LT1054L

My LT1054L circuit is not operating.  Vref is at approx 2.5V and the FB pin is above 2V, Vcc and GND are connected but the output voltage is not going negative.

Will a large inductance (100uH) with low DCR (0.083 ohms) at the Vin pin (5V = Vin), which has a 22uF low ESR tantalum cap as Cin, cause the part to be damaged?
I ask because when I simulate my circuit, there are considerable voltage spikes at the Vin pin when the part starts up and these spikes increase with higher Cin.
Bringing down Cin to 1uF keeps these transients to about 12V but that could still damage the LT1054L which is only rated for 7V.

I could try reducing my Cin to 1uF or so, but I'd like to understand if I need to change my entire topology and do the inductive filtering downstream rather than trying to inductively isolate the LT1054L power.  Also, I need to know if I should be ordering more parts, because I've blown the ones I have.  (I will check the CAP+ and CAP- pins today, but if they turn out to be operative, I'm not sure if the part is still good--I don't know the exact operation failure that would occur from overvoltage spikes.)

The circuit always simulates fine and gives me pretty much the expected output (-Vout is not quite the calculated value). 
It is only in the real world that things are going wrong.

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  • 0
    •  Analog Employees 
    on Jul 29, 2020 1:01 PM

    Hello, For the S8 package CIN, (C4 in this case) should be about equal to COUT (C5). This is explained on page 8 of the datasheet but it does not specifically specify the "L" version. 

    The LT1054 basically connects C4 across VIN and GND to charge up C4. Then C4 is disconnected from VIN and GND, C+ is connected to GND and C- is connected to VOUT to supply the output. The regulator does also control the voltage across C4. 

    When C4 is connected to VIN a large slug of current is used to charge C4 from VIN and C1. The current will come from the input to replenish C1 which in this case is thru the 100uH inductor. The simulation should show what that current is. When C1 is replenished and the current into C1 stops there can be a voltage across L1 causing the voltage spike. If the voltage of V+ is above the absmax voltage then it can break the LT1054L. 

    Somethings that might help. Decrease C4 to match C5. If a 100uH inductor is needed for a filter, place a large bulk cap next to the inductor to help absorb some of the charge. When C1 sends slugs of current to charge C4, C1 can be replenished with the bulk cap and reduce the large current in the inductor causing the voltage spike. I hope your simulation can show this.

    Please also make sure C1, C4 and C5 are as close to the LT1054L as possible. Inductance in the traces can also contribute to the voltage spike.

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  • 0
    •  Analog Employees 
    on Jul 29, 2020 1:01 PM

    Hello, For the S8 package CIN, (C4 in this case) should be about equal to COUT (C5). This is explained on page 8 of the datasheet but it does not specifically specify the "L" version. 

    The LT1054 basically connects C4 across VIN and GND to charge up C4. Then C4 is disconnected from VIN and GND, C+ is connected to GND and C- is connected to VOUT to supply the output. The regulator does also control the voltage across C4. 

    When C4 is connected to VIN a large slug of current is used to charge C4 from VIN and C1. The current will come from the input to replenish C1 which in this case is thru the 100uH inductor. The simulation should show what that current is. When C1 is replenished and the current into C1 stops there can be a voltage across L1 causing the voltage spike. If the voltage of V+ is above the absmax voltage then it can break the LT1054L. 

    Somethings that might help. Decrease C4 to match C5. If a 100uH inductor is needed for a filter, place a large bulk cap next to the inductor to help absorb some of the charge. When C1 sends slugs of current to charge C4, C1 can be replenished with the bulk cap and reduce the large current in the inductor causing the voltage spike. I hope your simulation can show this.

    Please also make sure C1, C4 and C5 are as close to the LT1054L as possible. Inductance in the traces can also contribute to the voltage spike.

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