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Understanding LTC4008-1

Category: Hardware
Product Number: ltc4008-1
Dear community,
I'm an engineer in charge of the development of a truck sensor testing device. Right now i'm tackling the battery management system. For that purpose, I'm using IC LTC4008-1, but I'm not sure I'm getting it to work properly.
This is the current schematic design I'm running:
 

As you can see in the picture, I'm using a 8,4V / 1A configuration. Current limiter is set to 1,8A. I'm using a switch to soft-reset the IC.

My questions are the following:
1. How does inductance affect the circuit?
I tried datasheet's recommended value, 40 uH for an 1A approach, and charging current keeps switching bettwen 0A and 0,09A. At that point, only ICL flag is lowered (it reads 1,45V) even though the voltage drop across RCL is 35mV, so the limiter shouldn't trigger. If i lower the inductance value to 10 uH, charging current rises up to 0,999A and LTC4008-1 goes into Constant-Current mode, as all flags are high.

2. What does CHARGE mosfet switch really do?
I've noticed that this mosfet shorts the gate of the upper mosfet to ground, activating it. But i can't seem to notice the effect of it, and I can't find its function on the datasheet either. Charging current doesn't change when the switch is pushed. What is it the switch does?

3. How does de thermistor circuit work?
I disabled this feature in my schematic design, as i couldn't make it work. Following datasheet's intructions i used a 24K3 Ohm resistor in series with the parallel result of a 10K NTC (BETA=3950) and a 100K Ohm resistor, as well as a 270 nF cap. FAULT flag was low with this configuration, so i just disabled it. Is there another configuration that might work? Is there a recommended NTC set of parameters for this application?

Thanks for your time, I'm looking forward to your response.
Best regards,
  • Update #2 on question Nº2:

    After analyzing in detail the schematic i figured this Switch_CHARGE must be an SYSTEM_ON/OFF switch, as it allows current to flow from the batteries to the system load.

    I performed a full charge test with a dead battery. In constant current mode, current flow was 1A and Switch_CHARGE did nothing noticeable when pressed. Upon reaching Constant voltage mode, current flow is 0.13 A and FLAG pin is low. In this state, VBAT is 8,34V and VCELL+ is 8,14V, pressing Switch_CHARGE rises current flow to 0.5A into the batteries and VCELL+ to 8,20V.

    What could this mean? Is this meant to happen? does it need to be pressed for the batteries to keep charging or would it damage the batteries (as they need low current flow in constant voltage mode)?

    Why won't VCELL+ ever reach 8,4V? from what i understand, constant voltage charge should happen at 8,4V (or 8,34 given resistor tolerances and VBAT voltage).

    Thanks in advance for your response.

    Best regards,

    Tomas.

  • Update: New question Nº4.

    As constant voltage charging eventually drops charge current to 0A, is that itself an overvoltage protection measurement?

    Say I leave this device charging for 3 whole days without an overvoltage protection IC. Would my cells suffer from overvoltage?

    Thanks for yout time and attention!

    Best regards, 

    Tomas.