Dual Level Float Charger Operations

The UC2906 is shown configured as a dual level float

charger in Figure 1. All high currents are handled by the

external PNP pass transistor with the driver supplying

base drive to this device. This scheme uses the TRICKLE

BIAS output and the charge enable comparator to give

the charger a low current turn on mode. The output cur-

rent of the charger is limited to a low-level until the battery

reaches a specified voltage, preventing a high current

charging if a battery cell is shorted. Figure 2 shows the

state diagram of the charger. Upon turn on the UV sense

circuitry puts the charger in state 1, the high rate bulk-

charge state. In this state, once the enable threshold has

been exceeded, the charger will supply a peak current

that is determined by the 250mV offset in the C/L ampli-

To guarantee full re-charge of the battery, the charger’s

state 1 and state 2. When the battery voltage reaches

state 2. The charger stays in this state until the OVER-

CHARGE TERMINATE pin goes high. In Figure 1, the

charger uses the current sense amplifier to generate this

signal by sensing when the charge current has tapered to

have been controlled by an external source, such as a

timer, by using the OVER-CHARGE INDICATE signal at

Pin 9. If a load is applied to the battery and begins to dis-

charge it, the charger will contribute its full output to the

load. If the battery drops 10% below the float level, the

charger will reset itself to state 1. When the load is re-

moved a full charge cycle will follow. A graphical repre-

sentation of a charge, and discharge, cycle of the dual

lever float charger is shown in Figure 3.

UC2906

UC3906

OPERATION AND APPLICATION INFORMATION

Internal Reference Temperature Characteristic and

Tolerance

4