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Feedback divider impedances too high on TPS62822 and TLV61220 regulators

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Valutazione

Difficoltà
3/5
Tempo stimato
1-2 giorni
Idoneità per principianti
48/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Tranquilla
Ambito
embedded-iot

Direzione di ricerca

Start by locating U1 (TLV61220DBVR) and U2 (TPS62822DLCT) and their feedback-divider resistors in the board schematic. Read the relevant datasheet sections, verify the placed resistor values and intended output voltages, then confirm that any reduced values preserve the divider ratios and meet the quiescent-current requirements.

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Descrizione

bug

Summary

Both switching regulators on the board use feedback resistor dividers with impedances in the 1–2 MΩ range, which is above or at the upper limit recommended in the respective datasheets. High feedback impedance degrades output voltage regulation accuracy and increases susceptibility to board leakage and noise.

TPS62822 (U2, 3V3_AUX step-down)

The output voltage is set by the FB divider per:

V_OUT = V_ref × (1 + R_top / R_bot)

TI's TPS62822 datasheet recommends feedback resistor values in the 100 kΩ range. Values approaching or exceeding 1 MΩ are explicitly flagged as increasing output voltage error due to:

  • Bias current into the FB pin creating a voltage error (I_bias × R_top)
  • PCB surface leakage becoming significant relative to divider current
  • Increased noise coupling into the FB node

The TPS62822 FB pin bias current is specified as up to 100 nA. With a 1 MΩ top resistor, this creates up to 100 mV of voltage error on the feedback node — which directly shifts the output voltage.

Action: Reduce the TPS62822 feedback divider resistors by a factor of 10 (e.g., from ~1 MΩ/x to ~100 kΩ/x). The output voltage ratio is unchanged; only the absolute impedance decreases.

TLV61220 (U1, boost converter)

The TLV61220 feedback divider uses resistors in the 1.8 MΩ range. The TLV61220 datasheet (§9.2.2) states: "To minimize quiescent current, it is recommended to use large resistors in the feedback network" — however, it also states that values beyond ~2 MΩ risk regulation accuracy degradation from input bias current (I_B ≈ 5–30 nA typical, up to 100 nA worst case).

At 1.8 MΩ and I_B = 100 nA: V_error ≈ 180 mV on the FB node, which can shift the output voltage by a comparable amount depending on the divider ratio. This is especially problematic if the TLV61220 is boosting to a well-defined voltage (e.g., 5 V) that other circuits depend on.

Action: Review the intended output voltage and quiescent current budget. If quiescent current is critical, 500 kΩ–1 MΩ is a reasonable compromise. If quiescent is not the bottleneck, reduce to 100–200 kΩ for better regulation accuracy.

Affected components

  • U2: TPS62822DLCT (3V3_AUX), feedback divider resistors (verify designators)
  • U1: TLV61220DBVR (boost), feedback divider resistors (verify designators)

Additional note

Verify the actual placed resistor values match the intended output voltages. If the feedback divider was designed for the correct V_OUT but uses high-impedance resistors, reducing the resistor values (maintaining the same ratio) will fix the issue with no functional change.

🤖 Generated with Claude Code

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