Design Log DEC 073 ltc3350 rt resistor correction 332 kω 133 kω amends dec 030 - thepinkmile/Enigma-NG GitHub Wiki
DEC-073 - LTC3350 RT Resistor Correction: 33.2 kΩ → 133 kΩ (Amends DEC-030)
Date: 2026-05-15 Status: Accepted Author: Izzyonstage & GitHub Copilot Amends: DEC-030
Decision
R23 (LTC3350 RT frequency-setting resistor) is changed from 33.2 kΩ to 133 kΩ (E96 nearest value to 133.75 kΩ). BOM part changed from ERA-2AEB3322X (33.2 kΩ, Panasonic ERA-2, 0.1%, 0402) to ERJ-PC3B1333V (133 kΩ, Panasonic ERJ-PC3, 0.1% thick-film, 0603).
Rationale
DEC-030 stated that R23 = 33.2 kΩ sets the LTC3350 switching frequency to 400 kHz. This was an arithmetic error.
Formula (from LTC3350 datasheet RT programming table): SW(kHz) = 53,500 / RT(kΩ)
Verification at datasheet calibration points:
| RT (kΩ) | Calculated fSW | Datasheet typical |
|---|---|---|
| 267 | 53,500 / 267 = 200 kHz | 200 kHz ✓ |
| 107 | 53,500 / 107 = 500 kHz | 500 kHz ✓ |
| 53.6 | 53,500 / 53.6 = 998 kHz | 1 MHz ✓ |
Error in DEC-030: 33.2 kΩ → 53,500 / 33.2 = 1,611 kHz (61% over the rated 1 MHz maximum frequency). Operating the LTC3350 switching converter above its rated maximum frequency is out-of-spec and risks converter instability, elevated losses, and component damage.
Correct calculation for 400 kHz target:
- Required: RT = 53,500 / 400 = 133.75 kΩ
- Nearest E96 standard value: 133 kΩ
- Actual fSW: 53,500 / 133 = 402 kHz (within 0.5% of target) ✔
- Package/footprint: 0603 thick-film — footprint change from 0402 required.
The 400 kHz switching frequency target (≥4 switching cycles within the 10.2µs backup switchover window) is unchanged from DEC-030. Only the resistor value and MPN are corrected.
Files Changed
- design/Electronics/Power_Module/Design_Spec.md - DR-PM-11 updated: 33.2 kΩ → 133 kΩ; formula added with calculation; BOM R23 MPN updated; Last Updated 2026-05-15