SC908 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC908' PDF : 30 Pages View PDF
SC908
Applications Information (continued)
Step 3
With the example thermistor, there is no choice of R
NPU
that will yield the specified results at both hot and cold
limits. A more sensitive thermistor, one with a wider per-
centage variation in resistance at the desired threshold
temperatures, may provide a better solution. Steps 1 and
2 are repeated using other devices from the same vendor,
seeking a closer match at the cold threshold.
The Mitsubishi TH11-4C153F was the final selection. Its
characteristics are: RHOT is 7.73kΩ (at 40°C), RCOLD is 53.94kΩ
(at 0°C). Its dissipation constant DC = 3.0mW/°C. Step 1
yields R = 18.2kΩ, with the result that NTC /V =
NPU
COLD VSYS
0.748 ≈ RTNTC_CR, NTCHOT/VVSYS = 0.298 ≈ RTNTC_HF. The NTC
resistances that give the exact cold and hot thresholds
RT and RT are 54.6kΩ (which is R at approxi-
NTC_CR
NTC_HF
NTC
mately -0.5°C) and 7.80kΩ respectively, closely matching
the resistance of the thermistor at the targeted threshold
temperatures.
Step 4
Verify acceptable thermistor self heating. The dissipation
constant is the power rating of the thermistor resulting in
a 1°C self heating error. Since accuracy is important only
at the thresholds, self heating is assessed only at 0°C and
40°C.
For V = 4.6V, the 0°C NTC network current is
VSYS
I
= V /(R + R ) = 63.8μA
NTC_COLD
VSYS NPU
COLD
Power dissipation in the thermistor at this temperature is
P = R × (I
)2 = 0.219mW
COLD
COLD
NTC_COLD
The self heating error is
TSH _ COLD
0.219mW
3 mW C
0.073qC
The 40°C NTC network current
I = V /(R + R ) = 0.177mA
NTC_HOT
VSYS NPU
HOT
Power dissipation in the thermistor at this temperature is
P = R × (I )2 = 0.243mW
HOT
HOT
NTC_HOT
for self heating of approximately 0.081°C. The actual cold
and hot thresholds will be 0.073 and 0.081 degrees lower
than designed, respectively, which are negligible errors.
Logical CC-to-CV Transition
The SC908 differs from most monolithic linear single cell
Li-Ion chargers, which implement a linear transition from
CC to CV regulation. The linear transition method uses
two simultaneous feedback signals — output voltage and
output current — to the closed-loop controller. When the
output voltage is sufficiently below the CV regulation
voltage, the influence of the voltage feedback is negligible
and the output current is regulated to the desired current.
As the battery voltage approaches the CV regulation
voltage (4.2V), the voltage feedback signal begins to influ-
ence the control loop, which causes the output current to
decrease although the output voltage has not reached
4.2V. The output voltage limit dominates the controller
when the battery reaches 4.2V and eventually the control-
ler is entirely in CV regulation. This system may be
characterized as a dual-constraint (voltage and current)
controller, with a soft transition between constraints. The
soft transition effectively reduces the charge current
below that which is permitted for a portion of the charge
cycle, which increases charge time.
In the SC908, a logical transition is implemented from CC
to CV to recover the charge current lost due to the soft
transition. The controller regulates only current until the
output voltage exceeds the transition threshold voltage.
It then asynchronously switches to CV regulation. The
transition voltage from CC to CV regulation is typically less
than 10mV higher than the CV regulation voltage, which
provides a sharp and clean transition free of chatter
between regulation modes. The difference between the
transition voltage and the regulation voltage is the CC/CV
overshoot. While in CV regulation, the output current is
limited to approximately 105% of the fast-charge current
programmed by the IPRGM pin or the IPUSB pin, depend-
ing on the charging input selected, providing mode
transition hysteresis. If the output current exceeds this
current limit threshold, the controller asynchronously
reverts to current regulation.
The logical transition from CC to CV results in the fastest
possible charging cycle that is compliant with the speci-
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