PT8A2707 Datasheet, PDF (22/26 Pages) Pericom Semiconductor Corporation – Charger Controller with Fuel-Guage & LCD/LED Display

PT8A2707-12/17-19
Charger Controller
with Fuel-Guage & LCD/LED Display
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Application instruction
1. Self-Discharge Calibration
During the idle state, the self-discharge rate of batteries is calibrated in the fuel-gauge counter. The self-discharge calibration rate
is fixed at 1/128C per day, which corresponds to the NiMH battery characteristics at which the battery pack will be completely
self-charged to zero in about 4 months in a storage temperature of about 20-35C.
VEE
2. Setting Battery Temperature Measurement
RX
The PT8A2707-12/2717-19 employs a thermistor with negative temperature coefficient to
measure the battery‟s temperature. Since most thermistors are inherently nonlinear with respect _VT
to temperature, in order to reduce the effect of nonlinearity, a resistor-divider network in
parallel with the thermistor is recommended. A typical application circuit is shown on the right
RY
side. The resistor-divider consists of RX and RY, which can be selected to adjust the peak
temperature threshold (e.g. 55C) as well as dT/dt slope according to the characteristics of the
employed thermistors. (The calculation methods will be provided in datasheet later)
3. Setting Charging Current
Figure 10
The PT8A2707-12/2717-19 employs two internal references for DC/DC bulk converter to achieve constant current charging
disregards to the input power supply voltages. For fast-charging, an internal reference voltage of 70mV is employed to compare
with the voltage drop across the sensing resistor. For pre-charging or maintenance trickle-charging, an internal reference voltage
of 7mV is employed to compare the voltage drop across the sensing resistor to achieve 1/10 of the fast-charging current. For
example, when controlling a 700mA charging control, a 0.10 1/8W sensing resistor is required. When it is in the pre-charging or
trickle-charge status, the current is controlled at 70mA (This is already excluding the additional current that may flow from the
adapter to LCD screen or LEDs or even to the motor when loaded unless motor is drawing all current from adapter). When a
higher charging current would be used for 1-hr charging for a higher capacity battery, the sensing resistor should be scaled
proportionally to maintain the same reference voltages. For examples:
Table 9: Charge Sensing Resistor Selection
Charging Current Pre-Charging & Maintenance Current
Sensing Resistor
700mA
70mA
0.10, 1/8W
1.03A
100mA
0.068, 1/8W
1.3A
130mA
0.054, 1/8W
1.6A
160mA
0.044, 1/8W
Basically, the sense resistor is inversely proportional to the charging current. The power dissipation is maintained at about 0.07W.
4. Setting dT/dt value to stop fast-charge
The PT8A2707-12/2717-19 employs thermistor (NTC) as battery temperature sensor. When the rate of temperature change (dT/dt)
reaches certain value, fast-charge is terminated. When design external NTC sampling circuit, user should consider that appropriate
dT/dt value is depended on fast charge time and battery type. Table 16 shows the dT/dt value for different fast charge time.
Suggested NTC sampling circuit is based on this table and dT/dt characteristics of the devices.
Table 10: Rate of Temperature Change (dT/dt) to Terminate Fast-Charge
Fast charge time and measure time
dT/dt
T_FCMAX =90mins, Measure 90s
T_FCMAX =60mins, Measure 60s
T_FCMAX =30mins, Measure 30s
T_FCMAX =150mins, Measure 150s
0.9C/min
1.38C/min
2.76C/min
0.54C/min
5. Setting Motor-Over-Current Protection Threshold
By setting the VMSTOP pin, three motor-over-current voltage thresholds can be selected as follows:
Table 11: Motor-Over-Current Setting
VMSTOP
Over-Current Reference Voltage
GND
300mV across sensing resistor
0.6-1.0V
350mV across sensing resistor
VDD
210mV across sensing resistor
12-07-0005
22
PT0268-1 07/05/12