eFuse V2.1 fix: datasheet-derived OVLO ladder + ILIM correction

Works the TPS25961 fix out from the datasheet rather than from memory, and
re-solves it against resistors already on the BOM.

OVLO: TI's 3-resistor ladder (eq 7/8) -> R46 620k->150k, R47 100k->10k, new
R48 12k + R49 2.2k (=14.2k).  UVLO 8.93 V typ, unchanged from today; OVLO
15.21 V typ, 15.81 V worst case.  The OVLO ceiling is set by U3 (TPS562201,
4.5-17 V input), not by the eFuse -- that rules out a tempting 3-resistor
solution tripping at 17.24 V.

ILIM: R45 100k -> 33k.  0.516 A sat BELOW the board's own 621 mA inrush, so
the part entered current limiting on every power-on (-> thermal shutdown ->
retries -> latch-off).  Inrush control comes from the fixed 5.17 V/ms slew
rate, not ILIM, so raising ILIM costs no inrush protection; it is pure fault
protection at ~27x the measured 56 mA load.

Every value is already stocked and 620k loses its only user: one FEWER unique
part, +2 placements.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

hardware/re-bba-rb/REVIEW.md

@@ -315,10 +315,34 @@ datasheet extraction, WebSearch/WebFetch for parts and reference designs.
   device sits in overvoltage lockout with the FET off (bench-confirmed: 12V_EXI =
   0 V, GC_ON = 0 V, W5100 unpowered). The earlier review note ("OVLO tied to GND
   for the internal fixed threshold, abs max 6.5 V fine") caught the pin abs-max
-  but MISSED that the fixed VIN cutoff is ~6 V. **Fix (V2.1):** drive OVLO from a
-  divider off 12V_RAW so the trip sits ~14–16 V (above 12 V, below the 19 V max /
-  the D3 clamp): e.g. **R_top ≈ 1.1 MΩ (12V_RAW→OVLO) + R_bottom ≈ 100 kΩ
-  (OVLO→GND)** → OVLO pin = 1.24 V at ~15 V VIN, and ~1.0 V at 12 V (stays on).
+  but MISSED that the fixed VIN cutoff is ~6 V.
+
+  **FIX (V2.1), datasheet-derived 2026-10-02** — TI's 3-resistor ladder
+  (§8.2.3.2) from 12V_RAW to GND, EN/UVLO tapped at R1–R2 and OVLO at R2–R3:
+      VIN(UV) = VUVLO(R) × (R1+R2+R3)/(R2+R3)      (eq 7)
+      VIN(OV) = VOVLO(R) × (R1+R2+R3)/R3           (eq 8),  both thresholds 1.24 V
+  | ref | was | becomes | LCSC | already on board as |
+  |---|---|---|---|---|
+  | R46 (R1, 12V_RAW→EN/UVLO) | 620 k | **150 k** | C93947 | R3 |
+  | R47 (R2, EN/UVLO→OVLO)    | 100 k | **10 k**  | C25744 | R2, R9, R11, … |
+  | R48 (R3a, OVLO→GND) NEW   | —     | **12 k**  | C25752 | R10 |
+  | R49 (R3b, series w/ R48) NEW | —  | **2.2 k** | C25879 | R30 |
+  R3 = 12 k + 2.2 k = 14.2 kΩ. Results across tolerance (VOVLO 1.20/1.24/1.27):
+  **UVLO 8.93 V typ (8.64–9.14)** — identical to the current board, so turn-on
+  behaviour is unchanged — and **OVLO 15.21 V typ (14.72–15.58)**, worst case
+  with 1% resistors **15.81 V**.  String current 68.9 µA at 12 V (TI requires
+  ≥20× the 0.1 µA pin leakage, i.e. ≥2 µA).  OVLO pin sits at 0.98 V at 12 V and
+  1.55 V at 19 V — far under its 6.5 V abs max, the lowest-rated pin on the part.
+
+  **The OVLO ceiling is set by U3, not by the eFuse.** The TPS562201 buck is
+  rated **4.5–17 V** input (19 V abs max), so OVLO must disconnect before the
