ACT8810
Active- Semi
Rev 4, 01-Oct-09
ActivePathTM CHARGER
FUNCTIONAL DESCRIPTION CONT’D
Finally,
at low temperatures by connecting a resistor in
parallel with ITH. The desired resistance can be
found using the following equation:
R = 5kꢀ - 3.9kꢀ = 1.1kꢀ
This result shows that adding 1.1kꢀ in series with
the thermistor sets the net resistance from TH to G
to be 0.5V at 40°C, satisfying VTHH at the correct
temperature. Adding this resistance, however, also
impacts the lower temperature limit as follows:
(ITH + (VCHG_IN - VTHL)/R) × kCOLD(@0°C) × RNOM = VTHL
Rearranging yields
R = (VCHG_IN - VTHL)/(VTHL/(kCOLD(@0°C) × RNOM) - ITH)
R = (5V - 2.5V)/(2.5V/(2.816 × 6.8kꢀ) - 100µA)
R = 82kꢀ
VTHL/ITH = kCOLD(@TC) × RNOM + R
k
COLD(@TC) = (VTHL/ITH) - R)/RNOM
Finally,
COLD(@TC) = (25kꢀ - 1.1kꢀ)/6.8kꢀ = 3.51
Adding 82kꢀ in parallel with the current source
increases the net
current flowing into the
k
thermistor, thus increasing the voltage at TH.
Adding this resistance, however, also impacts the
upper temperature limit:
Reviewing the characteristic curves, the lower
threshold is found to move to -5°C, a change of only
1°C. As a result, the system satisfies the upper
threshold of 40°C with an operating temperature
range of -5°C to 40°C, vs. our design target of 0°C
to 40°C. It is informative to highlight that due to the
NTC behavior of the thermistor, the relative impact
on the lower threshold is significantly smaller than
the impact on the upper threshold.
V
THH = (ITH + (VCHG_IN - VTHH)/R) × kHOT(@40°C) × RNOM
Rearranging yields,
HOT(@TC) = VTHH/(RNOM × (ITH + (VCHG_IN - VTHH)/R))
HOT(@TC) = 0.5V/(6.8kꢀ × (100µA + (5V - 0.5V)/82kꢀ))
k
k
= 0.4748
Reviewing the characteristic curves, the upper
threshold is found to move to 45°C, a change of
about 14°C. Adding the parallel resistance has
allowed us to achieve our desired lower threshold of
0°C with an operating temperature range of 0°C to
45°C, vs. our design target of 0°C to 40°C.
Fix VTHL
Following the same example as above, the
"unadjusted" results yield an operating temperature
range of -6°C to 33°C vs. the design goal of 0°C to
40°C. In applications that favor VTHL over VTHH
,
however, one can control the voltage present at TH
Figure 12:
Figure 13:
Fix VTHL Configuration
Fix VTHH Configuration
ACT8810
CHG_IN
100µA
ACT8810
100µA
+
+
VTHH
VTHH
Li+ Battery
Pack
Li+ Battery
+
+
–
R
Pack
R
–
TH
TH
+
+
–
NTC
NTC
–
VTHL
VTHL
–
–
Innovative PowerTM
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ActivePMUTM and ActivePathTM are trademarks of Active-Semi.
Copyright © 2009 Active-Semi, Inc.
I2CTM is a trademark of Philips Electronics.
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