5

UCC2956

UCC3956

APPLICATION INFORMATION (cont.)

tery voltage is above a user set threshold, the charger

will initiate a constant current bulk charge state. Once the

battery reaches 95% of it’s final voltage, the charger will

enter an over charge state. During the over charge state,

the converter will transition from a constant current to a

constant voltage mode of operation. Figure 2 shows typi-

cal current, voltage, and capacity levels of a Lithium-Ion

battery during a complete charge cycle.

A Block Diagram of the UCC3956 is shown on the first

page of the data sheet, while Figure 1 shows a typical

application circuit for a Buck derived

switch mode

charger. The UCC3956 can be used for charging a single

cell or multiple cells in series. If more than two cells are

stacked in series, however, a level shifting gate drive will

be needed to operate the buck switch. The application

circuit charges a 1200mAh 2 cell stack at a 1C rate.

Setting the Oscillator Frequency

The frequency of operation for the converter is set by

picking values for RSET and COSC.

fOSC =

3.475

(COSC+ 20pF)• RSET

The UCC3956 is capable of operating at frequencies

higher than 200kHz. However, the actual operating fre-

quency of the buck converter will ultimately be deter-

mined by the usual tradeoffs of size, cost and efficiency.

The application circuit frequency is set at 100kHz with

COSC = 180pF and RSET = 162k.

Trickle Charge State

When the battery’s voltage is below a predetermined

threshold, the battery is either deeply discharged or has

shorted cells. The trickle charge state offers a low charg-

ing current to bring the battery up above zero capacity. In

the case of shorted cells, the trickle charge state pre-

vents the charger from delivering high currents during

this fault condition. Stacking several cells makes the de-

tection of a shorted cell more difficult.

For Lithium-Ion batteries, the trickle charge threshold is

typically set to a value around 2.5V per cell (this corre-

sponds to near zero capacity). When the cell voltage is

below the threshold, only a trickle current will be applied

to the battery. The threshold is established by program-

ming CHGENB to 2.05V when the battery (or stack) volt-

age is at the threshold. Referring to the application circuit

Figure 1. Typical Application Circuit

UDG-96198-1

= 1.2A

= 100kHz

Timeout = 120 minutes