change r282 to 70K instead of 10 #6

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opened 2026-01-20 17:19:19 +00:00 by allspice-carah · 0 comments

allspice-carah commented

2026-01-20 17:19:19 +00:00

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Component U29, pin 11: FB2 feedback pin has incorrect voltage divider values - R282 should be approximately 70K instead of 10K, resulting in 0.94V output instead of the intended 1.8V.

Pin 11 (FB2) is the feedback pin for DCD2 (datasheet page 3). It is connected to net N25830704 as part of a voltage divider network for DCD2 output regulation. The datasheet specifies a feedback regulation voltage of 0.8V typical (page 5: 0.785V min, 0.8V typ, 0.815V max). The voltage divider consists of R282 (10K) from output N25830712 to FB2 (N25830704), and R281 (56.2K) from FB2 to GND. C244 (0.047µF) provides compensation. Using the voltage divider equation: VOUT = VFB × (1 + R282/R281) = 0.8V × (1 + 10K/56.2K) = 0.8V × 1.178 = 0.942V. However, the schematic text indicates '1.8V @ 1.5A max' and R287 (0Ω) connects the output to the 1P8V rail, clearly indicating the intended output is 1.8V. For a 1.8V output with 0.8V feedback: 1.8V = 0.8V × (1 + R282/R281), solving gives R282 = 1.25 × R281 = 1.25 × 56.2K = 70.25K. The current value of R282 = 10K is incorrect and will result in an output voltage of approximately 0.94V instead of 1.8V, a critical 48% error. Recommendation: Replace R282 with a 70K or 71.5K resistor (standard E96 value) to achieve the correct 1.8V output voltage.

Originally posted by @DRCY in https://hub.allspice.io/AllSpice-Demos/Honeycrisp-SDAX-Demo/pulls/1/files#diff-dcdcb94003e38cba372049b9bb4e30b1d34e6b5f-issuecomment-42123

**Component `U29`, pin `11`: FB2 feedback pin has incorrect voltage divider values - R282 should be approximately 70K instead of 10K, resulting in 0.94V output instead of the intended 1.8V.** !thumbnail[](parallella.sdax){ diff="AllSpice-Demos/Honeycrisp-SDAX-Demo:e3bd227a2e06a51e009f1a792a46dc8301aad123...d86f880abf4c72a7a39d3337390ac1a0a22b177c" pr="1" doc-id="899dffe5006059bf8b9b" diff-visibility="full" variant="default" view-coords="63.08,37.70,70.58,45.20" aspect-ratio="1.33" } Pin 11 (FB2) is the feedback pin for DCD2 ([datasheet](https://www.renesas.com/us/en/document/dst/isl9307-datasheet#page=3) page 3). It is connected to net N25830704 as part of a voltage divider network for DCD2 output regulation. The datasheet specifies a feedback regulation voltage of 0.8V typical (page 5: 0.785V min, 0.8V typ, 0.815V max). The voltage divider consists of R282 (10K) from output N25830712 to FB2 (N25830704), and R281 (56.2K) from FB2 to GND. C244 (0.047µF) provides compensation. Using the voltage divider equation: VOUT = VFB × (1 + R282/R281) = 0.8V × (1 + 10K/56.2K) = 0.8V × 1.178 = 0.942V. However, the schematic text indicates '1.8V @ 1.5A max' and R287 (0Ω) connects the output to the 1P8V rail, clearly indicating the intended output is 1.8V. For a 1.8V output with 0.8V feedback: 1.8V = 0.8V × (1 + R282/R281), solving gives R282 = 1.25 × R281 = 1.25 × 56.2K = 70.25K. The current value of R282 = 10K is incorrect and will result in an output voltage of approximately 0.94V instead of 1.8V, a critical 48% error. Recommendation: Replace R282 with a 70K or 71.5K resistor (standard E96 value) to achieve the correct 1.8V output voltage. _Originally posted by @DRCY in https://hub.allspice.io/AllSpice-Demos/Honeycrisp-SDAX-Demo/pulls/1/files#diff-dcdcb94003e38cba372049b9bb4e30b1d34e6b5f-issuecomment-42123_