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Questions about LTM4622A

Hi AD team, please answer the following questions:

1. The LTM4622A datasheet says (in the Description section):

Fault protection features include input overvoltage, output overcurrent and overtemperature protection.

However, no explanation is given on the overcurrent protection mechanism. Hence the question: is the LTM4622A module output protected against short-circuit condition? If yes, how this protection works (shut-down with soft start upon short-circuit elimination, etc.) If the protection is implemented as output current limiting, then what is the current limit when both channels are paralleled, and the output voltage is in the range 4 to 10V?

2. Suppose the module is operating at 10V output, the output current being 1 to 1.5A (once again, both channels are paralleled). What happens if the external feedback resistor is instantly changed from 3.86 kOhm (corresponding to 10V output) to 10.6 kOhm (corresponding to 4V output)? Is it safe for the module to perform an output voltage transition in this way?

3. Will the LTM4622A module stay in production in the coming years?

Thanks in advance and best regards,

Michael

  • Hi Michael, 

    Thanks for the message. Please see my comments / replies below. Thanks. 

    1) The LTM4622A has a cycle by cycle over-current protection, if current limit is exceeded such as in a short circuit event the inductor valley current will be limited to the current limit so the output voltage will fall according to this current limit level and the short impedance. Once the short is removed so current limit is no longer exceeded the output voltage will immediately ramp back to regulation without soft-start. Some example short circuit and start-up captures are shown on page 6. From the electrical table the output current limit is listed for a typical operating condition (ie 12Vin to 1.5Vo). The LTpowerCAD design tool (LTpowerCAD | Analog Devices ) can also help you to estimate the output current limit for other operating conditions. If both channels are paralleled the total current limit will be 2x of a single phase (each phase provides half the current). 

     

    2) This is a large step change in output voltage, but is fine for the LTM4622A, the output will transition based on the control loop responding to this transient kind of event. You can also consider using another scheme such as DAC for better control of the transition between the desired output voltages. 

     

    3) Yes the LTM4622A is a popular module expected to stay in production for the foreseeable future.

  • Hi SYoung,

    thank you very much for your reply. It was really helpful. In fact, I'm using a DAC to control the output voltage of LTM4622A. I just tried to describe the physical situation rather than specific solution.

    Best regards,

    Michael

  • Hi Michael, 

    Thanks. Also for more reference, if you would like to check your design in LTpowerCAD attached is an example LTpowerCAD design tool for the LTM4622A configured with a starter setup for a 12Vin to 4Vo or 10Vout application with the LTM4622A in 2-phase single output configuration. You can also simulate in LTSpice for more details. A demo board is available (DC2568A) as well for bench evauation, the demo board includes optional jumpers that can be installed to configure for paralleling of both phases together. 

    Thanks. 

    LTM4622Poly-phase12Vin4Voor10Vo.ltpc.zip
  • Hi SYoung,

    thank you very much and sorry for the delay. Finally I had time to install LTSpice and try it. It looks impressive, but there are a few questions:

    1. Specific capacitors were used for the simulations. Can I change them? Does the program really account for all capacitor characteristics, including ESR vs. frequency, capacitance vs. DC bias and temperature, etc.?

    2. In my application, I use paralleled outputs to draw a maximum of 1.5 A, i.e. less than a single output can supply. How can I configure the simulation for this case? (I assume that for currents 3 - 4 A the simulation "connects" the outputs automatically, but for currents < 2A there are two possibilities, hence the question.) 

    3. For voltages above 5 V, the datasheet recommends the frequency of 1.5 MHz. But the simulation shows that for a 8 V output 964 kHz is more efficient than 1.5 MHz (97.28% vs. 96.86%). Is this correct? Are there other factors (except for the output voltage) that affect the required operating frequency? For instance, shouldn't the frequency be less than maximum operating frequency of input and output capacitors?

    Thanks in advance and best regards,

    Michael

  • Hi Michael, 

    Note the attachment provided in the previous reply was for LTpowerCAD. In case you were actually referring to LTpowerCAD (instead of LTSpice) in your questions, replies were included for both cases below. Thanks.  

    For LTpowerCAD : 

    (1) The simulation does not account for all characteristics, but does try to account for DC bias characteristics if the output cap. was chosen from the built in cap. library and the setting to "Estimate Actual Capacitance at DC Bias" is selected from the (gear shaped) menu towards the top of the schematic page. Otherwise it would be recommended the actual parameters be entered by the user in the schematic for the operating condition according to the cap. datasheet specs.

    (2) If you are using a parallel design tool (ie like the example forwarded previously) then it already accounts for the parallel connections in its estimations.  

    (3) Same comment as # '(3)' below. 

    For LTSpice : 

    1) The simulation does not account for all capacitor characteristics (ie capacitance vs. DC bias, AC bias, temperature,...) so it would be preferred these parameters (ie actual C, ESR, ESL) be entered by you using the cap. datasheet's provided curves for the expected operating conditions.

    2) In LTSpice the circuit should be configured for parallel outputs as you would for your actual schematic (ie by drawing the connections in the LTSpice schematic to connect respective VIN, VOUT, FB, COMP, SS, RUN pins together among phases)

    3) The recommended operating frequency in the LTM4622A datasheet also considers having reasonable inductor ripple current. In this case (ie 12Vin to 8Vo, 964kHz) you may get a little higher efficiency with the lower frequency. The part will also operate find under that condition although switching freq. is not at recommended. The recommended switching freq. is also concerned with making sure iL ripple is not excessively large that it would cause the reverse current limit to be tripped at max. Vin an no load in steady state. 

    Thanks. 

  • Hi SYoung,

    In fact, I did install LTpowerCAD, as it was mentioned in your previous reply, but then I looked somewhere else and made a mistake while writing my answer. Thank you very much for the explanations, they are very helpful.

    Best regards,

    MIchael