LT1787 datasheet(16/24 Pages) AD

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LT6118

6118f

For more information www.linear.com/LT6118

The comparator has one input available externally. The

other comparator input is 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 Input

ThecomparatorinputcanswingfromV–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 input is 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.

applicaTions inForMaTion

Setting Comparator Threshold

Thecomparatorhasaninternal400mVprecisionreference.

In order to set the trip point of the LT6118 comparator as

configured in Figure 12, the input sense voltage at which

the comparator will trip, VSENSE(TRIP) must be calculated:

VSENSE(TRIP) = ISENSE(TRIP) • RSENSE

The selection of RIN is discussed in the Selection of Exter-

nal Input Gain Resistor RIN section. Once RIN is selected,

ROUT can be calculated:

ROUT = RIN

400mV

VSENSE(TRIP)

Since the amplifier output is connected directly to the

comparator input, the gain from VSENSE to VOUT is:

AV =

400mV

VSENSE(TRIP)

As shown in Figure 13, R2 can be used to increase the

gain from VSENSE to VOUT without changing VSENSE(TRIP).

As before, R1 can be easily calculated:

= RIN

400mV

VSENSE(TRIP)

OUTA

IOUTA

–

V+

SENSEHI

INC

6118 F12

V+

V–

LT6118

SENSELO

OUTC

VLE

VPULLUP

LOAD

VSUPPLY

VSENSE

RSENSE

OVERCURRENT

FLAG

RIN

–

V–

ISENSE =

VSENSE

RSENSE

ROUT

VOUT

400mV

REFERENCE

CLC

Figure 12. Basic Comparator Configuration

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