SC908 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC908' PDF : 30 Pages View PDF
SC908
Applications Information (continued)
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the battery voltage falls below the recharge threshold
(VCV - VReQ), the charger will clear the charge timer and ini-
tiate a charge cycle. The status of the charger output as a
function of the Charge Complete timer status and I is
BAT
shown in Table 1.
Table 1 — Charger Output Status
Timer
t < Timeout
t > Timeout
Disabled
Iout
N/A
N/A
<I
termination
Output State
On
Off
Off
0
6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14
RITERM (kΩ)
Figure 2b — Pre-charge and Termination Current
Variation vs. ITERM Resistance, High Resistance Range
Charge Timer
The timer provides over-charging protection in the event
of a faulty battery and maximizes charging capacity. The
RTIME pin is connected to VSYS to select the internal
(default) time duration of three hours, and to GND to
disable the timer. Connecting a resistor between RTIME
and GND will program the Charge Complete Time-Out, in
hours, according to the equation
t QComp
RRTIME u 1
3.334 3600
The timer is programmable over the range of 2 to 6 hours.
The output is automatically turned off when the charge
timer cycle ends.
If the charge cycle remains in precharge for longer than
one fourth of the Charge Complete Time-Out period, a
charging fault is detected and the charger turns off. The
Precharge Fault Time-Out period, in minutes, is
tPr eQF
tQComp u 60
4
Monitor Mode
When a charge cycle is complete (termination if the timer
is disabled, charge timeout if the timer is enabled), the
output turns off and the device enters monitor mode. If
Remote Kelvin Sensing at the Battery
The BSEN pin provides for Kelvin sensing of the battery
positive terminal voltage. This prevents feedback error
due to charging, battery load, and switching regulator
input currents flowing over resistive PCB traces.
Optimal PCB layout routes the BSEN trace directly to the
battery positive terminal connection on the PCB to
achieve the most accurate sensing of battery cell voltage.
Connecting BSEN to BAT directly at the SC908 will intro-
duce battery voltage measurement error that can cause
an improper transition from CC to CV regulation, length-
ening the charge time. This error could also raise or lower
the final battery voltage, and may alter the final
state-of-charge.
EN_NTC Interface
The EN_NTC pin is the interface to a battery pack temper-
ature sensing Negative Temperature Coefficient (NTC)
thermistor, which can be used to suspend charging if the
battery pack temperature is outside of a safe-to-charge
range. It is also the charger-disable input. The typical
EN_NTC network is a fixed resistor from VSYS to the
EN_NTC pin, and the battery pack EN_NTC thermistor
from the EN_NTC pin to ground. In this configuration, an
increasing battery temperature produces a decreasing
NTC pin voltage.
When V is greater than the high (cold) threshold or
EN_NTC
less than the low (hot) threshold, the charge cycle is sus-
pended by turning off the output. This suspends but does
not reset the charge timer, and indicates a fault on the
FLTB pin. Hysteresis is included for both high and low
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