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Obtaining Optimum Performance with Summit's ADOC™/Marginer Family

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Figure 5. VMX/GND Connections to 2 Loads    Figure 6. Use a buffer to reduce/eliminate PUP currents    Figure 7. CAPX and VMX Filter Action

The PUP outputs: Pullup resistor selection
Excessive currents flowing into the PUP outputs can contribute to measurement errors. Minimize these currents to less than 3mA total by using the largest possible resistor value that will ensure a high noise margin to avoid false turn-on of the DC-DC converters. If the PUP currents are unavoidably excessive, use a buffer between the PUP outputs and the converter's ENABLE pin (Figure 6).

ADC Accuracy

The ADC is a separate functional block than the ADOC and is used for monitoring and reading channel supply voltages. As it is a separate block, certain external components are required to optimize its accuracy and separate it from the ADOC function.

CAP_X : Selection and Proper Placement

This component is used to filter switching noise on each VM_X input prior to inputting into the ADC. The capacitor forms a low-pass filter with the marginer's internal resistive divider (Figure 7).

The minimum recommended value for CAP_X is 80nF placing the -3dB filter point at 80Hz; well below the ripple and noise voltage caused by the converter's VM_X switching frequency.

The value of CAP_X is also important for eliminating last sequence position overshoot. Overshoot during sequence on of the power supplies can occur on the last sequence position if ADOC is enabled on the channel(s) and if the UV1 setting of that channel(s) is less than 80% of the channel's nominal voltage. This is due to the TRIM_CAP_X capacitor not reaching the nominal value of the converter's TRIM pin voltage before the UV1 setting is reached by the converter's output.

The charging current available from the TRIM_CAP_X node during sequencing is 1mA and assuming a final TRIM voltage of 1.25V the time to fully charge a 1µF TRIMCAP_X capacitor is:



To prevent overshoot choose the RC time constant consisting of the internal R = 25k and the CAP_X capacitor so the input to the ADC lags the converter output by 2ms. This allows the TRIM_CAP_X node to be fully charged before the device enables ADOC thus preventing overshoot (See Figures 9-12). For TRIM_CAP_X capacitors larger than 1µF the 2ms time constant should be multiplied by: