LTC4020

Reviewing the demo-board layouts as you are doing is very worthwhile.  If you wish to review them further, may I recommend the PADS free viewer at https://www.pads.com/downloads/pads-pcb-viewer/.  Gerbers are awful to work with.

1. In the DC2134A the connection between SGND and PGND is on layer 3 as you state.  In the DC2044A the SGND-PGND connection is on layer 1 between pin 35 and the pad for the device.  That said, I am not impressed with the connection between SGND on layer 1 (split plane on the pin 13-19 side of the LTC4020) and the SGND split planes on layers 2, 3, 4.  The only connection I see is at the SGND turret, E9.  There should be vias in the SGND plane around the pin 1-31 sides of the LTC4020.  I like the DC2044A configuration, but there really should be more SGND vias.

2. For 10A charge current you will want to use 2oz. (71µm)  copper on all layers.  I cannot say whether 4 layers are adequate,  The main consideration will be current-handling capability of the connection from battery PGND to input PGND.

3. Again, I cannot give a definite answer.  You will need to consider the power dissipation of the FETs you choose.  Keep in mind that in buck-only mode the FET driven by TG2 will be on nearly 100% of the time.  Likewise, in boost-only mode the FET driven by TG1 will be on nearly 100% of the time.  Furthermore, with 20V in and 29.2V out the current in the TG1 FET (and inductor) will be on the order of 16A.    If you can find FETs that will dissipate no more than 1-2W you should be fine.  It may make sense to lay out pads for a second FET, especially the TG1 FET.

4. Charge current is set using the current-sense resistor placed between the power-stage output and the source of the battery PFET.  A 5mΩ resistor will set a charge current of 10A.  The full-current-charge value can be reduced with a resistor at RNG/SS pin.

5. I do not have direct experience using all parallel FETs in the DC2134A.  There really is no limit on the current capability of the LTC4020, since it is a controller and does not directly process power.  The main practical limitation is how much current will the PCB copper handle.  Pay attention to the current and power limitations in the gate-drive circuits.  Gate-drive is provided through the INTVCC regulator in the LTC4020.  Total gate current is limited to 85mA (worst-case),  Power dissipation in the INTVCC regulator is limited to ~500mW.  See datasheet, pp. 25, 26 for power-limitation information.  You will likely have more than 500mW power dissipation in the INTVCC regulator.  You can address this problem with an external regulator for INTVCC.  Do not use the circuit in Figure 9 or 10.  Instead, use the regulator to regulate the voltage at PVIN to 5-10V and allow the internal INTVCC regulator to do its work with a lower input voltage.  The 85mA total-gate-drive current limit is a hard limit.

Regards

Concerning the external regulator for INTVCC, I will repeat the warning, "Do not use the circuit in Figure 9 or 10.  Instead, use the regulator to regulate the voltage at PVIN to 5-10V and allow the internal INTVCC regulator to do its work with a lower input voltage."

Here is an example of the circuit I have used.

In my experience I have not needed to use external gate drivers for my 250kHz designs.  With higher switching frequencies or very large MOSFETs the external drivers may become necessary to support the higher gate-drive currents.  I do not have direct experience working with gate drivers in LTC4020 applications, but it should be possible to use them.  Be sure to consider any effects of propagation delay.

Concerning the external regulator for INTVCC, I will repeat the warning, "Do not use the circuit in Figure 9 or 10.  Instead, use the regulator to regulate the voltage at PVIN to 5-10V and allow the internal INTVCC regulator to do its work with a lower input voltage."

Here is an example of the circuit I have used.

In my experience I have not needed to use external gate drivers for my 250kHz designs.  With higher switching frequencies or very large MOSFETs the external drivers may become necessary to support the higher gate-drive currents.  I do not have direct experience working with gate drivers in LTC4020 applications, but it should be possible to use them.  Be sure to consider any effects of propagation delay.

Dear all,

In my design of a LTC4020 battery charger I found the next problem:

When I charge a battery at 3 amps, everything is ok, The controller works as expected. And the Stat1 signal is at 0v, so led is ON. This design is same as we can see in datasheet page 37 (two leds connected to Intvcc and series resistor of 2K7 to each led). But when the battery is fully charged, the stat1 should change to high impedance, so led should be off. In my case, the battery current is dropped to 0, but  the Stat1 does not go to high impedance. Instead, I can see an oscillation in Stat1, so led is on in a dimmed mode. The oscillation in Stat1 is a square wave, 28 hz, 70% duty cycle

I checked the Intvcc with oscilloscope and it is correct (5 volts)

Please send me your suggestions Thanks

I recommend checking the output of the power stage with a scope to see whether the converter is oscillating.  Feel free to post a schematic and power-stage-output oscilloscope image.  If posting a schematic please see the attachment for instructions.PDF

Hi, I have more information regarding the problem:

When the circuit is charging a battery, Stat1 is ok, to 0V. But when the charge is finished, charge current is dropped, and stat1 oscillates. 

The same effect arises when I use only as a power supply (without battery to charge or discharge). when output current is 0, Stat 1 oscillates, When ouptut current is increased to 1A, Stat1 is stabled, not oscillating

A similar situation occurs when I increase the input voltage. At 27v input,  stat1 is ok, but at 30v oscillates

Sometimes I solve the problem by switching on or off the load or the input voltage

In all cases, Int1=5v and pvin=7v, both with stable shape, correct.

Attached is included the stat1 signal when it is oscillating

PDF

PDF

PDF

Can you post the following?

• Schematic in text-searchable .pdf form
• Scope shot of power-stage-output voltage, waveform at BAT pin, inductor current.

Thanks for your suggestions.  May I send you the information in private email?