ADM2914

Rev. 0 | Page 10 of 16

THRESHOLD ACCURACY

The reset threshold accuracy is fundamental, especially at lower

voltage levels. Consider an FPGA application that requires a 1 V

core voltage input with tolerance of ±5%, where the supply has a

specified regulation, for example, ±1.5%. As shown in Figure 19, to

ensure that the supply is within the FPGA input voltage require-

ment range, its voltage level must be monitored for UV and OV

conditions. The voltage swing on the supply itself causes the

voltage band available for setting the monitoring threshold to be

quite narrow. In this example, the threshold voltages, including

the tolerances, must fit within a monitor region of only 0.035 V.

The ADM2914 device with 0.1% resistors can achieve this level

of accuracy.

1.05V

TIME

VOLTAGE

1.015V

1V CORE

VOLTAGE

0.985V

0.95V

UV

+5% TOLERANCE

3.5% RANGE FOR

OV MONITORING

3.5% RANGE FOR

UV MONITORING

+1.5% SUPPLY REGULATION

–1.5% SUPPLY

REGULATION

–5% TOLERANCE

Figure 19. Monitoring Threshold Accuracy Example

VOLTAGE MONITORING EXAMPLE

To illustrate how the ADM2914 device works in a real application,

consider the 1 V input example shown in Figure 19, with the

addition of a −12 V rail.

The first step is to choose the nominal current flow through

both voltage divider circuits, for example, 5 μA.

For the 1 V ± 5% input, due to the specified ±1.5% regulation of

the supply, the UV and OV thresholds should be set in the middle

of the voltage monitoring band. In this case, on the ±3.25%

points of the supply, the UV threshold is 0.9675 V and the OV

threshold is 1.0325 V.

Input these values into Equation 1.

 





kΩ

5

.

96

10

5

0325

.

1

1

)

5

.

0

(

6









Z

R

 





kΩ

42

.

6

kΩ

5

.

96

10

5

9675

.

0

1

)

5

.

0

(

6











Y

R

Equation 3.

kΩ

5

.

96

kΩ

42

.

6

kΩ

5

.

96

10

5

1

6













X

R

This design approach meets the application specifications. As

described previously, the 1 V rail is specified with an input

requirement of ±5% and a supply tolerance of ±1.5%. This

effectively means that the OV threshold of the monitoring

device, including all the tolerance factors, must fit within the

1.015 V to 1.05 V range. Similarly, the UV threshold range must

be between 0.95 V and 0.985 V.

The four worst-case scenarios of minimum and maximum

undervoltage and overvoltage thresholds are calculated as follows:

Minimum overvoltage threshold



V

015

.

1

V

016

.

1

)

001

.

1

)(

500

,

96

(

)

999

.

0

)(

6420

500

,

96

(

1

4925

.

0

%

1

.

0

%)

1

.

0

(

%)

1

.

0

(

1

%)

5

.

1

V

5

.

0

(

_





















































Z

Y

X

MIN

OV

R

R

R

V

Maximum overvoltage threshold



V

05

.

1

V

049

.

1

%

1

.

0

%)

1

.

0

(

%)

1

.

0

(

1

%)

5

.

1

V

5

.

0

(

_



































Z

Y

X

MAX

OV

R

R

R

V

The maximum and minimum overvoltage threshold values lie

within the 1.015 V to 1.05 V range specified. The minimum and

maximum undervoltage thresholds are calculated as follows:

Minimum undervoltage threshold

 



V

95

.

0

V

953

.

0

%

1

.

0

%

1

.

0

%)

1

.

0

(

1

%)

5

.

1

V

5

.

0

(

_



































Z

Y

X

MIN

UV

R

R

R

V

Maximum undervoltage threshold





V

985

.

0

V

984

.

0

%

1

.

0

%

1

.

0

%)

1

.

0

(

1

%)

5

.

1

V

5

.

0

(

_































Z

Y

X

MAX

UV

R

R

R

V

Again, these values fit within the specified undervoltage

monitoring range. All four worst-case scenarios satisfy the

tolerance requirement; therefore, the design approach is valid.

ADM2914

VH1

5V

1V RAIL

GND

VL1

VL3

VH3

REF

UV

SEL

OV

–12V RAIL

2.49MΩ

23.4kΩ

89.8kΩ

96.5kΩ

6.42Ω

96.5kΩ

Figure 20. Positive and Negative Supply Monitor Example