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TPS54627 TINA Model and Physical Application Issues

Other Parts Discussed in Thread: OPA2313, INA195, TPS54627

Hello,

I went ahead and simulated the circuit in TI TINA, with a basic functional summary outlined below:

I'm currently working with the TPS54627, INA195 and the OPA2313, to construct a voltage controlled adjustable current source capable of 6A.

- The TPS54627 device output is connected to the load through a 0.010 current sense resistor. The INA195 uses the voltage across this sense resistor to provide the Feedback to the TPS54627

-The OPA2313 is connected to provide current control, using a resistor divider of 10k and 10k between the OPA2313 and the INA195. The center of this divider is connected to the TPS54627's feedback.  

ASSUMING THE OPA2313 OUTPUT IS 0V, 

- For the INA195, this would be a V_out = 100 * 0.010 * I_load

- Or V_out = I_load, i.e. 1V = 1A in this case

The target voltage of the TPS54627 is 0.765V, and my target current is 6A, so I have the INA195 output connected across another resistive divider with a 10k and 3.4k resistor.

Through the first resistor divider: V_fb = 0.765 = V_d * (10k)/(10k+10k)

V_d = 0.765 * (10+10)/(10) = 2*0.765 = 1.53 V

Through the second INA divider:

V_d = 1.53 = V_out * (3.4k/(3.4k+10k))

V_out = 1.53*(10+3.4)/(3.4) = 6.03 V

All of this works correctly in my simulations.

I have gone ahead and assembled a test circuit. This is where my issue is. I was getting an output of a mere 14mA. So I verified my connections and confirmed by result. SS was left unconnected in my simulations (and works fine in the model), so I started experimenting.

I was successful by simply increasing the SS capacitance on the TPS54627 brought the current up higher, to about 1.2A, at which point it visually shuts down
for a SS capacitance 10uF, this is a little over 1 second per the datasheet:

t_ss_(ms) = C_(nF) * 0.765 * 1.1 / 6

t_ss_(ms) = 10000 * 0.765 * 1.1 / 6 = 1402.5 ms = 1.4 sec


I wanted to ask about the TPS54627's hiccup mode and current limiting and how it works. Again per the datasheet, the internal current limit is a minimum of 6.3A, which my application is below.


My belief is, the target voltage of the INA195's V_out is demanding 6.03V, but because the TPS54627's feedback voltage is less than 65% of the target 0.765, it never actually reaches the target, and thus dramatically reduces the current, but NEVER shuts down. The voltage rises fine in either case of load or no load, but the current appears to be self limiting.

I believe something about the way the TPS54627 is modeled in the provided TI TINA simulation models does not include the behavior of the Over Current Protection or Under Voltage Protection behavior of the TPS54627. 

Where should I go from here?

  • Hi, 

    I briefly looked at your posts, and found out maybe three questions,

    1) How the Hiccup function work?

    when some scenario, such as hard short happens, VFB drops, circuit detects if for a short time (us level), and Hiccup function is working to protect circuit, low side Power MOSFET turns off, and then there's Hiccup off time for roughly 20msec (related to soft start time). after Hiccup off time, if the short remains, IC enter into Hiccup again.

    2)VFB is only 65%, i think it means your circuit's large signal behavior (DC) hasn't established correctly. 

    3) for TINA simularion model on OVP and UVP, need to consult modeling team for this answer, will keep you updated.

  • Nick,

    I've been working with this a bit more.

    In case (1) where you described a hard short -- lets say for instance the load across the the TPS54627 is a diode and the sense resistor I described (0.010 ohm, in series with the diode).

    Effectively in the case where the V_f of the diode is great enough, and the diode is turned on. Lets suppose for easy math, V_f is 1V @ 6A. 

    1/6 = 0.167 ohm @ 6A. 

    0.167 + 0.010 = 0.177

    In series, the total load resistance would be expected to be approximately 0.177 ohm @ 6A. 

    I believe now a I have a better question:

    1. What is the minimum load resistance necessary for the TPS54627 to maintain good load regulation without entering hiccup mode?

    Since it seems like <0.200 ohms or less is too low.

    2. What is the maximum SS capacitance?

    By increasing the SS capacitance, I have been able to increase the current, HOWEVER I believe this is only because of the hiccup delay time. 100uF is very large for this, and it would likely require another order of magnitude (>1000uF) to rise to the correct level.

    Thanks