Datasheet

LTC2996
11
2996f
applicaTions inForMaTion
Output Noise Filtering
The V
PTAT
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:
2






t
AVG
=
0.01
[ ]
°
C Hz
T
NOISE
where t
AVG
is the averaging time and T
NOISE
the desired
temperature noise in °C RMS. For example, if the desired
noise performance is 0.01°C RMS, set the averaging time
to one second. See Typical Performance Characteristics.
Temperature Monitoring
The LTC2996 continuously compares the voltage at V
PTAT
to the voltages at the pins VTH and VTL to detect either an
overtemperature (OT) or undertemperature (UT) condition.
The VTH comparator output drives the open-drain logic
output pin OT and the VTL comparator output drives the
open-drain logic output pin UT. The voltage at V
PTAT
must
exceed a threshold for five consecutive temperature update
intervals (3.5ms each) before the respective output pin is
pulled low. Once the V
PTAT
voltage crosses the threshold
with an additional 20mV of hysteresis, the respective output
pin is released after a single update interval.
Temperature Monitor Design Example
The LTC2996 can be configured to give an alert if the
temperature of the internal sensor falls below 0°C or rises
above 90°C. Tie the D
+
pin to V
CC
to select the internal
sensor. The voltages at VTL and VTH are set to:
VTL =(0K + 273.15K) • 4
mV
K
= 1.093V
VTH =(90K + 273.15K) • 4
mV
K
= 1.453V
When V
PTAT
falls below 1.093V, UT is pulled low. Once the
temperature rises again and V
PTAT
reaches 1.093V plus
a hysteresis of 20mV, UT is released high again. Accord-
ingly, OT is pulled low if temperature increases to 90°C as
V
PTAT
reaches 1.453V and is released high if V
PTAT
drops
again below 1.433V.
Temperature Thresholds
The threshold voltages at VTL and VTH can be set with
the 1.8V reference voltage (V
REF
) and a resistive divider
as shown in Figure 5.
Figure 5. Temperature Thresholds
The following design procedure can be used to size the
resistive divider.
1. Calculate Threshold Voltages:
VTL = T1• 4
mV
K
•
η
ACT
1.004
VTH = T2 • 4
mV
K
•
η
ACT
1.004
2996 F05
V
PTAT
R
TC
R
TB
R
TA
V
REF
= 1.8V
1.8V
SLOPE = • 4
1.004
ACT
η
O
T
200K T
1
T
2
450K
VT2
VT1
O.8V
K
mV