LTC4121EUD charge when Vin<12V

Only at an input voltage >12V PWM occurs and the charging current appears (0.14A input current). Otherwise, there are no signals. The charge status and error are read at <12 volts as well as at >12 volts at the input: nCHG = 0, nFLT = 1.

I need Vfloat = 4.0-4.2V, batery voltage which I try to charge V=3.5V. Vin=6-8.4V (two LiPo cells), Vout=4V (single cell LiFePO4).

May be only LTC4121-4.2 can work with <12Vin? But I need Adj Vfloat.

What should I do to start PWM on low voltage with LTC4121EUD?

PS. When Vin < 12V Vfloat avg = 2.74V, voltage ripple with a frequency of 1/280 ms, Vp-p=2.7-2.8V looks like a saw.

  • I occure the similar question.the LTC4121 can't work with input under 15V .only the input is above 15V ,it works

  • 0
    •  Analog Employees 
    on Jun 27, 2018 11:25 PM

    We have seen strong sensitivity of the operation of the LTC4121 to the physical placement of the capacitor at IN, capacitor at INTVcc and at BOOST.  Improper placement can lead to the INTVcc voltage being pumped down by chip operation, leading to a UV lockout on the INTVcc voltage.  In this scenario, the LTC4121 may rapidly cycle between on and off.  A typical symptom of this problem is limited range of the input voltage; over certain input voltages, the LTC4121 will appear to stop working completely, or may charge at a much lower rate than programmed.  The presence of this problem can be confirmed by applying an independent 4.3-5.0V, bypassed power supply between INTVcc and GND which will overpower the internal INTVcc.  If the circuit behaves as expected with the independent supply, the problem is due to less-than-ideal placement of one or more of the 3 capacitors.  The designer can review the DC1977A demo-board layout for a successful capacitor-placement strategy. 

    Here’s the location of the DC1977A design files.

  • 0
    •  Analog Employees 
    on Aug 2, 2018 4:06 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin
  • I have the same problem but it depends on the type of battery connected. 
    Some are loaded without problems, others do not load at all. Vfloat is about 4.2-4.3V.
    When I power LTC4121-4.2 from an external power supply, everything works fine (always).
    The distance of the capacitors is less than 5mm.
    This is another one of the LT inverter systems that causes problems not described in the data sheet.
    I think that this is another reason for returning to BQ24xxx chargers...
  • Unlike the LTC4121 chip, the 4.2V fixed voltage IC (LTC4121-4.2) works well at low voltage. Only the FB resistor divider for the new chip was removed, BATSNS connected to BAT.