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LTC2995

2995f

APPLICATIONS INFORMATION

Output Noise Filtering

The VPTAT output typically exhibits 0.6mV RMS (0.25°C

RMS) noise. For applications which require lower noise

digital or analog averaging can be applied to the output.

Choose the averaging time according to:

⎛

⎝

⎜

⎞

⎠

⎟

t AVG =

0.01[]

°CHz

TNOISE

where tAVG is the averaging time and TNOISE the desired

temperature noise in °C RMS. For example, if the desired

noise performance is 0.015°C RMS, set the averaging time

to one second. See Typical Performance Characteristics.

Temperature Monitoring

The LTC2995 continuously compares the voltage at VPTAT

to the voltages at the pins VT1 and VT2 to detect either an

overtemperature (OT) or undertemperature (UT) condition.

The VT1 comparator output drives the open-drain logic

output pin TO1 and the VT2 comparator output drives the

open-drain logic output pin TO2. The polarity of these

comparisons is configured via the three-state polarity

select pin (PS) (Table 3).

Table 3. Temperature Polarity Selection

PS PIN

FUNCTION

CONDITION

OUTPUT

VCC

VT1 Undertemperature

Threshold

VPTAT < VT1

TO1 Pulled Low

VT2 Overtemperature

Threshold

VPTAT > VT2

TO2 Pulled Low

Open

VT1 Overtemperature

Threshold

VPTAT > VT1

TO1 Pulled Low

VT2 Overtemperature

Threshold

VPTAT > VT2

TO2 Pulled Low

GND

VT1 Undertemperature

Threshold

VPTAT < VT1

TO1 Pulled Low

VT2 Undertemperature

Threshold

VPTAT < VT2

TO2 Pulled Low

If pin PS is connected to VCC, the voltage on VT1 becomes

an undertemperature threshold and the voltage on VT2

an overtemperature threshold. In this configuration TO1 is

pulled low if the voltage VPTAT falls during five consecutive

conversions below the undertemperature threshold VT1.

Once pulled low, TO1 is released high again if VPTAT rises

above VT1 plus an additional hysteresis of about 20mV.

Accordingly, T02 is pulled low if the voltage VPTAT rises

above the overtemperature threshold VT2 and –once pulled

low– TO2 is released high if VPTAT falls below VT2 minus

an additional hysteresis of about 20mV. Leaving PS uncon-

nected configures both VT1 and VT2 as overtemperature

thresholds and connecting PS to GND configures them

both as undertemperature thresholds. If the internal and

external sensors are monitored alternately by leaving DS

unconnected, VT1 becomes a dedicated threshold for the

internal sensor and VT2 becomes a dedicated threshold

for the external sensor.

Temperature Monitor Design Example

The LTC2995 can be configured to give an early warning

if the temperature of the internal sensor rises above 60°C

and an alarm if the temperature passes 90°C. Tie the DS

pin to VCC to select the internal sensor and leave the pin

PS unconnected to configure both input voltages VT1 and

VT2 as overtemperature thresholds. The voltages at VT1

and VT2 are set to:

VT1

=(60K + 273.15K) • 4

= 1.332V

VT2

=(90K + 273.15K) • 4

= 1.452V

When VPTAT reaches the threshold voltage on pin VT1, TO1

is pulled low indicating an overtemperature early warning.

If the temperature reaches 90°C TO2 is also pulled low,

indicating an overtemperature alarm.

Once the temperature drops below each threshold, the

corresponding TO pins will return high after a time-out-

period (tUOTO) set by the capacitor connected to TMR.

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