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SMM665, 605, 205, 105 Supply Voltage Marginers and Active DC Output Controllers COMPONENT SELECTION FOR OPTIMUM PERFORMANCE

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Finding the Margin Settling Time

Once the C_TRIMCAP value is chosen the MARGIN settling time is found:


Where:




The MARGIN time can be reduced by 1/10th by selecting the Fast Convergence feature of the device family or choosing a slightly larger ripple voltage.

Setting the Value of R_TRIM (Figure 2)

The device TRIM pin is connected to the Trim or VADJ pin of a DC-DC converter or adjustable switching regulator (PWM or LDO). Typically, the converter's Trim pin is internally connected to a resistor and the gain is determined either by inspecting the data sheet or empirically finding the value. In this example, the TRIM pin is connected to an external resistor (R3, R_TRIM) that is used as the equivalent resistor found in the converter module. R3 connects to the negative input terminal of the error amplifier. This input is held at the internal reference voltage used by the converter. Another resistor (R5) is connected from the negative input terminal to ground. Resistor (R4) is connected from the negative input terminal to the positive sense line of the converter (output voltage) completing the resistive divider feedback network.

The current in R5 is always V_{REF(converter)}/R5. The part controls the voltage on the Trim pin that in turn controls the current through R3. The current through R3 is (V_TRIM - V_{REF(converter)})/R3. The current in R4 (R_TRIM) is equal to (V_{OUT(converter)} - V_{REF(converter)})/R4 where V_{OUT(converter)} is the converter output voltage. With this configuration, the current in R3 will change the converter output from its nominal output. The change in V_{OUT(converter)} in Volts is equal to the current in R3 times R4 or ((V_TRIM- V_{REF(converter)})/R3) * R4.

 
In order to minimize the ripple on the converter output due to changes in the TRIM pin voltage, R3 can be chosen such that the maximum swing of the TRIM pin causes a maximum change of +/-10% in the converter output.

For the converter described above, R3 is chosen as (with safety margin) less than: (V_{REF(converter)} - 0.3V)/(10% of voltage supply) * R4.

Once R3 is chosen, the ripple on the converter output (usually set under 1mV) can be calculated as the ripple on the TRIM pin times R4/R3.

TRIM_CAP Selection

The ADOC function requires attention be paid to minimize the leakage current from the TRIM_CAP pin. The maximum allowable leakage from this pin is:


Allowing for PCB and other leakage sources use 50nA as the maximum leakage allowed from the TRIM_CAP node capacitor. For a circuit having an average voltage of 5V on the TRIM_CAP capacitor, the maximum allowable leakage (IR) resistance of the capacitor is:


Many vendors offer an X7R type ceramic of value 1uF with adequate IR (insulation resistance) to be suitable for the ADOC TRIM_CAP (see list below). Be certain the ceramic capacitor chosen also meets or exceeds the IR requirements at elevated temperatures. Film capacitors do offer much higher IR ratings but at both a cost and space premium. These may be used as an alternative but are generally not required.

Kemet: C0805C105K9RAC, 1uF, 0805, +-10%, 6.3V
http://www.kemet.com

AVX: 08056C105KAT4A, 1uF, 0805, +-10%, 6.3V
http://www.avxcorp.com

TDK p/n: C2012X7R1C105K, 1uF, 2012, +-10%, 16V
http://www.component.tdk.com

NIC Components Corporation: p/n
NMC0805X7R105K16TRP, 1uF, 0805, +-10%, 16V
http://www.niccomp.com