LTC2977

After reading the LTC2977, does this device actually allow user to monitor and control up to 8 power supply outputs.
I'm using eight of AC/DC and DC/DC in the range of 5v to 40v. Both positive and negative voltages.

The device can track all eight power supply within this range? for example output voltages and current?

Does it allow power turn on sequencing.

Does this device get connected in series with my DC/DC and AC/DC converter?

Regard,

Tom

  • 0
    •  Analog Employees 
    on Jun 12, 2020 4:36 PM 3 months ago

    Hi  Tom,

    Yes, the 2977 is able to monitor and control eight power supplies. The chip only monitors DC voltages.  Maybe you can provide more info on the AC/DC converter, or send a diagram.

    The 'control' aspect is enabling and disabling the individual supplies. The supplies can be enabled/disabled either with a CONTROL pin, with a PMbus command, or it will just respond to Vin.  The enable signal is a logic level signal.  There's also a sequencer built into the 2977 that allows you to set the time when enables fires to each supply. 

    The 'monitor' aspect takes a bit more explanation, especially when high voltage is involved. The 2977 has eight voltage sensing pins, and the max voltage is limited to 6V.  If you need to sense a 40V rail, you'll need to divide down 40V to something less than 6V to satisfy the 2977 ratings. This can be accomplished with a simple voltage divider, or with an op-amp that drives a single-ended voltage into the Vsense pin.  The advantage of the op-amp is its low impedance output that drives Vsense. The resistor approach is fine but you need to select the values carefully, so as to avoid errors introduced due to Vsense pin current being injected into the divider. The demo board DC2518 covers some of these issues.

    When discussing HV power management, we will shortly introduce a 2-ch PSM that supports voltages as high as +/-60V.  The part number is LTC2971. It controls and monitors 60V supplies and also measures output current.  You would need four 2971 devices to monitor/measure 8 power supply rails.  It also measures/monitors the input supply's voltage and current.

    Whether you use a single 2977 or multiple 2971, the supplies can be sequenced based on the timing settings that you set. A timing signal called SHARE_CLK is tied across all PSM devices and is used to synchronize the on/off delay times.

    I hope this gives better insight into the workings of a PSM device.

    Mike

  • Thanks MPETERS for the reply, I have some follow-up questions:

    Since the control portion of the LTC2977 is use to control the ON/OFF function of the power supply device then most likely I must select to use a DC/DC converter instead of a AC/DC converter because most AC/DC converter / regulator does not have an ON/OFF pin, ...do you agree. (maybe I'm missing something)

    My system power distribution consist of 8 power supply: +/- 5v, +/- 15v, +/-20v, +40v and another master +5v power supply, They are all AC/DC switching converter from Acopian. Since they do not have an ON/OFF input, I probably have to switch them all to DC/DC converter instead if I were to perform power Sequencing. 

    In this case, my power topology would change to include a high power AC/DC then the output would drive my eight DC/DC.

    Originally, I thought the LTC2297 controls the distribution of all eight power by connecting the power supply voltages to the LTC2297 device input pin then performing the ON/OFF function internally and then sending the voltage out of the  LTC2297 device pin then to the "User Load". This is NOT the case of the ON/OFF function of the LTC2977,... correct ?

    As far as monitoring a Negative power supply voltages, does the LTC2297 support negative voltage, example -5v, -15,....

    Lastly, Can the LTC2977 monitor input and output current? If it can not then are there any current LTC / ADI device that can perform this?

    I appreciate your prompt attention to this matter.

    Regard.

    Tom

  • 0
    •  Analog Employees 
    on Jun 15, 2020 8:40 PM 3 months ago in reply to tomc123

    Hi Tom, I am not familiar with Acopian power supplies but I looked a few of their products. It looks like a few have an INHIBIT input that could be used with the VOUT_EN of the 2977.  Not sure about the polarity of INHIBIT. 

    The bigger issue, and you point it out, is that you need to decide on the power topology. It would seem that one AC/DC converter would be a better choice and select your 7 or 8 switching regulators to provide DC power. Maybe buy an AC/DC converter that drives +48V which would be your input supply for the 7 switchers.

    The 2977 can measure negative supply voltages.  We show a simple way to sense negative voltage in Fig 29 on pg 85.

    Finally, the 2977 is a power manager, does not provide power itself.  If you need to measure input supply current and output load current, I suggest you look at the 2975 (4ch) or 2972 (2ch).

    Regards

    Mike

  • Hi MPETER,

    The LTC2977 has a separate VIN and VOUT: sensing, monitoring nomenclature.  Function wise, shouldn't they be the same. Why are they designated separately. sensing should be the same regardless if it come from VIN or VOUT of the DC/DC converter unless the function if different, and if they are different then can you explain,

    Regard,

    Tom

  • 0
    •  Analog Employees 
    on Jun 18, 2020 8:29 PM 3 months ago in reply to tomc123

    On the VIN side, you power the chip typically with the VPWR pin and use VIN_SNS to sense the input power supply voltage.  The VIN_ON and VIN_OFF are user defined thresholds that the chip reacts to start many of the management functions, such as sequencing and fault detection.  The VIN_SNS pin is one of the inputs to the mux'd ADC.  Both VPWR and VIN_SNS pins are allowed to go as high as 15V.

    For the Vout side, the 2977 manages eight power supplies.  Each supply is enabled, monitored, supervised, and servo'd.  The VSENSE pins are allowed to directly touch output voltages up to 6V.  If higher voltages are present, you may use a voltage divider to keep the pin below 6V.  The mux'd ADC does a round robin measurement of these eight VSENSE pins.  Each pin also has a high speed supervisor that uses a comparator that trips if the OV or UV output voltages are exceeded.

    Typically you would tie VIN_SNS to the system's input power supply which powers the eight 'output' supplies.  The VIN_SNS pin does not have a supervisor to quickly fault off the eight channels. Instead it uses the ADC result to determine if there's an OV on Vin.  It is common for Vin to be +12V, and this supply powers a number of buck regulators that may be at various 'low' voltages, such as 3.3V, 2.5V, 1.8V, etc.

    I hope this gives you more context.

    Mike