LTC2938IDE datasheet(12/20 Pages) LINER

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LTC2938/LTC2939

293839ff

APPLICATIONS INFORMATION

Figure 3. Setting the Positive Adjustable Trip Point

Using the Adjustable Thresholds

The reference inputs on the V3 and/or V4 comparators are

set to 0.5V when the positive adjustable modes are se-

lected (Figure 3). The LTC2939 V5 and V6 comparators are

always in positive adjustable mode with a 0.5V reference.

The tap point on an external resistive divider, connected

between the positive voltage being sensed and ground, is

connected to the high impedance, adjustable inputs (V3,

V4, V5 and V6). Calculate the trip voltage from:

TRIP =+

⎛

⎝⎜

⎞

⎠⎟

.•

In a negative adjustable application, the minimum value for

R4 is limited by the sourcing capability of VREF (±1mA).

With no other load on VREF, R4 (minimum) is:

1 210

Tables 2 and 3 offer suggested 1% resistor values for

various adjustable applications assuming 5% monitor

thresholds.

Table 2. Suggested 1% Resistor Values for the ADJ Inputs

VSUPPLY (V)

VTRIP (V)

R3 (kΩ)

R4 (kΩ)

11.25

2150

100

9.4

1780

100

7.5

1400

100

7.5

1300

100

5.6

1020

100

4.725

845

100

3.3

3.055

511

100

2.82

464

100

2.5

2.325

365

100

1.8

1.685

237

100

1.5

1.410

182

100

1.2

1.120

124

100

0.933

86.6

100

0.9

0.840

68.1

100

Table 3. Suggested 1% Resistor Values for the Negative

ADJ Inputs

VSUPPLY (V)

VTRIP (V)

R3 (kΩ)

R4 (kΩ)

–2

–1.87

1.87

121

–5

–4.64

464

121

–5.2

–4.87

487

121

–10

–9.31

931

121

–12

–11.30

1130

121

Although all of the supply monitor comparators have

built-in glitch immunity, bypass capacitors on V1 and V2

are recommended because the greater of V1 or V2 is also

the supply for the device. Filter capacitors on the V3, V4,

V5 and V6 inputs are allowed.

In the negative adjustable mode, the high impedance,

adjustable input on the V4 comparator is connected to

ground (Figure 4). The tap point on an external resistive

divider, connected between the negative voltage being

sensed and the VREF output, is connected to the high

impedance adjustable input (V4). VREF provides the nec-

essary level shift required to operate at ground. The nega-

tive trip voltage is calculated from:

VV No

TRIP

REF

= −

•;

min

1 210

Figure 4. Setting the Negative Adjustable Trip Point

– 0.5V

293839 F03

V3, V4, V5 OR V6

VTRIP

LTC2938/LTC2939

293839 F04

VREF

VTRIP

LTC2938/LTC2839

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