SLVSDT1C July   2017  – June 2020

PRODUCTION DATA.

1. Features
2. Applications
3. Description
1.     Device Images
4. Revision History
5. Pin Configuration and Functions
6. Specifications
7. Detailed Description
1. 7.1 Overview
2. 7.2 Functional Block Diagram
3. 7.3 Feature Description
4. 7.4 Device Functional Modes
8. Application and Implementation
1. 8.1 Application Information
2. 8.2 Typical Application
1. 8.2.1 Design Requirements
2. 8.2.2 Detailed Design Procedure
3. 8.2.3 Application Curves
3. 8.3 System Examples
9. Power Supply Recommendations
10. 10Layout
11. 11Device and Documentation Support
12. 12Mechanical, Packaging, and Orderable Information

• MOP|24

#### 8.2.2.7 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 7, VOUT tracks VTR simultaneously.

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Equation 7.

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Following the example of Setting the Output Voltage with VOUT = 1.8 V, R1 = 240 kΩ and R2 = 120 kΩ, Equation 8 and Equation 9 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 8.

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R3 calculates now rearranging Equation 7:

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Equation 9.

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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 approxiimately 0.6V. Because 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, ΔtDELAY, delays the ramp of VOUT. The internal ramp starts after ΔtDELAY 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).