LTC4110
25
4110fb
Where:
C = C rate of the battery
I
CHG
= Programmed charging current
For Example, if we charge a 3Ah battery with 1A current,
then x = 15.
SAFETYSIGNAL DECODER
Table 3. SafetySignal State Ranges (Except SLA)
SafetySignal
CHARGE
RESISTANCE
CHARGE STATUS BITS
DESCRIPTION
0Ω to 500Ω
RES_UR, RES_HOT,
BATTERY_PRESENT
Under range
500Ω to 3k
RES_HOT, BATTERY_PRESENT
Hot
3kΩ to 30k
BATTERY_PRESENT
Ideal
30k to 100k
RES_COLD, BATTERY_PRESENT
Cold
Above 100k
RES_OR, RES_COLD
Overrange
Note: The under range detection scheme is a very important feature of the
LTC4110. The R
THA
/R
SafetySignal
divider trip point of 0.307 • 4.75V = 1.46V
is well above the 0.047 • V
DD
= 140mV threshold of a system using a 10k
pull-up. A system using a 10k pull-up would not be able to resolve the
important under range to a hot transition point with a modest 100mV of
ground offset between battery and SafetySignal detection circuitry. Such
offsets are anticipated when charging at normal current levels.
Table 4. SafetySignal for SLA (7.256k Between THA and THB)
SafetySignal
CHARGE
RESISTANCE
CHARGE STATUS BITS
DESCRIPTION
0
Ω
to 3.1k
RES_HOT,
BATTERY_PRESENT
Hot
3.1k to 114k
BATTERY_PRESENT
Ideal
114k
RES_COLD, RES_OR
Battery Removal
This decoder measures the resistance of the SafetySignal
and features high noise immunity at critical trip points.
The SafetySignal decoder is shown in Figure 10.
The value of R
THA
is 1.13k and R
THB
is 54.9k. SafetySignal
sensing is accomplished by a state machine that reconfi gures
the switches of Figure 10 using THA_SELB and THB_SELB,
a selectable reference generator, and two comparators.
The state machine successively samples the SafetySignal
value starting with the RES_OR ≥ RES_COLD threshold,
then RES_C0LD ≥ RES_IDEAL threshold, RES_IDEAL ≥
RES_HOT threshold, and fi nally the RES_HOT ≥ RES_UR
threshold. Once the SafetySignal range is determined, the
lower value thresholds are not sampled. The SafetySignal
decoder block uses the previously determined SafetySignal
value to provide the appropriate adjustment in threshold to
add hysteresis. The R
THB
resistor value is used to measure
the RES_OR ≥ RES_COLD and RES_COLD ≥ RES_IDEAL
thresholds by connecting the THB pin to an internal voltage
and measuring the voltage resultant on the THA pin. The
R
THA
resistor value is used to measure the RES_IDEAL ≥
RES_HOT and RES_HOT ≥ RES_UR thresholds by con-
necting the THA pin to the internal voltage and measuring
the resultant voltage on the THB pin. The SafetySignal
impedance is interpreted according to Table 3.
When the DCIN supply is present, a full sampling of the
SafetySignal is performed every 27ms. When the supply is
absent, a low power limited sampling of the SafetySignal is
performed every 218ms. A full sampling of the thermistor
state is performed only if a change of battery presence is
detected when the supply is not present.
GPIO AND STATUS FUNCTIONS
All of the GPIO pins are open drain with N-MOSFET drivers
capable of sinking current suffi cient to drive an LED (see
V
OL
). The pins are not capable of sourcing any current and
instead enter a Hi-Z mode when the output is not low. An
external pull-up will be required to create any high output
logic state.
OPERATION
Figure 10. Battery Safety Decoder (Except SLA)
V
INT
SafetySignal
CONTROL
R
SafetySignal
C
SS
LATCH
RES_OR
TH_HI
MUX
THB_SELB
4110 F10
+
–
TH_LO
HI_REF
REF LO_REF
+
–
V
INT
R
THB
54.9k
THA_SELB
THB
R
THA
1.13k
THA
RES_COLD
RES_HOT
RES_UR
