SC908 datasheet(19/30 Pages) SEMTECH

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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: R

HOT

is 7.73kΩ (at 40°C), R

COLD

is 53.94kΩ

(at 0°C). Its dissipation constant DC = 3.0mW/°C. Step 1

yields R

NPU

= 18.2kΩ, with the result that NTC

COLD

VSYS

0.748 ≈ RT

NTC_CR

, NTC

HOT

VSYS

= 0.298 ≈ RT

NTC_HF

. The NTC

resistances that give the exact cold and hot thresholds

NTC_CR

and RT

NTC_HF

are 54.6kΩ (which is R

NTC

at approxi-

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

VSYS

= 4.6V, the 0°C NTC network current is

NTC_COLD

= V

VSYS

/(R

NPU

+ R

COLD

) = 63.8μA

Power dissipation in the thermistor at this temperature is

COLD

= R

COLD

× (I

NTC_COLD

)2 = 0.219mW

The self heating error is

073

219

COLD

The 40°C NTC network current

NTC_HOT

= V

VSYS

/(R

NPU

+ R

HOT

) = 0.177mA

Power dissipation in the thermistor at this temperature is

HOT

= R

HOT

× (I

NTC_HOT

)2 = 0.243mW

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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