8

UC2909

UC3909

During the bulk charge state, the voltage error amplifier

is now operational and is commanding maximum charge

tempts to force the battery to VOC.

C) Overcharge State

STAT0 = STATLV = logic 0, STAT1 = logic 1

The battery voltage surpasses 95% of VOC indicating

the UC3909 is in its overcharge state.

During the overcharge charge state, the voltage loop be-

comes stable and the charge current begins to taper off.

As the charge current tapers off, the voltage at CSO in-

creases toward its null point of 2.3V. The center connec-

tion of the two resistors between CSO and VLOGIC sets

the overcurrent taper threshold (OVCTAP). Knowing the

(5)

()

RI

RS

R

OVC

OCT

OVC

12

18518

=•

•

•

.

D) Float State

STAT0 = STAT1 = STATLV = logic 1

The battery charge current tapers below its OVCTAP

threshold, and forces STATLV high increasing the volt-

age sense divider ratio. The voltage loop now forces the

(6)

()

()

VV

RS

RS

RS

RS

FREF

=

++

123

3

charger goes back to the bulk charge state, STATE 1.

OFF LINE APPLICATIONS

For off line charge applications, either Figure 3 or Figure

4 can be used as a baseline. Figure 3 has the advan-

tage of high frequency operation resulting in a small iso-

lation transformer. Figure 4 is a simpler design, but at

the expense of larger magnetics.

APPLICATION INFORMATION (cont.)

Figure 3. Off line charger with primary side PWM

UDG-95009