Shown below is the internal block diagram of a switched mode battery charger IC (LT1510). The LT1510 is a current mode, PWM (pulse-width modulated), positive buck switcher that operates in one of two states: constant current or constant voltage.
I am having difficulty understanding the operation and internal block diagram of IC (especially the highlighted sections). I have only beginner level knowledge on current mode control and op-amp compensation.
Divya K S
First, do not confuse voltage-mode-/current-mode-control (of switchmode power supplies) with constant-current/constant-voltage battery-charging technique (CC/CV).
In the block diagram which you show with a red circle, be aware that diagram gives a very high-level, heuristic view of the circuit operation. The amplifiers with a diode in series simply means that the amplifier can source current, not sink current. (The diode is not really present.) The current coming out of the PROG pin is equal to the sum of currents from the outputs of CA1, VA, so the circuit adds the current signal and OVP signal together, and the impedance at the PROG pin changes that current to a voltage. When a battery is deeply discharged, all the current into PROG will come from CA1; the switchmode controller will regulate the charging current, not the battery voltage. This is the "constant-current" part of CC/CV charging. When the battery is charged so the voltage at OVP ≥ 2.465V, the output of VA will source more current, and dominate the control function. This is the "constant-voltage" part of CC/CV charging.
As I mentioned at the beginning, do not confuse current-/voltage-mode control and CC/CV charging. However, in the block diagram CA1 provides current feedback for the current-mode-control portion of the LT1510 and the constant-current-charging function. When the charging is nearly complete and the charger changes to CV mode, CA1 functions only as the current amplifier for current-mode control.
I have a question for you. Where did you find the text in your posting describing the operation of the LT1510. It's not from the datasheet!
Thanks a ton for replying. You really helped me clear the basics.
What I understood is as follows.
CA1 acts as a differential amplifier/Current Amplifier. VA acts as a differential amplifier/voltage amplifier. The gains of both these amplifiers are constant. The real compensator is CA2. It ensures the voltage at PROG pin is same as Vref always - during both CC and CV mode.
Till OVP Pin voltage<2.465, the circuit is in CC Mode. During CC operation Iprog (current output of CA1) would be maintained constant and no current flows out of VA.
Once the OVP> 2.465, then the circuit should enter CV mode. During CV Mode, the VA starts sourcing current and hence current output from CA1( as well as Ibat) decreases so that the sum of CA1 and VA currents still remain the same.
Regarding the text attached, it is from the following design manual.
We understood the high level functionality of CC/CV. We are still a lil confused about how duty ratio is adjusted appropriately for CC and CV mode. During CC Mode, the output voltage from buck converter has to be continuously increased( as the battery is getting charged) to maintain constant current. During CV mode, the output voltage from converter has to be constant for which (ideally) the duty ratio has to be constant. How does this happen with the above IC? I dont see much change in duty ratio during CC mode.If someone can guide me through what happens at the comparator C1 +/- terminals during CC/CV mode it would be of great relief.
We are working on a university project trying to build an efficient battery charger for Li-ion batteries.
I have stared at the LT1510 block diagram for a little while, and I can't construct a coherent understanding of what is going on at the input of C1 either. Sorry. I do not know whether this is due to a poor/confusing/unclear block diagram or my own cognitive limitations.