LT3045 datasheet(40/45 Pages) AD

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Data Sheet

LT3097

APPLICATIONS INFORMATION

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Rev. 0 | 40 of 45

Negative Side Current Limit

=3.75 A×kΩ

RILIMN

(7)

For example, a 7.5 kΩ resistor programs the current limit to 500

mA, and a 15 kΩ resistor programs the current limit to 250 mA. For

good accuracy, Kelvin connect this resistor to the GND pin (pin 13)

of the LT3097.

When the INN-to-OUTN differential is greater than 7 V, the foldback

circuitry of the negative regulator of the LT3097 decreases the

internal current limit. As a result, the internal current limit can

override the externally programmed current-limit level to keep the

LT3097 within its SOA. See Figure 57.

ILIMN is not designed to serve as a current monitoring pin. If the

external current limit is not used, connect ILIMN to GND.

POSITIVE OUTPUT OVERSHOOT RECOVERY

During a load-step change from full load to no load (or light

load), the positive output voltage overshoots before the regulator

responds to turn the power transistor off. Given that there is no load

(or a light load) present at the positive output, it takes a long time to

discharge the output capacitor.

As shown in the Figure 122, the LT3097 incorporates an overshoot

recovery circuitry that turns on a current sink to discharge the

output capacitor in the event OUTSP is higher than SETP. This

current is typically about 4 mA. No load recovery is disabled for

positive input voltages less than 2.5 V or positive output voltages

less than 1.5 V.

If OUTSP is externally held more than SETP, the current sink turns

on in an attempt to restore OUTSP to its programmed voltage. The

current sink remains on until the external circuitry releases OUTSP.

NEGATIVE OUTPUT OVERSHOOT RECOVERY

During a load-step change from full load to no load (or light

load), the negative output voltage overshoots before the regulator

responds to turn the power transistor off. Given that there is no load

(or a light load) present at the negative output, it takes a long time

to discharge the output capacitor.

As illustrated in the Figure 123, the LT3097 incorporates an over-

shoot recovery circuitry that turns on a current source to discharge

the output capacitor in the event OUTSN is higher than SETN. This

current is typically about 3.5 mA.

If OUTSN is externally held more than SETN, the current source

turns on in an attempt to restore OUTSN to its programmed volt-

age. The current source remains on until the external circuitry

releases OUTSN.

PCB LAYOUT CONSIDERATIONS

Given the high bandwidth and ultra-high PSRR of the LT3097,

a careful PCB layout must be employed to achieve full device per-

formance. Figure 130 shows the EVAL-LT3097-AZ evaluation board

with a layout that delivers the full performance of the regulator.

For more details refer to the LT3097 evaluation board user guide

(EVAL-LT3097-AZ).

Figure 130. EVAL-LT3097-AZ Evaluation Board

THERMAL CONSIDERATIONS

The positive and negative regulators of the LT3097 have internal

power and thermal limiting circuits that protect the device under

overload conditions. The thermal shutdown temperature is nominal-

ly 165°C for the positive regulator and 167 °C for the negative

regulator, with about 8°C of hysteresis for each regulator. For

continuous normal load conditions, do not exceed the maximum

junction temperature of 125°C. It is important to consider all sour-

ces of thermal resistance from junction to ambient, which includes

junction to case, case to heatsink interface, heatsink resistance,

or circuit board to ambient as the application dictates. Additionally,

consider all heat sources close to the LT3097.

The underside of the DFN package has exposed metal from the

lead frame to the die attachment. Note that the exposed-pad

pin 23 is electrically connected to the ground (pin 19), and the

exposed-pad pin 24 is electrically connected to INN (pins 6 and 7).

This package allows heat to directly transfer from the die junction to

the PCB metal to limit the maximum operating junction temperature.

The dual, inline pin arrangement allows the metal to extend beyond

the ends of the package on the topside (component side) of the

PCB.

For surface-mount devices, heat sinking is accomplished by using

the heat-spreading capabilities of the PCB and its copper traces.

Copper board stiffeners and plated throughholes can also be used

to spread the heat generated by the LDO regulator.

Table 5 lists the thermal resistance as a function of the copper

area on a fixed board size. All measurements were taken in still

air on a 4-layer FR4 board with 1 oz solid internal planes and

2 oz top and bottom planes with a total board thickness of 1.6

mm. The four layers were electrically isolated with no thermal vias

present. PCB layers, copper weight, board layout, and thermal

vias affect the resultant thermal resistance. For more information

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