Board L — Driver, Control, LED Array

The locked Board L design — 24 warm-white 2835 LEDs in 8 parallel 3-series strings at 500mA, an AL8860 constant-current buck, an AP63203 3.3V rail, an STM32G031 MCU and an EC11 knob, with a complete pin-level net table and worked component math.

Board L is everything that is not USB-PD negotiation: the constant-current LED driver, the LED array itself, the 3.3 V logic rail, the control MCU, and the knob. It consumes the switched 15 V rail from Board P and nothing else.

Block diagram

LED array and the brightness number

The LED candidates page left CCT open. For a fire/candle effect the answer is warm, and warm is exactly where the JLCPCB stock sits: HL-AM-2835H421W-S1-08-HR3 (C210315) at 399,813 units, in the 2800–3100 K bin.

Its manufacturer specification (Honglitronic B-17-A-0596 Rev A/2) gives real optical numbers, which the parts DB does not:

Arrangement

8 parallel strings of 3 series LEDs = 24 emitters, 500 mA total, 62.5 mA per string.

3-series was chosen over 4-series. The headroom analysis called 4s "tight"; with this part's datasheet Vf ceiling of 3.2 V a 4s string is 12.8 V, which puts the buck at 85% duty — outside the AL8860's recommended 25–75% duty band (DS39014 Rev 5, "Recommended Duty Cycle Range"). 3s plus a ballast resistor lands at 74%, just inside it.

Brightness

For scale: a 60 W incandescent bulb is about 800 lm, and a typical decorative table lamp is 300–600 lm. ~700 lm through a diffuser is a genuinely bright ambient lamp, and the modulation spends most of its time below full scale.

The flux figures are read at the datasheet's 60 mA test point; the design runs 62.5 mA (+4%), so the real number is marginally higher. It is quoted at the 60 mA value rather than extrapolated.

Ballast and current sharing

The parallel strings need ballast — led-candidates lists three ways, and per-string resistors are the one that fits: one CC driver per string would mean eight AL8860s and eight passive sets, and Vf-binned procurement is not something the JLCPCB catalog can guarantee.

R30–R37 = 33 Ω, 1206 (C2907384, ±1%):

(The 0.27 W figure holds Vf at the nominal 3.0 V as a conservative bound. The LED that actually draws 90 mA is by definition a low-Vf one, dropping closer to 2.6 V, so its real dissipation is nearer 0.23 W — 81% of rating. The conservative number is the one carried into the hot-spot table.)

The last row is the check that matters: even at the datasheet's full Vf spread, no LED exceeds its package power rating. The constant-current driver holds the total at 500 mA, so one string running hot forces the others down rather than compounding — there is no runaway path. The 1.03 W of ballast loss is the price paid for that guarantee, and it is a deliberate trade against the alternative of eight driver ICs.

Ballast resistors sit on the anode side (between LED_P and each string's top LED), so the eight split nets are on the quiet DC rail and the common LED_N node — which is the driver's switching node — stays as one compact net.

AL8860 constant-current driver

AL8860MP-13 (C500782), MSOP-8EP.

The AL8860 is a hysteretic buck with high-side current sensing: the sense resistor sits between VIN and SET, and the LED string floats above an internal low-side NDMOS switch. That topology is the reason LED_N (the common cathode) is a switching node.

Duty cycle across the LED's full Vf bin:

Input rating vs the clamp ceiling. AL8860's recommended VIN maximum is 40 V and its absolute maximum is 42 V (DS39014 Rev 5, "Absolute Maximum Ratings"). Board P's SMAJ20A clamps at ≤32.4 V worst case, so the driver clears the ceiling by 7.6 V on the operating rating and 9.6 V on the absolute maximum. This is the reason PT4115 was rejected — see Decisions.

AP63203 logic rail (not an LDO)

AP63203WU-7 (C780769), TSOT-23-6, fixed 3.3 V.

