LTC2903 データシートの表示（PDF） - Linear Technology

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LTC2903

Precision Quad Supply Monitor in 6-Lead SOT-23

Linear Technology 

LTC2903-1

APPLICATIO S I FOR ATIO

Calculate the trip voltage from:

VTRIP = 0.5V1+ RR21

Table 1 contains suggested 1% resistor values for the ADJ

input to obtain nominal –11.5% thresholds.

Table 1. Suggested 1% Resistor Values for the ADJ Input

VSUPPLY (V)

12

VTRIP (V)

10.75

R1 (kΩ)

2050

R2(kΩ)

100

10

8.95

1690

100

8

7.15

1330

100

7.5

6.7

1240

100

6

5.38

976

100

5

4.435

787

100

3.3

2.935

487

100

3

2.66

432

100

2.5

2.2

340

100

1.8

1.605

221

100

1.5

1.325

165

100

1.2

1.065

113

100

1

0.884

76.8

100

0.9

0.795

59

100

Connect unused supervisor inputs to the highest supply

voltage available (typically V1). On the LTC2903-C1, the

negative V4 input must always be applied.

Implications of Threshold Accuracy

Specifying system voltage margin for worst-case opera-

tion requires consideration of three factors: power supply

tolerance, IC supply voltage tolerance and supervisor re-

set threshold accuracy. Highly accurate supervisors ease

the design challenge by decreasing the overall voltage

margin required for reliable system operation. Consider a

5V system with a ±10% power supply tolerance band.

System ICs powered by this supply must operate reliably

within this band (and a little more, as explained below).

The bottom of the supply tolerance band, at 4.5V (5V –

10%), is the exact voltage at which a perfectly accurate

supervisor generates a reset. Such a perfectly accurate

supervisor does not exist—the actual reset threshold may

vary over a specified band (±1.5% for the LTC2903-1 su-

pervisors). Figure 4 shows the typical relative threshold

accuracy for all four inputs, over temperature.

With this variation of reset threshold in mind, the nominal

reset threshold of the supervisor resides below the mini-

mum supply voltage; just enough so that the reset thresh-

old band and the power supply tolerance bands do not

overlap. If the two bands overlap, the supervisor could

generate a false or nuisance reset when the power supply

remains within its specified tolerance band (say, at 4.6V).

Adding half of the reset threshold accuracy spread (1.5%)

to the ideal 10% thresholds puts the LTC2903-1 thresh-

olds at 11.5% (typ) below the nominal input voltage. For

example, the 5V typical threshold is 4.425V, or 75mV

below the ideal threshold of 4.500V. The guaranteed

threshold lies in the band between 4.500V and 4.350V

over temperature.

The powered system must work reliably down to the

lowest voltage in the threshold band or risk malfunction

before the reset line falls. In the 5V example, using the

1.5% accurate supervisor, the system ICs must work

down to 4.35V. System ICs working with a ±2.5% accurate

supervisor must operate down to 4.25V, increasing the

required system voltage margin and the probability of

system malfunction.

1.5

1.0

0.5

0

–0.5

–1.0

–1.5

–50

–25 0 25 50

TEMPERATURE (°C)

75 100

29031 F04

Figure 4. LTC2903 Typical Threshold Accuracy vs Temperature

29031f

11

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