3086 regulator is stable with a wide range of output

capacitors. The ESR of the output capacitor affects stabil-

ity, most notably with small capacitors. Use a minimum

output capacitor of 10µF with an ESR of 0.1Ω or less to

prevent oscillations. The output load transient response is

a function of output capacitance. Larger values of output

capacitance decrease the peak deviations and provide im-

proved transient response for larger load current changes.

For applications with large load current transients, a low

ESR ceramic capacitor in parallel with a bulk tantalum

capacitor often provides an optimally damped response.

For example, a 47µF tantalum capacitor with ESR = 0.1Ω

in parallel with the 10µF ceramic capacitor with ESR <

0.01Ω reduces output deviation by about 2:1 for large

transient loads and increases loop phase margin.

Give extra consideration to the use of ceramic capacitors.

Manufacturers make ceramic capacitors with a variety of

dielectrics, each with different behavior across tempera-

ture and applied voltage. The most common dielectrics

are specified with EIA temperature characteristic codes

of Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics

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