LTC2977 Triming status when DC/DC load transient

Hi,

We use LTC2977 to trim DC/DC(LTM4650) with an external feedback network.

When the load transient, if LTC2977 exactly monitor the pulse (for example, 6mV diviation), if LTC2977 adjust its DAC to trim DC/DC ? Because the DC/DC will return back after some microsecond, but if LTC2977 trim the DC/DC during load transient, it will degrade the power supply performance.

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  • The LTC2977 servo loop acts to adjust Vout over relatively long periods, approx. 100ms.  There are two feedback loops in such a system.  The fast loop is the 4650 and associated top/bottom feedback resistors.  The slow loop is the 2977's ADC and DAC.  A DAC resistor adds/subtracts a small DC current into the feedback node of the switcher.  This acts to 'trim' the output. The servo loop updates at 100ms and is intentionally slow so as not to conflict with the fast loop.  For fast transients, the 2977 servo loop doesn't see pulses because the ADC takes 6ms to accurately measure Vout and moves the DAC one LSB, and is essentially 'blind' to small glitches. 

    To improve the load step performance of the 4650 (fast loop), you may consider adding components to the COMP pins and/or possibly consider layout changes if you already have hardware and are taking measurements.

    Mike

Reply
  • The LTC2977 servo loop acts to adjust Vout over relatively long periods, approx. 100ms.  There are two feedback loops in such a system.  The fast loop is the 4650 and associated top/bottom feedback resistors.  The slow loop is the 2977's ADC and DAC.  A DAC resistor adds/subtracts a small DC current into the feedback node of the switcher.  This acts to 'trim' the output. The servo loop updates at 100ms and is intentionally slow so as not to conflict with the fast loop.  For fast transients, the 2977 servo loop doesn't see pulses because the ADC takes 6ms to accurately measure Vout and moves the DAC one LSB, and is essentially 'blind' to small glitches. 

    To improve the load step performance of the 4650 (fast loop), you may consider adding components to the COMP pins and/or possibly consider layout changes if you already have hardware and are taking measurements.

    Mike

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