Datasheet

 2001-2012 Microchip Technology Inc. DS21439D-page 5
TC620/TC621
3.0 DETAILED DESCRIPTION
The TC620 has a positive temperature coefficient tem-
perature sensor and a dual threshold detector. Temper-
ature set point programming is accomplished with
external resistors from the HIGH SET and LOW SET
inputs to V
DD.
The HIGH LIMIT and LOW LIMIT outputs
remain low as long as measured temperature is below
set point values. As measured temperature increases,
the LOW LIMIT output is driven high when temperature
equals the LOW SET set point (±3°C max). If tempera-
ture continues to climb, the HIGH LIMIT output is driven
high when temperature equals the HIGH SET set point
(Figure 3-1). The CONTROL (hysteresis) output is
latched in its active state at the temperature specified
by the HIGH SET resistor. CONTROL is maintained
active until temperature falls to the value specified by
the LOW SET resistor.
FIGURE 3-1: TC620/TC621 Input vs.
Output Logic
3.1 Programming the TC620
The resistor values to achieve the desired trip point
temperatures on HIGH SET and LOW SET are
calculated using Equation 3-1:
EQUATION 3-1:
For example, a 50°C setting on either the HIGH SET
or LOW SET input is calculated using Equation 3-2 as
follows:
EQUATION 3-2:
Care must be taken to ensure the LOW SET program-
ming resistor is a smaller value than the HIGH SET
programming resistor. Failure to do this will result in
erroneous operation of the CONTROL output.
Care must also be taken to ensure the LOW SET
temperature setting is at least 5°C lower than the HIGH
SET temperature setting.
Figure 3-2 can help the user obtain an estimate of the
external resistor values required for the desired LOW
SET and HIGH SET trip points.
FIGURE 3-2: TC620 Sense Resistors vs.
Trip Temperature
3.2 Built-in Hysteresis
To prevent output “chattering” when measured
temperature is at (or near) the programmed trip point
values, the LOW SET and HIGH SET inputs each have
built-in hysteresis of -2°C below the programmed
settings (Figure 3-3).
FIGURE 3-3: Built-In Hysteresis on Low
Limit and High Limit Outputs
As shown, the outputs remain in their active state
(hysteresis) until temperature falls an additional 2°C
below the user’s setting.
Low Set Point
High Set Point
Low Limit Output
High Limit Output
Control Output (Cool Option
Control Output (Heat Option)
Temperature
R
TRIP
= 0.5997 x T
2.1312
Where:
R
TRIP
= Programming resistor in Ohms
T = The desired trip point temperature in degrees
Kelvin.
R
SET
= 0.5997 x ((50 + 273.15)
2.1312
) = 133.6 k
TEMPERATURE (°C)
RESISTANCE, R
TRIP
(kΩ)
-55 -35 -15 5 25 45 65 85 105 125
50
100
150
200
250
Set Point
(Set Point 2°C)
High Limit
or Low Limit
Output