ACT2801/ACT2801C
Rev 2, 15-Apr-14
Table 4: Battery Impedance
LEDs on in Boost Mode
RIMC(Kꢀ)
50
50
100
100
200
200
LEDs can be always on during boost mode, the
schematic is shown in Figure17.
Battery
Impedance
R(mꢀ)
Rcs=25mꢀ
Rcs=50mꢀ
200
100
400
Boost Output Plug-in Auto Detection
Figure 14 provides a solution for auto plug-in
detection.
Figure 17. LEDs on in boost mode circuit
Inductor and Capacitor Selection
ACT2801/ACT2801C supports SMD components.
2.2uH inductor is recommended. Input side, 4.7uF
ceramic capacitor in series with 2.7Ω resistor are
recommended, on battery and output sides, 22uF
ceramic capacitors are recommended.
Figure 14. Boost output auto detection circuit
Battery Temperature Monitoring
Input Over Voltage Surge
ACT2801 continuously monitors the temperature of
the battery pack by sensing the resistance of its
thermistor, and suspends charging if the
temperature of the battery pack exceeds the safety
limits.
In the case of pure ceramic input capacitor is
chosen, if the input cable is long, stray inductance
may cause over voltage spikes as twice as the
steady-state voltage when input source is plugged
in. Below input circuit is recommended to avoid
input voltage surge. R1 resistor is added in series
with capacitor C1 to damp the potential LC
resonance as shown in Figure 15.
In a typical application, the TH pin is connected to
the battery pack's thermistor input as shown in
Figure 18. The ACT2801 injects a 60µA current out of
the TH pin into the thermistor, so that the thermistor
resistance is monitored by comparing the voltage at
TH to the internal VTHL and VTHH thresholds of 0.3V
and 1.5V, respectively. When VTH > VTHH or VTH < VTHL
charging and the charge timers are suspended. When
VTH returns to the normal range, charging and the
charge timers resume.
The threshold is given by:
Figure 15. Input over voltage surge protection circuit
60µA×RNOM×kHOT=0.3V→ RNOM×kHOT=5kꢀ
60µA×RNOM×kCOLD=1.5V → RNOM×kCOLD= 25kꢀ
External Input Over Voltage Protection
where RNOM is the nominal thermistor resistance at
room temperature, and kHOT and kCOLD are the ratios
of the thermistor's resistance at the desired hot and
cold thresholds, respectively.
Considering the maximum voltage rating at VIN pin,
the external OVP circuit as shown in Figure 16 is
recommended if input voltage may go higher than
7V. With the enhanced OVP circuit, input voltage
can be up to 18V.
Figure 18. Battery thermal circuit
Figure 16. Input over voltage protection
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