UB2016 Datasheet, PDF (8/13 Pages) Unisonic Technologies – ONE-CELL STANDALONE LINEAR LITHIUM BATTERY CHARGER

UB2016
Preliminary
CMOS IC
„ APPLICATION INFORMATION
Stability Considerations
The constant-voltage mode feedback loop is stable without an output capacitor provided a battery is connected to
the charger output. With no battery present, an output capacitor is recommended to reduce ripple voltage. When
using high value, low ESR ceramic capacitors, it is recommended to add a 1W resistor in series with the capacitor.
No series resistor is needed if tantalum capacitors are used. In constant-current mode, the PROG pin is in the
feedback loop, not the battery. The constant-current mode stability is affected by the impedance at the PROG pin.
With no additional capacitance on the PROG pin, the charger is stable with program resistor values as high as 20kΩ.
However, additional capacitance on this node reduces the maximum allowed program resistor thus it should be
avoided. Average, rather than instantaneous, charge current may be of interest to the user. For example, if a
switching power supply operating in low current mode is connected in parallel with the battery, the average current
being pulled out of the BAT pin is typically of more interest than the instantaneous current pulses. In such a case, a
simple RC filter can be used on the PROG pin to measure the average battery current as shown in Fig. 1 A 10kΩ
resistor has been added between the PROG pin and the filter capacitor to ensure stability.
Fig. 1 Isolating Capacitive Load on PROG Pin
Thermal Limiting
An internal thermal feedback loop reduces the programmed charge current if the die temperature attempts to rise
above a preset value of approximately 120°C. This feature protects the UTC UB2016 from excessive temperature
and allows the user to push the limits of the power handling capability of a given circuit board without risk of
damaging the UTC UB2016. The charge current can be set according to typical (not worst-case) ambient
temperature with the assurance that the charger will automatically reduce the current in worst-case conditions.
The conditions that cause the UTC UB2016 to reduce charge current through thermal feedback can be
approximated by considering the power dissipated in the IC. Nearly all of this power dissipation is generated by the
internal MOSFET. This is calculated to be approximately:
( ) P_D = V_CC − V_BAT × I_BAT
Where P_D is the power dissipated, V_CC is the input supply voltage, VBAT is the battery voltage and I_BAT is the
charge current. The approximate ambient temperature at which the thermal feedback begins to protect the IC is:
T_A = 120^o C − P_D × θ_JA
( ) T_A = 120^o C − V_CC − V_BAT × I_BAT × θ_JA
Moreover, when thermal feedback reduces the charge current, the voltage at the PROG pin is also reduced
proportionally as discussed in the Operation section. It is important to remember that UTC UB2016 applications do
not need to be designed for worst-case thermal conditions since the IC will automatically reduce power dissipation
when the junction temperature reaches approximately 120°C.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
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QW-R502-869.a