LTC7871: ITHLOW pin functionality and Burst mode

Hi, 

I'm confused a bit by the ITHLOW pin's functionality. I'm trying to operate the chip in buck mode with both DCM and Burst mode operation. Two LTC7871 are also synchonized to provide a 12 phase operation.

  1. From the block diagram on page 12, (SETCUR + 1.25) would override the ITHLOW pins voltage through the diode. So it seems like the avg current set by SETCUR is the ITHLOW. But in the text, ITHLOW sounds like the peak inductor current. Could you explain ? 
  2. In Table 2, there is a column titled "When Parked" and specifies prebiased turn-on for buck operation of ITHLOW. Could you explain how to interpret this table along with explaining what prebiased turn-on is?
  3. What is the difference in ITHLOW pin's operation in DCM and Burst mode operation ? 
  4. It is suggested to connect ITHLOW pins of two 7871 together for buck operation. Is that true for both DCM and burst mode operation ? 
  5. Should ITHHIGH pins of two 7871 be connected if chip is operated in buck mode only ? 

Parents
  • +1
    •  Analog Employees 
    on Jun 7, 2021 4:54 AM

    Hello,

    I apologize for the late reply.

    My replies are below:


    1. ITHLOW controls the peak inductor current as you stated. When the LTC7871 is operating as a buck, the current amplifier will over-ride the VFBLOW error amplifier once the IMON pin (represents the low pass filtered sum of the six inductor currents) exceeds 1.25V + SETCUR. Once this happens, the current amplifier controls ITHLOW and the converter behaves as a constant current converter. The VFBLOW and current amplifiers are labeled below. 



    2. When turning on into a pre-biased VLOW, ITHLOW is parked at its 0A voltage while VFBLOW is greater than the soft-start pin (VFBLOW will track SS during ramp-up instead of the internal reference). Once the SS pin exceeds VFBLOW, switching will start and the inductor current will start at 0 Amps instead of a negative value. This prevents large reverse current. This is important when turning on into a battery or other high energy source. A pre-biased output means that VOUT is at some value greater than 0 Volts before it turns on.

    3. ITHLOW pin behaves the same in FCM, DCM and burst mode. However in burst mode, the inductor current is set to 1/3 of the maximum current sense voltage independent of the voltage on the ITHLOW pin.

    4. The ITHLOW pins of the two LTC7871 controllers in a 12-phase converter need to be tied together when operating in FCM, DCM and burst mode.


    5. If the LTC7871 is operating as a buck only then the VFBHIGH pin can be tied to SGND and ITHHIGH can be left open. The two ITHHIGH pins do not need to be tied together.

  • Thank you for the detailed answer. Few followup questions: 

    1. Could you explain the functioning of the compensation network of ITHLOW pin? Is that a low pass filter or a related network which may control the ripple current etc ? Any recommendation for the compensation network R/C. 
    2. Is it a right interpretation that ITHLOW is exposed on the chip with these 3 intensions ? 
      1. Enable multiple LTC7871 to "share" information on the current and synchronize their current 
      2. Precisely set a peak inductor current using an analog voltage if the ILIM pin based settings need to be overridden ? 
      3. Somehow control the VLOW through the compensation network ? 
Reply
  • Thank you for the detailed answer. Few followup questions: 

    1. Could you explain the functioning of the compensation network of ITHLOW pin? Is that a low pass filter or a related network which may control the ripple current etc ? Any recommendation for the compensation network R/C. 
    2. Is it a right interpretation that ITHLOW is exposed on the chip with these 3 intensions ? 
      1. Enable multiple LTC7871 to "share" information on the current and synchronize their current 
      2. Precisely set a peak inductor current using an analog voltage if the ILIM pin based settings need to be overridden ? 
      3. Somehow control the VLOW through the compensation network ? 
Children
  • +1
    •  Analog Employees 
    on Jun 8, 2021 1:04 AM in reply to ethicalHacker

    Hello,

    The ITHLOW network along with the gm error amplifier forms a type 2 error amplifier. Application note 149 goes into more detail. The link is below:
    AN149 Modeling and Loop Compensation Design of Switching Mode Power Supplies (analog.com)

    You can use LTspice or LTpowerCAD to help design the network. The LTC7871 LTpowerCAD model is not available now, but the LTC3871 LTpowerCAD model can be used instead.

    You asked a good question. These are the  purposes of the ITHLOW pin:

    1. The ITHLOW pin is the output of the VFBLOW error amplifier. The network tied to this pin allows the phase margin and load step response for the converter to be optimized through the selection of RTH, CTH and CTHP.

    2. ITHLOW controls the peak inductor current for the 6 phases. (Peak inductor current = IL(DC) + Ipp/2). This allows the current to be equally shared from no load to full load. If two or more LTC7871 converters are paralleled, then their ITHLOW pins should be tied together to allow all of the phases to equally share the current. Page 28 and 29 provides details on the other pins that need to be tied together for multi-chip applications.

    The ITHLOW pin could also be used to control the DC current by over-riding it with an external amplifier. However, this is not necessary. The DC current can be programmed with the SETCUR pin instead. The SETCUR voltage can be programmed with a resistor or a voltage. Pages 44 and 45 of the data sheet cover this. The SETCUR pin can also be programmed via the SPI interface also.

    Best regards,

    Mike