LTC2995CUD View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2995CUD' PDF : 20 Pages View PDF
LTC2995
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[°C Hz
TNOISE
]⎞⎟
⎠
2
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
VT1 Undertemperature
Threshold
VPTAT < VT1
VCC
VT2 Overtemperature
Threshold
VPTAT > VT2
Open
VT1 Overtemperature
Threshold
VT2 Overtemperature
Threshold
VPTAT > VT1
VPTAT > VT2
VT1 Undertemperature
Threshold
VPTAT < VT1
GND
VT2 Undertemperature
Threshold
VPTAT < VT2
OUTPUT
TO1 Pulled Low
TO2 Pulled Low
TO1 Pulled Low
TO2 Pulled Low
TO1 Pulled Low
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) • 4mV = 1. 332V
K
VT2 =(90K + 273.15K) • 4mV = 1.452V
K
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.
2995f
12
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