LT6105 datasheet(19/28 Pages) AD

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LT6119-1/LT6119-2

611912f

For more information www.linear.com/LT6119-1

applicaTions inForMaTion

of ROUT and any parasitic and/or load capacitance. Note

that reducing RIN or increasing ROUT will both have the

effect of increasing the voltage gain of the circuit. If the

output capacitance is limiting the speed of the system, RIN

and ROUT can be decreased together in order to maintain

the desired gain and provide more current to charge the

output capacitance.

The response time of the comparators is the sum of the

propagation delay and the fall time. The propagation de-

lay is a function of the overdrive voltage on the input of

the comparators. A larger overdrive will result in a lower

propagation delay. This helps achieve a fast system re-

sponse time to fault events. The fall time is affected by

the load on the output of the comparator as well as the

pull-up voltage.

The LT6119 amplifier has a typical response time of 500ns

andthecomparatorshaveatypicalresponsetimeof500ns.

When configured as a system, the amplifier output drives

the comparator input causing a total system response

time which is typically greater than that implied by the

individually specified response times. This is due to the

overdrive on the comparator input being determined by

the speed of the amplifier output.

Internal Reference and Comparators

The integrated precision reference and comparators

combined with the high precision current sense allow

for rapid and easy detection of abnormal load currents.

This is often critical in systems that require high levels of

safety and reliability. The LT6119 comparators are opti-

mized for fault detection and are designed with latching

outputs. Latching outputs prevent faults from clearing

themselves and require a separate system or user to

reset the outputs. In applications where the compara-

tor output can intervene and disconnect loads from the

supply, latched outputs are required to avoid oscillation.

Latching outputs are also useful for detecting problems

that are intermittent. The comparator outputs on the

LT6119 are always latching and there is no way to disable

this feature.

Eachofthecomparatorshasoneinputavailableexternally,

with the two versions of the part differing by the polarity

of those available inputs. The other comparator inputs are

connected internally to the 400mV precision reference.

The input threshold (the voltage which causes the output

to transition from high to low) is designed to be equal to

that of the reference. The reference voltage is established

with respect to the device V– connection.

Comparator Inputs

ThecomparatorinputscanswingfromV–to60Vregardless

of the supply voltage used. The input current for inputs

well above the threshold is just a few pAs. With decreas-

ing input voltage, a small bias current begins to be drawn

out of the input near the threshold, reaching 50nA max

when at ground potential. Note that this change in input

bias current can cause a small nonlinearity in the OUTA

transfer function if the comparator inputs are coupled to

the amplifier output with a voltage divider. For example,

if the maximum comparator input current is 50nA, and

the resistance seen looking out of the comparator input

is 1k, then a change in output voltage of 50µV will be

seen on the analog output when the comparator input

voltage passes through its threshold. If both compara-

tor inputs are connected to the output then they must

both be considered.

Setting Comparator Thresholds

The comparators have an internal precision 400mV refer-

ence. In order to set the trip points of the LT6119-1 com-

parators, the output currents, IOVER and IUNDER, as well as

the maximum output current, IMAX, must be calculated:

IOVER =

VSENSE(OVER)

RIN

, IUNDER =

VSENSE(UNDER)

RIN

IMAX =

VSENSE(MAX)

RIN

where IOVER and IUNDER are the over and under currents

through the sense resistor which cause the comparators

to trip. IMAX is the maximum current through the sense

resistor.

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