LTC2901-1IGN datasheet(11/16 Pages) LINER

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LTC2901

2901is sn2901

APPLICATIO S I FOR ATIO

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:

1.21V

÷ 1mA = 1.21kΩ.

Tables 2 and 3 offer suggested 1% resistor values for

various adjustable applications.

Although all four 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

VCC for the chip. Filter capacitors on the V3 and V4 inputs

are allowed.

Power-Down

On power-down, once any of the VX inputs drop below

their threshold, RST and COMPX are held at a logic low.

A logic low of 0.4V is guaranteed until both V1 and V2

drop below 1V. If the bandgap reference becomes invalid

(VCC < 2V typ), the part will reprogram once VCC rises

above 2.4V max.

Monitor Output Rise and Fall Time Estimation

All of the outputs (RST, COMPX, WDO) have strong pull-

down capability. If the external load capacitance (CLOAD)

for a particular output is known, output fall time (10% to

90%) is estimated using:

tFALL ≈ 2.2 • RPD • CLOAD

where RPD is the on-resistance of the internal pull-down

transistor. The typical performance curve (VOL vs ISINK)

demonstrates that the pull-down current is somewhat

linear versus output voltage. Using the 25

°C curve, RPD is

estimated to be approximately 40

Ω. Assuming a 150pF

load capacitance, the fall time is about 13.2ns.

Although the outputs are considered to be “open-drain,”

they do have a weak pull-up capability (see COMPX or RST

Figure 5. Setting the Positive Adjustable Trip Point

–

– 0.5V

2901 F05

V3 OR V4

VTRIP

LTC2901

Figure 6. Setting the Negative Adjustable Trip Point

–

2901 F06

VREF

VTRIP

LTC2901

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

VSUPPLY (V)

VTRIP (V)

R3 (k

Ω)

R4 (k

Ω)

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 –ADJ Input

VSUPPLY (V)

VTRIP (V)

R3 (k

Ω)

R4 (k

Ω)

–2

–1.87

187

121

–5

–4.64

464

121

–5.2

–4.87

487

121

–10

–9.31

931

121

–12

–11.30

1130

121

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