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LTC2995

2995f

APPLICATIONS INFORMATION

2. Find RB to set the UV trip point of the monitor:

RB =

0.5V

10μA

•

4.5V

– 453

≅ 10k

3. Determine RC to complete the design:

RC =

10μA

–453

Ω – 100Ω≈ 442k

Power-Up and Undervoltage Lockout

As soon as VCC reaches approximately 1V during

power-up, the OV as well as TO1 and TO2 weakly pull to VCC

while the UV output asserts low indicating an undervolt-

age lockout condition. Above VCC = 2V (typical), the VH

and VL inputs take control. Once both VH inputs and VCC

are valid, an internal timer is started. After an adjustable

delay time, UV weakly pulls high.

When VCC falls below 1.9V, the LTC2995 indicates again

an undervoltage lockout (UVLO) condition by pulling low

UV while OV is cleared.

Threshold Accuracy

Reset threshold accuracy is important in a supply sensitive

system. Ideally, such a system would only reset if supply

voltages fell outside the exact threshold for a specified

margin. All LTC2995 VHn/VLn inputs have a relative

threshold accuracy of ±1.5% over the full operating

temperature range. For example, when the LTC2995 is

configured to monitor a 5V input with a 10% tolerance,

the desired UV trip point is 4.5V. Because of the ±1.5%

relative accuracy of the LTC2995, the UV trip point can be

anywhere between 4.433V and 4.567V which is 4.5V ±1.5%.

Likewise, the accuracy of the resistances chosen for RA,

RB, and RC can affect the UV and OV trip points as well.

Using the previous example, if the resistances used to set

the UV trip point have 1% accuracy, the UV trip range can

grow to between 4.354V and 4.650V. This is illustrated in

the following calculations.

The UV trip point is given as:

VUV = 0.5V

RA + RB

⎛

⎝

⎜

⎞

⎠

⎟

The two extreme conditions, with a relative accuracy of

1.5% and resistance accuracy of 1%, result in:

VUV(MIN) = 0.5V 0.985

RC 0.99

(RA + RB) 1.01

⎛

⎝

⎜

⎞

⎠

⎟

and

VUV(MAX) = 0.5V 1.015

RC 1.01

(RA + RB) 0.99

⎛

⎝

⎜

⎞

⎠

⎟

For a desired trip point of 4.5V,

RA + RB

= 8

Therefore,

VUV(MIN) = 0.5V 0.985

+ 8

0.99

1.01

= 4.3545V

⎛

⎝

⎜

⎞

⎠

⎟

and

VUV(MAX) = 0.5V 1.015

+ 8

1.01

0.99

= 4.650V

⎛

⎝

⎜

⎞

⎠

⎟

Glitch Immunity

In any supervisory application, noise on the monitored DC

voltage can cause spurious resets. To solve this problem

without adding hysteresis to the VH/VL comparators, which

would add error to the trip voltage, the LTC2995 lowpass

filters the output of the comparator. This filter causes the

output of the comparator to be integrated before assert-

ing the UV or OV logic. Any transient at the input of the

comparator must be of sufficient magnitude and duration

before the comparator will trigger the output logic. The

Typical Performance Characteristics section shows a graph

of the Typical Transient Duration vs Comparator Overdrive.

In temperature monitoring, the voltage at VPTAT must

exceed a threshold for five consecutive temperature up-

date intervals before the respective TO pin is pulled low.

Once the VPTAT voltage crosses back the threshold with

an additional 20mV of hysteresis, the respective TO pin

is released after a single update interval and an additional

delay adjustable by the capacitor on TMR.

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