SC805
POWER MANAGEMENT
Applications Information (Cont.)
The NTC external component configuration is shown in
the typical application schematic on page 1 of the
datasheet. When the NTC voltage from the divider is
above or below the cold and hot temperature threshold
values the SC805 will suspend the charge cycle by turn-
ing off the output, freezing the charge timer, and indicat-
ing a fault on the CHRGB LED. Hysteresis is included for
both hot and cold thresholds to avoid chatter at the NTC
trip points. When the temperature returns to the valid
range the SC805 will automatically resume the charge
cycle. The charge timer will time-out when the SC805
output on-time exceeds the timer setting regardless of
how long it has been disabled due to the NTC tempera-
ture. The internal NTC thresholds of 30% and 75% VCC
were designed to work with Curve-2 type thermistors
available from numerous vendors.
Design Example based on the typical application sche-
matic of page 1 using a Curve 2 Vishay-Dale NTC.
Thot = 50
o
C
Tcold = 0
o
C
Find R2 and RT1
Step 1: Pick an RT1 value high enough to prevent self-
heating.
RT1
≥
The closest standard resistor value is 9.53k. RT(temp)/
RT(25
o
C) is often referred to as the ratio at the given
temp. For this NTC at 50
o
C the ratio is 0.4086.
Step 3: Calculate the Tcold value.
RT
COLD
RT
25
R2
=
3
•
=
2.86
10K
The ratio for the curve 2 NTC at 0
o
C is 2.816. The differ-
ence represents less than 1/2
o
C (i.e., actual trip would
be -0.5
o
C instead of 0
o
C.) For temperature trip values
other than 0
o
C and 50
o
C alternate curve-type NTC ele-
ments can be used or an additional resistor can be added
to the Rntc divider to alter the temperture coefficient.
CHRGB LED Flag
The CHRGB LED driver on the SC805 can be used to
drive two external LEDs: CHRGB=Low (Charging) and
CHRGB=High (Charge Complete). The CHRGB output
will be high impedance when the VCC is in a UV condi-
tion. The Table below defines the CHRGB LED output
states.
(
V (R T 1 )
)
2
DC
LE D
CHRGB
High
Iout < Iterm
Low
Iout > term
High-Z
VCC<UV
Blinking
PreCharge
OV or NTC
Fault
In general lower values of RT1 provide more noise immu-
nity for the NTC voltage at the expense of bias current
from the input adapter. The dissipation constant DC is
the power rating of the NTC resulting in a 1
o
C self heat-
ing error.
The DC value for this NTC is 3mW and the voltage across
the NTC is 2.5V for a 5V adapter. Therefore RT1 > 2K,
Set RT1 = 10k
Step 2: Find R2 to set the correct Hot level trip point,
R
T50
=
9.52k
R2
=
2.33
•
R
T25
*
R
T25
The CHRGB output can be used for a VCC indicator. When
the SC805 is enabled the CHRGB output is either high
or low providing the VCC input is above UVLO.
The CHRGB output signifies the charging status. When
the output current is > Itermination, CHRGB=low. CHRGB
is high when Iout is < Itermination. The CHRGB output is
latched high at the end of the charge cycle when the
output current is less than Itermination. The CHRGB
output will remain high until the timer expires and vbatt
enters a re-charge cycle, or if EN, or VCC are toggled.
2005 Semtech Corp.
8
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