LTC4020 preconditioning mode problem


I am trying to build a 10S3P Li-Ion charger based on the LTC4020IC. I am using the DC2044A demo board, which I modified to adapt it to my charge current (4.2A) and my input (55V max) / output (Li-Ion 10S3P) voltages.

Everything works great, I am abble to completely charge my battery, and the transition from CC to CV is done around 41.5V (I set up 42V as float voltage).

But I have a problem with the preconditioning mode, which should stop when the battery is above 29.4V (1.75V on FB pin), unfortunately it only stops when the voltage is above 34V (2.0V measured approx on the FB pin).

I don't have the possibility to set by myself the preconditioning threshold, do you have a clue on what is happening here?

Here are the relevant components modified on the demo board:

R8 = R9 = 680k, 0.1%

R10 = R11 = 43k 0.1%

Other components modified:

M1: Si7465

M2, M3, M4, M7: BSC098N10NS5

RCBR1, RCBR2: 10mOhms

L1: 22µH (WURTH 74435572200)

C14,C18,C27,C28: operating voltage at 63V minimum

C17,C15: operating voltage at 63V minimum

C19,C20: 1uF 50V

D7: 6V 0.1A Zener

And the header JP1 is of course set to CC/CV

Thank you for your help.
I look forward to hearing from you,


  • Note there is no JP1 on the DC2044A.  There is a turret connected directly to the MODE pin and to signal ground through R12.  For CC/CV mode, be sure there is a direct connection to ground at this pin.  (I.e., short R12).

    I believe you are encountering the "instant-on" feature when the voltage at VFB>1.75V, but the voltage at VFBMIN is less than 2.125V.  The instant-on feature establishes a minimum voltage at VOUT even when the battery is deeply discharged.  With instant on, the battery FET operates in linear mode, dropping the voltage at VOUT to the battery voltage.  Since this leads directly to power dissipation in the battery FET the charging current is reduced.  See the discussion in the datasheet for VFBMIN (p. 11-12) and the instant-on feature, p. 27-28.  Note the current-reduction plot in Figure 14.  

    The only reason to use the instant-on feature is if you want to guarantee a minimum voltage at VOUT regardless of battery voltage.  For example, if you are using the LTC4020 as a battery charger only, you can disable the instant-on feature by adding a second, separate resistor divider from VOUT to VFBMIN such that whenever 1.75V<VFB the voltage at VFBMIN>2.125V.  For example, a 361kΩ/43kΩ resistor divider from VOUT to VFBMIN would disable the instant-on feature as long as the battery voltage is greater than 20V.

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