SC802 Datasheet, PDF (9/16 Pages) Semtech Corporation – Fully Integrated Lithium-Ion Battery Charger System with Timer

SC802/SC802A
POWER MANAGEMENT
P_ApR_pE_lL_icIM_atI_iN_onA_sR_IY_{nformation Cont.}
perature returns to the valid range the SC802 automati-
cally resumes the charge cycle. The charge timer will
time-out when the SC802 output on-time exceeds the
timer setting regardles of how long it has been disabled
due to temperature. 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^oC
Tcold = 0^oC
Find R3 and RT1
Step 1: Pick an RT1 value high enough to prevent self-
heating.
RT1 ≥ ^(V(RT1))2
DC
In general lower values of RT1 provide more noise 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^oC self heating error.
than 0^oC and 50^oC alternate curve-type NTC elements
can be used or an additional resistor can be added to
the Rntc divider to alter the temperture coefficient.
LED Flags
There are two LED drivers on the SC802: CHRGB (Charge)
and CPB (Charger Present). These outputs are open drain
NMOS drivers for controlling LEDs directly. The Table be-
low defines each LED output state.
LED
FLAG
CPB
CHRGB
ON
UVLO
< VCC <
OVP
Iout >
Iterm
OFF
Blinking
Power
Not Good
X
Iout <
Iterm*
PreCharge
or NTC
Fault
* Output remains on when timer is enabled and t < timeout
The CPB output can be used for a VCC indicator. Re-
gardless of the state of EN, the CPB output reflects the
voltage of the VCC (adapter) input. When VCC is between
the UVLO and OVP thresholds the CPB output is low.
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 R3 to set the correct Hot level trip point
R3
=
2.33
×
⎜⎛
⎜⎝
R
T25
×
⎜⎜⎝⎛
R
R
T50
T25
⎟⎟⎠⎞⎟⎟⎠⎞
=
9.52k
The closest standard resistor value is 9.53k. RT(temp)/
RT(25^oC) is often referred to as the ratio at the given
temp. For this NTC at 50^oC the ratio is 0.4086.
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 during the charge cycle when the output current
is less than Itermination. This latch is reset when the
battery enters a re-charge cycle, or if BIPB, EN, or VCC
are toggled. The CHRGB indicator operates the same
way in both charge and ldo mode.
There are five fault modes detected by the SC802; (In-
put 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 below.
Step 3: Calculate the Tcold value.
⎜⎜⎝⎛
RT_COLD
RT₂₅
⎟⎟⎠⎞
=
3×
⎜⎛ ^R3
⎝ ^10k
⎟⎞
⎠
=
2.86
Fault
VCC UVLO
or
VCC OVP
NTC Fault
CPB
off
on
CHRGB
off
Blinking
The ratio for the Curve 2 NTC at 0^o is 2.816. The differ-
ence represents less than 1/2^oC (ie actual trip would be
-0.5^oC instead of 0^oC.) For temperature trip values other
Pre-Charge
Time-out
on
Tj > 150C
off
Blinking
off
© 2005 Semtech Corp.
9
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