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© 2008 Semtech Corp.
SC805A
www.semtech.com
POWER MANAGEMENT
Applications Information (Cont.)
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 schematic
of Page 1 using a Curve--2, Vishay-Dale NTC.
Thot = 50°C
Tcold = 0°C
Find R2 and RT1
Step 1: Pick an RT1 value high enough to prevent self-
heating.
2
DC
1) V(RT
1 RT
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°C self-heating 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,
9.52k
R
R
R 2.33 R2
T25
T50
T25
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The closest standard resistor value is 9.53k. RT(temp)/
RT(25°C) is often referred to as the ratio at the given temp.
For this NTC at 50°C, the ratio is 0.4086.
Step 3: Calculate the Tcold value.
2.86
10K
R2
3
RT
RT
25
COLD
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The ratio for the Curve-2 NTC at 0°C is 2.816. The differ-
ence represents less than 1/2°C (i.e., actual trip would be
-0.5°C instead of 0°C.) For temperature trip values other
than 0°C and 50°C, alternate curve-type NTC elements can
be used or an additional resistor can be added to the Rntc
divider to alter the temperature coeffi cient.
For detailed design guidance, including thermistor selec-
tion guidelines, see the Semtech Application Note
AN–PM–0801, NTC Thermistor Network Design for
Ratiometric Thresholds.
CHRGB LED Flag
The CHRGB LED driver on the SC805A 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 condition. The
table below defi nes the CHRGB LED output states.
LED High Low High-Z Blinking
CHRGB
Iout <
Iterm
Iout > term VCC < UV
Pre-charge
OV or NTC
Fault
The CHRGB output can be used for a VCC indicator. When
the SC805A is enabled, the CHRGB output is either high
or low providing the VCC input is above UVLO.
The CHRGB output signifi es the charging status. When
the output current is greater than termination current,
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. The CHRGB indicator operates the same way in
both charge and LDO mode.
The CHRGB output can be used for a VCC indicator. When
the SC805A is enabled the CHRGB output is either high or
low providing the VCC input is above UVLO. Alternately an
LED connected to VCCIL can be used for a VCC indicator as
shown in the typical application schematic on Page 1.
There are fi ve fault modes detected by the SC805A; (input
under-voltage, input over-voltage, NTC temperature, max
die temperature (or OT) and pre-charge timeout). The LED
states for these fault modes are shown in the following
table.
