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TPS62822 PG pullup to 5V drives MCP23018 GPIO input above absolute maximum

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Valutazione

Difficoltà
3/5
Tempo stimato
1-2 giorni
Idoneità per principianti
55/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Tranquilla
Ambito
embedded-iot

Direzione di ricerca

Start by locating the schematic containing U2, R1, U41, and the 3V3_AUX_PG net, then verify R1's pull-up supply and U41's VDD. Choose and document one approved fix, ensuring GPB2 remains within its absolute maximum voltage; recheck the net and power-sequencing implications before considering the issue done.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

bug

Summary

The power-good (PG) output of U2 (TPS62822, 3V3_AUX regulator) is pulled up to 5V_AUX through R1 (180 kΩ). This net (3V3_AUX_PG) is routed to pin GPB2 of U41 (MCP23018 I/O expander). The MCP23018 is a 3.3 V device with an absolute maximum input voltage of V_DD + 0.5 V ≈ 3.8 V. When PG is released (regulator output good), the net rises to 5V_AUX = 5 V, which exceeds the absolute maximum by ~1.2 V and can permanently damage U41.

Signal path

U2 (TPS62822) PG pin  ── 3V3_AUX_PG net ──  R1 (180 kΩ)  ──  5V_AUX (5 V pull-up)
                                         └──  U41 GPB2 (MCP23018 input, abs max = 3.8 V)

The PG pin is open-drain. When the TPS62822 output is in regulation, PG is high-impedance and the net is pulled to 5 V by R1. When the output is out of regulation or during start-up, PG pulls the net to GND. Therefore GPB2 alternates between 0 V (safe) and 5 V (unsafe).

References

  • TPS62822 datasheet §7.1: PG is open-drain, tolerates V_PG up to 6 V
  • MCP23018 datasheet §1.0: V_DD = 1.8–5.5 V; I/O pin abs max = V_SS − 0.5 V to V_DD + 0.5 V
  • Assuming U41 V_DD = 3V3_AUX (3.3 V): abs max input = 3.8 V

Affected components

  • U2: TPS62822DLCT (3V3_AUX step-down)
  • R1: 180 kΩ pull-up (verify pull-up supply net in schematic)
  • U41: MCP23018 (I/O expander), GPB2 pin
  • Net: 3V3_AUX_PG

Recommended fixes (choose one)

Option A — Level-shift with resistor divider (simplest, no new parts if resistors available):
Replace R1 with a resistor divider to 3.3V, or pull up to 3V3_AUX (3.3 V) instead of 5V_AUX.

Option B — Change pull-up supply:
If 5V_AUX pull-up is needed (e.g. for noise margin on the open-drain PG bus), add a series clamp: a 10 kΩ resistor in series with GPB2 plus a Schottky diode (e.g. BAT54) from GPB2 to 3V3_AUX. This clamps the input at V_DD + V_f ≈ 3.55 V.

Option C — Preferred: pull up to 3V3_AUX directly:
Change R1 pull-up supply from 5V_AUX to 3V3_AUX. This is the cleanest fix — PG is still logic-high when in regulation, and the MCP23018 input never exceeds V_DD.

Note: if the intent was to use 5V_AUX so the PG signal is valid before 3V3_AUX comes up (bootstrapping), a separate level translation or a power sequencing review is needed.

🤖 Generated with Claude Code

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