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Board Configuration

If no tracking feature is required, both TRK1 and TRK2 should then be tied to VDD in order to soft-start

each output voltage based on soft-start capacitor value (Condition 3). This can be done by keeping R24,

R26, and R27 open while adding a 10Ω resistor onto R25 and R28.

Please note that the slew rate of track signal should be lower than the soft-start slew rate which is set by

soft-start capacitor value.

The following are the summary of different tracking configuration on LM3000EVAL board:

Condition R24 R25 R26 R27 R28

1 Open 10Ω 10Ω Open Open

2 10Ω Open Open 10Ω Open

3 Open 10Ω Open Open 10Ω

Figure 3. Tracking with an External Ramp for 3.3V Output

3.4 Output Voltage Ripple

Output voltage ripple measurement should be taken directly across the output capacitor C21 or C22. Care

has to be taken to minimize the loop area between the scope probe tip and the ground lead in order to

minimize noise in the measurement. This can be achieved by removing the probe’s spring tip and ground

lead and then wire a bare wire around scope probe shaft. The bare wire should be in contact with the

probe shaft since this is the “new” ground lead for the probe. The measurement can be taken by

connecting the bare wire onto the ground side of the capacitor and the probe tip onto the other side of the

capacitor. Figure 4 shows a diagram of this measurement technique.

Figure 4. Output Voltage Ripple Measurement Setup

SNVA389A–July 2009–Revised May 2013 AN-1951 LM3000 Evaluation Board

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