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LM317: Battery charger solution

Sherry Liang
Genius 10255 points
Part Number: LM317
Other Parts Discussed in Thread: LP2951, TPS25921L

Dear team,

My customer wants to use this device to act as a battery charger circuit. Their input is 12V, and the NiH cell is 4.8V nominal value. But their chargering condition is >8.5V/100mA. Can I still use this circuit?

In the datasheet, it says VI should be larger than VBAT+4.25V, according to my condition, VI need to be larger than 12.5V, right?

Thanks & Best Regards,

Sherry

  • 0
    TI__Guru 55940 points

    Hi Sherry,

    For LM317 to be within the recommended operating conditions (the region where LM317 was designed, tested, and characterized to operate within), you need to have a minimum of 3 V of headroom (Vin - Vout).  Since the reference of LM317 is 1.25 V, this means that Vin will need to be 4.25 V (3 V + 1.25 V) above your intended output voltage for use as in Figure 18.  If you wish your output voltage from the LDO to be 8.5 V, then you will need 12.75 V (8.5 V + 4.25 V) at your input.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    TI__Genius 10255 points

    Hi Ryan,

    The input voltage is 12V in my customer' project. Could you please help recommend another device which could meet my conditions?

    Input: 12V,  Output: >8.5V/100mA

    Thanks & Best Regards,

    Sherry

  • 0 in reply to Sherry Liang
    TI__Intellectual 1146 points

    Hi Sherry,

    Do you also need a regulated output voltage to match the cells? It seems like adding a current limited load switch to any LDO like our LP2951 could work here. Do you know the maximum voltage the battery can take? Maybe the TPS25921L could be added to control the output current to the battery and also limit the amount of voltage drop.