Lead acid battery charging through UCC28740

You will need a logic circuit or controller to adjust the reference, but the output voltage of the UCC28740 can be adjusted during operation.

I recommend implementing the UCC28740 in a similar fashion to PMP11451.  This reference is for USB PD applications, where the output voltage can vary between 5V to 20V, well outside the range the battery requires.

Hello Rajesh,

As Eric mentioned in his reply, it is possible to use the MCU to monitor the output voltage and send a signal to control the feedback network of the TL431 (as in the schematic of PMP11452). This signal would drive a signal-level MOSFET or NPN transistor to adjust the divider-network resistance ratio such that Vout is regulated to 14.4 V when the transistor is ON and 13.5 V when the transistor is OFF.

If the MCU is unavailable or impracticable for this purpose, an alternative idea is to use a pair of comparators (such as LM393), or a TLC555 (which has the comparators built in), to implement a hysteretic control of Vout. Using the LM393 (or CMOS equivalent), it can run directly off the Vout rail but it will need a stable reference voltage to set up 2 thresholds. The TLC555 already sets up the thresholds but will need a stable VDD source to prevent changes in those thresholds. A simple Resistor-Zener from Vout to about 6V will do that job nicely. Then the reference voltages are 4 V and 2 V. (I suggest 6 V for VDD of the TLC555 simply to make the thresholds easy numbers to work with, but any stable voltage lower than Vout can work.) If you tie the THRES and TRIG inputs together the TLC555 essentially becomes a Schmitt-trigger inverter. Inputs below 2V force the OUT high and inputs above 4V force OUT low. For low OUT currents, the OUT driver is effectively rail-to-rail; that is, OUT = 6 V or 0 V.

A high OUT can turn on the TL431-adjusting transistor to command regulation to 14.4 V. A low OUT will turn off the adjustment to command regulation to 13.5 V. A separate resistor divider from Vout to GND is applied to the TLC555 inputs (THRES+TRIG tied together) to toggle the OUT depending on the voltage of Vout.

Now, the TLC555 references are 2 V apart and the 14.4-V and 10-V targets are 4.4V apart. A 2-resistor divider is not enough to reach each of the references at the appropriate Vout level, so a third hysteresis resistor is needed from OUT back to the THRES+TRIG input to make up the differences. Assuming Vout has exceeded 14.4 V, we know OUT goes low, so THRES+TRIG input effectively has 2 resistors in parallel to GND and 1 resistor up to Vout. (Also the adjustment-NPN is turned off, so TL431 sensing is set for 13.5 V.) We want the 555 input to be 2 V when Vout falls to 10 V. Once that happens, OUT goes to 6 V (TL431 retargets to 14.4 V) and we want the 555 input to be 4 V when Vout reaches 14.4 V. If we arbitrarily choose the upper resistor to be 100 Kohm, we are left with 2 unknown resistors with 2 conditions, so we can solve using 2 simultaneous node equations.

Without showing all the math, the result is Rup = 100K, Rdown = 33.7K, and Rhys = 107K which result in input voltages of 2.04 V and 4.08 V for 10 V and 14.4 V at Vout. So actual trip points are a little off, but the principle is established and values can be adjusted to suit the application.

Some other considerations:
1. If the load on the battery is less than or equal to the current capability of the charger (apparently targeted for 0.7 A), then the charger will carry that load indefinitely at 13.5 V. Be sure the thermal design of the charger can handle this.
2. When the output reaches 14.4 V and regulation toggles down to 13.5 V, it will take some time for the output voltage to settle down to this level. During this time, the voltage loop control will experience this condition as excess voltage and cut back the switching frequency to the UCC28740 minimum of ~170 Hz. This condition can also last for a long time, so be sure that the -740 controller has sufficient VDD capacitance to avoid UVLO shutdown.
3. Be sure that the secondary to auxiliary turns-ratio is high enough to avoid UVLO of the UCC28740 when the battery voltage is at its lowest level.
4. To extend the life of the battery, the lead-acid float voltage target may be subject to a temperature co-efficient correction, which is beyond the scope of this reply. But be sure to consider this in the TL431 feedback network design.

I hope this idea can be of help to you.

Regards,
Ulrich