SC908 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC908' PDF : 30 Pages View PDF
SC908
Applications Information (continued)
NTC thresholds to avoid chatter at the NTC fault thresh-
olds. When V returns to the valid range, the charge
EN_NTC
timer resumes and the charge cycle continues. The charge
timer will expire when the output on-time exceeds the
timer setting, regardless of how long it has been disabled
due to an NTC fault.
Using the recommended NTC external network, the
EN_NTC pin voltage and the internal hot and cold NTC
thresholds are all ratios of V , rather than absolute volt-
VSYS
ages. This ensures that the hot and cold OK-to-charge
thresholds are insensitive to the VSYS pin output voltage.
The ratiometric thresholds are given by the parameters
RTNTCH and RTNTCC. EN_NTC pin voltage VEN_NTC between
RTNTCH×VVSYS
and
RTNTCC×VVSYS
enables
charging.
When
V
EN_NTC
is outside this range, charging is suspended and the FLTB
output is asserted (pulled low).
sheet for the proposed NTC thermistor, the Mitsubishi
TH11-3T223F, indicates that R = 11.93kΩ at 40°C, and
NTC
R = 69.41kΩ at 0°C, with a dissipation constant
NTC
DC = 3.0mW/°C.
So
R
HOT
=
11.93kΩ
and
R
COLD
=
69.41kΩ.
Step 1
Select R to obtain one of the desired temperature
NPU
thresholds. This example will solve for the hot threshold
for the normal (NTC thermistor to ground) configuration,
then evaluate the cold threshold. Solve the NTC network
voltage divider for RNPU to place the NTC voltage at
RT × V when R = R .
NTC_HF
VSYS
NTC
HOT
RTNTC _ HF u VVSYS
or, solving for R ,
NPU
VVSYS u RHOT
RNPU RHOT
When
V
EN_NTC
<
VTNTCDIS
(nominally
0.6V
),
the
SC908
charger
is disabled. The EN_NTC pin can be pulled to ground by
an external n-channel FET or microprocessor GPIO to
asnychronously disable or reset the device. When V <
EN_NTC
VT , the charger is turned off, the charge timer is reset,
NTC_DIS
and the CHRGB status output is turned off. While disabled,
the VAD input UVLO and OVP threshold detectors remain
active, and the CPB pin continues to indicate whether the
VAD input voltage is valid for charging.
The response of the SC908 to an EN_NTC pin voltage
above the high threshold or below the low threshold (but
above VTNTCDIS) is the same. Therefore the EN_NTC network
can be configured with the battery pack thermistor
between EN_NTC and VSYS, and a fixed resistor between
EN_NTC and ground. This configuration may be used to
reset the charge timer (and the CHRGB output) when the
battery pack is removed; the fixed resistor pulls the NTC
pin to ground to disable the charger without indicating a
fault.
NTC Design Example
This example uses the conventional NTC network configu-
ration shown in the block diagram. A fixed resistor (R )
NPU
is connected between EN_NTC and VSYS, and a battery
NTC thermistor (R ) is connected between the EN_NTC
NTC
pin and ground. The battery temperature range over
which charging is permitted is from 0°C to 40°C. The data-
RNPU
1 RTNTC _ HF
RTNTC _ HF
u RHOT
Using RT = 0.3, we obtain R = 27.837kΩ exactly.
NTC_HF
NPU
The closest 1% standard nominal value is R = 28.0kΩ.
NPU
Step 2
Evaluate the NTC network at the cold threshold. Compute
the NTC network resistor divider voltage as a function of
V at the desired cold threshold.
VSYS
NTC COLD
VVSYS u RCOLD
RNPU RCOLD
0.7126 u VVSYS
The value 0.7126 should be close to the nominal value of
RT = 0.75. To evaluate the significance of the discrep-
NTC_CR
ancy, an estimate of the actual cold threshold is obtained
by evaluating the value of R
that produces
NTC_Cold_Actual
the nominal value of RT = 0.75.
NTC_CR
RTNTC _ CR
RNTC _ Cold _ Actual
R R NTC _ Cold _ Actual
NPU
The solution shows R
= 84.0kΩ. Examination of
NTC_Cold_Actual
the thermistor specification resistance versus temperature
data indicates that the resulting actual cold threshold is
approximately -4°C, compared to the target of 0°C.
18
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