8.2.2.6 Tracking

For values up to 0.6V, an external voltage, connected to the SS/TR pin, drives the voltage level at the FB pin. In doing so, the voltage at the FB pin is directly proportional to the voltage at the SS/TR pin.

When choosing the resistive divider proportion according to Equation 8, V_OUT tracks V_TR simultaneously.

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Equation 8.

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Figure 9. Voltage Tracking

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Following the example of Setting the Adjustable Output Voltage with V_OUT = 1.8 V, R₁ = 240 kΩ and R₂ = 120 kΩ, Equation 9 and Equation 10 calculate R3 and R4, connected to the SS/TR pin. Different to the resistive divider at the FB pin, a larger current must be chosen, to avoid a tracking offset caused by the 5.25 µA current that flows out of the SS/TR pin. Assuming a 250 µA current, R₄ calculates as follows:

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Equation 9.

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R₃ calculates now rearranging Equation 8:

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Equation 10.

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However, the following limitations can influence the tracking accuracy:

• The upper limit of the SS/TR voltage that can be tracked is about 0.6V. Since it is detected internally by a comparator, process variation and ramp speed can cause up to ±30 mV different threshold.
• In case that the voltage at SS/TR ramps up immediately when VIN is supplied or EN is set High, the internal startup delay, Δt_DELAY, delays the ramp of V_OUT. The internal ramp starts after Δt_DELAY at the voltage level, which is actually present at the SS/TR pin.
• The tracking down speed is limited by the RC time constant of the internal output discharge (always connected when tracking down) and the actual load with the output capacitance. Note: The device tracks down with the same behavior for MODE High (Forced PWM) and Low (Auto PSM).