UB2016

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

8 of 13

www.unisonic.com.tw

QW-R502-869.a

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:

(

)

BAT

BAT

CC

D

I

V

V

P

×

−

=

charge current. The approximate ambient temperature at which the thermal feedback begins to protect the IC is:

JA

D

A

P

C

T

θ

×

−

=

o

120

(

)

JA

BAT

BAT

CC

A

I

V

V

C

T

θ

×

×

−

−

=

o

120

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.