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LM3423-Q1: Using LED Driver as Constant Current Battery Charger

Seth Berggren
Prodigy 70 points

Part Number: LM3423-Q1
Other Parts Discussed in Thread: LM3423, BQ76942, TPS54560B

Hello, I intend to use the LM3423 as a constant-current charger for a battery pack.

I am using the datasheet buck-converter example as a design reference (figure 42, pg. 59).

I removed the PWM dimming circuitry and I replaced the LED string with my 'system'.

The 'system' consists of a battery pack (with BQ76942 monitor) and a microcontroller.

The 'system' microcontroller needs to control the LM3423 EN pin. This will be achieved by controlling the gate of an NFET that will pull-down the EN pin (NOT shown in above diagram). It also needs to control a few other NFETs that will connect to the CSH pin to add additional current settings (NOT shown in above diagram).

So, all of the microcontroller signals will be controlling the gate of some NFET that has its source referenced to the LM3423 ground. The 'system' ground in the above diagram is referenced to the positive terminal of the inductor (L1) and therefore it will be at higher voltage than the LM3423 ground.

How can I reliably control the LM3423 signals? Do I need to use level-shifter IC or opto-coupler? Would it help to relocate the L1 inductor above 'system' so that the 'system' ground is referenced to the Q1 NFET drain?

What happens if the system cannot sink the current output of the LM3423? For example, if the LM3423 is sourcing an average of 500 mA, but, the battery pack charge path is disconnected and only the microcontroller is sinking 10 mA of current, there will be an excess of 490 mA.

  • 0
    TI__Mastermind 18210 points

    This is really innovate solution. As you know, LM3423 is constant current driver. My concern is that how you can control LM3423 for constant voltage charging with termination current. Like you mentioned, if there is excess current, output voltage will still be charged until OVP is triggered.  

  • 0
    Prodigy 70 points

    Appreciate the response. I got side-tracked for a bit so I apologize for not responding sooner. I'd rather not innovate and reinvent the wheel but I could not find a good solution to provide constant current of about 5A and constant voltage of about 30V. This is the solution that I came up with but if there is a less-inventive solution to the problem that somebody has already figured out then I would definitely prefer to go with that one :) 

    Do you have an alternative suggestion that might be more fitting for this application?

  • 0 in reply to Seth Berggren
    TI__Mastermind 18210 points

    Maybe you can consider using general converter/controller with external constant current/voltage control loop. For example, TPS54560B+ work around solution. Just a thought.