LTC2930

7

2930fa

APPLICATIONS INFORMATION

Supply Monitoring

The LTC2930 is a low power, high accuracy configurable six

supply monitoring circuit with a common reset output. An

external capacitor sets the reset timeout period. An external

resistive divider between VREF, VPG and GND selects 1 of

16 possible input voltage monitor combinations. All six volt-

age inputs must be above their predetermined thresholds

for the reset not to be activated. The LTC2930 asserts the

reset output during power-up, power-down and brownout

conditions on any one of the voltage inputs.

Power-Up

The greater of V1 and V2 serves as the internal supply

for the RST pin. This ensures that the RST output will be

low as soon as either V1 or V2 reaches 1V. The RST output

remains low until the part is configured. Once voltage

thresholds are set, if any of the supply monitor inputs is

below its configured threshold, RST will be a logic low.

Once all the monitor inputs rise above their thresholds,

an internal timer is started and RST is released after the

input impedance will be low (10kΩ typical).

Threshold Accuracy

Consider a 5V system with ±5% tolerance. The 5V supply

may vary between 4.75V to 5.25V. System ICs powered by

this supply must operate reliably within this band (and a

little more as explained below). A perfectly accurate su-

Table 1. Voltage Threshold Modes*

MODE V1 (V) V2 (V) V3 (V) V4 (V) R1 (kΩ) R2 (kΩ)

0

5.0

3.3

ADJ

ADJ

Open

Short

0.000

1

5.0

3.3

ADJ

–ADJ

93.1

9.53

0.094

2

3.3

2.5

ADJ

ADJ

86.6

16.2

0.156

3

3.3

2.5

ADJ

–ADJ

78.7

22.1

0.219

4

3.3

2.5

1.5

ADJ

71.5

28.0

0.281

5

5.0

3.3

2.5

ADJ

66.5

34.8

0.344

6

5.0

3.3

2.5

1.8

59.0

40.2

0.406

7

5.0

3.3

2.5

1.5

53.6

47.5

0.469

8

5.0

3.0

2.5

ADJ

47.5

53.6

0.531

9

5.0

3.0

ADJ

ADJ

40.2

59.0

0.594

10

3.3

2.5

1.8

1.5

34.8

66.5

0.656

11

3.3

2.5

1.8

ADJ

28.0

71.5

0.719

12

3.3

2.5

1.8

–ADJ

22.1

78.7

0.781

13

5.0

3.3

1.8

–ADJ

16.2

86.6

0.844

14

5.0

3.3

1.8

ADJ

9.53

93.1

0.906

15

5.0

3.0

1.8

ADJ

Short

Open

1.000

*V5 and V6 are always adjustable (ADJ).

pervisor for this supply generates a reset at exactly 4.75V,

however no supervisor is this perfect. The actual reset

threshold of a supervisor varies over a specified band;

the LTC2930 varies ±1.5% around its nominal threshold

voltage (see Figure 1) over temperature.

The reset threshold band and the power supply tolerance

bands should not overlap. This prevents false or nuisance

resets when the power supply is actually within its speci-

fied tolerance band.

The LTC2930 has a ±1.5% reset threshold accuracy, so a

“5%” threshold is typically set to 6.5% below the nominal

input voltage. Therefore, a typical 5V, “5%” threshold is

4.675V. The threshold is guaranteed to lie in the band be-

tween 4.750V and 4.600V over temperature. The powered

system must work reliably down to the low end of the

threshold band, or risk malfunction before a reset signal

is properly issued.

A less accurate supervisor increases the required system

voltage margin and increases the probability of system

malfunction. The LTC2930 ±1.5% specification improves

the reliability of the system over supervisors with wider

threshold tolerances.

5V

4.75V

4.675V

±1.5%

THRESHOLD

BAND

4.6V

NOMINAL

SUPPLY

VOLTAGE

SUPPLY TOLERANCE

MINIMUM

RELIABLE

SYSTEM

VOLTAGE

IDEAL

SUPERVISOR

THRESHOLD

REGION OF POTENTIAL MALFUNCTION

–5%

–6.5%

–8%

Figure 1. 1.5% Threshold Accuracy Improves System Reliability