+  buck is overstressed. This rules out an otherwise attractive 3-resistor
+  solution (150 k/10 k/12.4 k) that trips at 17.24 V typ / ~18 V worst case —
+  it would leave U3 running out of spec. Do not "simplify" to that.
+
+  **Cost:** every value above is already on the BOM, and 620 k (C137952) had
+  R46 as its only user, so it drops out — **net one FEWER unique part**, +2
+  placements.
   BENCH WORKAROUND to keep bring-up moving: inject 12 V straight onto 12V_EXI
   (C65+ pad or U3.3 VIN), bypassing the mis-set eFuse.
   **D3 (SMAJ12A TVS) stays on the INPUT side on purpose** — it clamps
@@ -332,12 +356,28 @@ datasheet extraction, WebSearch/WebFetch for parts and reference designs.
     dV/dt = 5.17 V/ms (fixed, typical) ⇒ I = C·dV/dt = 120 µF × 5.17 V/ms =
     **621 mA** over a 2.3 ms ramp. R_ILIM only matters if it is set *below*
     that figure.
-  - **R45 = 100 kΩ ⇒ I_LIM ≈ 500 mA** (datasheet Equation 6: R_ILIM =
-    50000/I_LIM; 100 kΩ is one of TI's own characterised points, 0.516 A typ,
-    ±18% ⇒ 0.42–0.61 A). This both caps inrush at 500 mA and gives **3.8×
-    headroom** over the ~130 mA steady-state 12 V draw. 100 kΩ is deliberately
-    inside a valid ON-resistance bin — **Table 7-1 forbids 58.8–66.7 kΩ,
-    111–142 kΩ and 250–500 kΩ**; do not "round" R45 into those bands.
+  - ⚠️ **R45 = 100 kΩ ⇒ I_LIM ≈ 0.516 A typ (0.42–0.61 A) is WRONG — it sits
+    BELOW this board's own 621 mA inrush.** The consequence is that the part
+    enters active current limiting on *every* power-on; sustained current
+    limiting drives thermal shutdown → 3 auto-retries → latch-off. **Change to
+    R45 = 33 kΩ (C25779, already on the board as R1/R39/R40) ⇒ 1.52 A typ
+    (1.24–1.79 A)**, which clears the inrush even if the slew rate runs 30%
+    fast (0.81 A).
+    **Inrush protection does NOT come from ILIM — it comes from the fixed
+    5.17 V/ms output slew rate**, which ramps the rail in 2.3 ms at 621 mA
+    regardless of the ILIM setting. (The TPS25961DRV is 6-pin: OUT/OVLO/ILIM/
+    GND/EN·UVLO/IN — there is NO dVdt pin, so the slew rate cannot be changed;
+    the only other lever on inrush is reducing C65.) ILIM is therefore pure
+    fault protection, and 1.52 A is ~27× the measured 56 mA load while staying
+    far inside the console's 3.25 A supply.
+    Measured 2026-10-02: the board draws **0.67 W at 12 V ≈ 56 mA** with the
+    link up — about half the 130 mA originally budgeted. Note the W5100S spec
+    is per LINK STATE, not traffic (100M Link 93 mA typ / 110 mA max vs Unlink
+    40 mA): in 100BASE-TX the PHY drives idle symbols continuously, so an idle
+    measurement with the link up already includes the dominant load and running
+    traffic adds very little.
+    **Table 7-1 forbids 58.8–66.7 kΩ, 111–142 kΩ and 250–500 kΩ** — 33 kΩ is
+    clear of all three; do not "round" R45 into those bands.
   - **Startup dissipation is safe**: FET energy = ½CV² = **8.65 mJ** regardless
     of ramp rate ⇒ ~3.0 W average over 2.9 ms. Figure 6-31 puts time-to-thermal-
     shutdown at ~3 W far beyond 100 ms. Steady-state loss is 106 mΩ × 0.13² =