TPS7A91 Low-Noise LDO: 7 Checks for Soft Start and Power-Good Timing
TPS7A91 Low-Noise LDO: 7 Checks for Soft Start and Power-Good Timing
Plan TPS7A91 regulation with real dropout margin, SS_CTRL selection, capacitor derating, load-step behavior and power-good timing. This is a documentation-based planning guide, not a report of independent hands-on testing.
1. Start with a feasible voltage pair
TPS7A91 supports a 1.4V to 6.5V input range and an adjustable output from 0.8V to 5.2V. Those ranges do not mean every input-output pair is possible: regulation also requires dropout headroom at the intended current and temperature. Check source tolerance and transient droop before selecting a nominal rail. The 1A rating is conditional on thermal and electrical limits. Begin with a modest load and identify the exact package and feedback network before energizing a custom board. Manufacturer documentation.
2. Set the output and thermal budget together
Choose feedback resistors using the manufacturer equation and recommended values, then include resistor tolerance in the output-voltage budget. Place sensing and quiet-ground connections deliberately so trace drop does not become an unnoticed regulation error. Estimate pass-device dissipation from input minus output voltage multiplied by load current. For example, dropping 5V to 3.3V at 0.5A produces about 0.85W before smaller losses. Confirm that the actual package mounting and copper can handle the resulting junction-temperature rise. Manufacturer documentation.
3. Preserve effective input and output capacitance
The device is designed and characterized with ceramic input and output capacitors of 10 microfarads or greater under the documented recommendations. Account for DC-bias and temperature effects instead of equating a printed capacitance with its effective operating value. Keep the components close to the pins and avoid long inductive supply leads. More capacitance is not automatically better: cable inductance and low ESR can create ringing, while increased output capacitance changes both transient depth and recovery time. Manufacturer documentation.
4. Choose the soft-start charging mode deliberately
SS_CTRL selects the NR/SS capacitor charging current. Connecting it to ground gives a typical 6.2-microamp current; connecting it to IN increases the typical value to 100 microamps. A larger current shortens the initial ramp for a given capacitor, but the design must include tolerance and the load behavior during startup. If no NR/SS capacitor is used, the pin guidance requires SS_CTRL at ground to avoid output overshoot. Do not leave this configuration to an unspecified module default. Manufacturer documentation.
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5. Allow the final settling tail
The initial ramp estimate does not describe the entire precision-settling interval. The documentation explains that the final portion of the NR/SS voltage charges through an internal resistance, producing an additional RC tail. Large noise-reduction capacitance can therefore extend the time before the highest output accuracy is reached. Define a valid-measurement delay based on the required error, not just the first apparently stable waveform. Record the capacitor and startup mode with any measurement that depends on early post-enable accuracy. Manufacturer documentation.
6. Qualify PG alongside feedforward capacitance
PG is an open-drain status output and needs a compatible pullup. A feedforward capacitor can improve noise and PSRR, but larger values can lengthen startup so PG incorrectly appears to indicate a fully settled output. Observe PG and the output voltage together through startup, shutdown and realistic load changes. If a downstream converter needs tighter settling than the status threshold provides, add an explicit delay or independent qualification. Do not use an unexamined PG edge as a precision timing guarantee. Manufacturer documentation.
7. Test noise and recovery as different behaviors
The low-noise headline applies to a specified frequency band and circuit conditions; it is not a measurement of every attached load. Keep probing noise, bandwidth and grounding under control when evaluating the rail. Test both load application and removal because the regulator cannot actively sink excess output charge in normal regulation. Save supply, load, capacitor network, startup configuration and temperature with the results. A useful handover distinguishes steady-state noise, dropout, transient response and settling instead of summarizing them with one attractive number. Manufacturer documentation.
Related reading: TPS7A4701 RF LDO: 7 Checks for ANY-OUT Settings, Noise and Startup.
Catalog note: The exact-part search did not return a matching Epro listing during preparation. Check the current catalog; this guide makes no price, availability or included-accessory claim.