ACT8810
Active- Semi
Rev 4, 01-Oct-09
ActivePathTM CHARGER
FUNCTIONAL DESCRIPTION CONT’D
where RNOM is the nominal thermistor resistance at
room temperature, and kHOT and kCOLD are the ratios
of the thermistor's resistance at the desired hot and
cold thresholds, respectively.
Simple Solution
The ACT8810 was designed to accommodate most
requirements with very little design effort, but also
provides flexibility when additional control over a
design is required. Initial thermistor selection is
accomplished by choosing one that best meets the
following requirements:
Figure 11:
Simple Configuration
R
R
NOM = 5kꢀ/kHOT, and
NOM = 25kꢀ/kCOLD
ACT8810
100µA
where kHOT and kCOLD for a given thermistor can be
found on its characteristic tables.
+
VTHH
Li+ Battery
+
Pack
Taking a 0°C to 40°C application using a "curve 2"
NTC for this example, from the characteristic tables
one finds that kHOT and kCOLD are 0.5758 and 2.816,
respectively, and the RNOM that most closely
satisfies these requirements is therefore around
8.8kꢀ. Selecting 10kꢀ as the nearest standard
value, calculate kCOLD and kHOT as:
–
TH
+
NTC
–
VTHL
–
k
COLD = VTHL/(ITH × RNOM) = 2.5V/(100µA × 10kꢀ) = 2.5
HOT = VTHH/(ITH × RNOM) = 0.5V/(100µA × 10kꢀ) = 0.5
Design Procedure
k
When designing with thermistors it is important to
keep in mind that their nonlinear behavior typically
allows one to directly control no more than one
threshold at a time. As a result, the design
procedure can change depending on which
threshold is most critical for a given application.
Identifying these values on the curve
2
characteristic tables indicates that the resulting
operating temperature range is 2°C to 44°C, vs. the
design goal of 0°C to 40°C. This example
demonstrates that one can satisfy common
operating temperature ranges with very little design
effort.
Most application requirements can be solved using
one of three cases,
Fix VTHH
1) Simple solution
For demonstration purposes, supposing that we
had selected the next closest standard thermistor
value of 6.8kꢀ in the example above, we would
have obtained the following results:
2) Fix VTHH, accept the resulting VTHL
3) Fix VTHL, accept the resulting VTHH
The ACT8810 was designed to achieve an
operating temperature range that is suitable for
most applications with very little design effort. The
simple solution is often found to provide reasonable
results and should always be used first, then the
design procedure may proceed to one of the other
solutions if necessary.
k
COLD = VTHL/(ITH × RNOM) = 2.5V/(100µA × 6.8kꢀ) = 3.67
kHOT = VTHH/(ITH × RNOM) = 0.5V/(100µA × 6.8kꢀ) = 0.74
which, according to the characteristic tables would
have resulted in an operating temperature range of
-6°C to 33°C vs. the design goal of 0°C to 40°C.
In this case, one can add resistance in series with
the thermistor to shift the range upwards, using the
following equation:
In each design example, we refer to the Vishay
NTHS series of NTCs, and more specifically those
which follow a "curve 2" characteristic. For more
information on these NTCs, as well as access to the
resistance/temperature characteristic tables referred
to in the example, please refer to the Vishay
website at http://www.vishay.com/thermistors.
(VTHH/ITH) = kHOT(@40°C) × RNOM + R
R = (VTHH/ITH) - kHOT(@40°C) × RNOM
R = (2.5V/100µA) - 0.5758 × 6.8kꢀ
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