The logic-rail research says an LDO is fine below ~30 mA, and this rail draws ~25 mA worst case. By that rule alone the LDO wins. The clamp ceiling overrides it:

Board P's audit accepted two transient-only gaps because the parts in question had no better alternative. Board L has one, for about $0.60, so it takes it. Zero new residual risk is worth a three-part BOM increase.

Component values from DS41326 Table 2 (AP63203, 3.3 V): L = 3.9 µH, C1 = 10 µF, C2 = 2 x 22 µF, C3 = 100 nF. Implemented as L2 = 4.7 µH (C167874, 78 mΩ DCR — the datasheet asks for under 100 mΩ, and 2.2–10 µH is its stated acceptable range), C14 = 10 µF, C16/C17 = 2 x 22 µF, C15 = 100 nF bootstrap. EN is left open: the block diagram (DS41326 Figure 3) shows an internal current source on EN and the pin description states "leave open for automatic startup".

MCU, knob, and dimming path

STM32G031F8P6 (C529334), TSSOP-20, 64 KB flash / 8 KB SRAM, Cortex-M0+, 12-bit ADC.

EC11L1525G01 (C2991196), 15-detent quadrature encoder, panel-mount bushing.

Analog dimming, not PWM dimming

This is the single most consequential control decision, and the AL8860 datasheet decides it. DS39014 Rev 5 states that for PWM on the CTRL pin, "the PWM frequency is recommended to be lower than 500 Hz" for high resolution. But the modulation research sets the flicker-perception floor at ~300 Hz for peripheral vision. A 500 Hz PWM carrier clears that by only 1.7x — thin margin for a lamp that is specifically meant to be seen out of the corner of the eye. Pushing the carrier up to get flicker margin is exactly what the datasheet says costs dimming accuracy.

Analog dimming escapes the conflict entirely. A DC level on CTRL scales the LED current continuously; the LED current never chops, so there is no flicker frequency to argue about and no PWM modulation of the inductor to make audible noise. The datasheet's "Recommended Analog Dimming Range" is 5% to 100%, a 20:1 span.

The DC level is produced by low-pass filtering an MCU PWM output — the "case (b)" the modulation research anticipated:

PA6 (TIM3_CH1, 20 kHz PWM, 0-3.3 V)
   |
  R20 10k
   +---- R21 33k ---- GND      (divider: 3.3 V x 33/43 = 2.53 V at 100% duty)
   |
  C21 100nF ---- GND           (filter + AL8860 soft-start capacitor)
   |
  U2.CTRL

Safe default-OFF

The AL8860's CTRL pin floats to ON ("leave floating for normal operation"). That is the opposite of what control-safety requires, so the default-off has to be built deliberately. Two independent mechanisms do it:

1. R21, 33 kΩ pull-down on CTRL. With PA6 in its reset high-impedance state, CTRL is tied toward ground rather than allowed to float up.

2. C21's 150 ms soft-start. Even in the worst case where the pull-down fails to hold CTRL below the 0.2 V off threshold, the driver's output cannot reach full current for 150 ms. The STM32G031 reaches firmware GPIO configuration in roughly 1–2 ms after power-on reset — two orders of magnitude inside that window. The same protection covers brown-out and watchdog resets.

Knob mapping

The encoder drives TIM2 in hardware quadrature-encoder mode (PA0 = TIM2_CH1, PA1 = TIM2_CH2), so edge counting costs no CPU time and the invalid-transition rejection in control-safety is handled in silicon. Firmware reads the counter each animation frame and clamps it.

The counts-per-detent figure assumes one full quadrature cycle per detent, which is the usual EC11 arrangement — the parts DB reports "30/15" for this SKU, consistent with 30 pulses and 15 detents per revolution. Confirm it on the received part; the firmware scale factor is a one-line change if it turns out to be 8 counts per detent instead. The scale exists because unscaled counting would need 4.3 turns to sweep 0–255, which is far too much travel for a lamp knob.

An encoder has no absolute position, so the boot ramp exists to tell the user the lamp is alive. It happens strictly after firmware has taken control of CTRL, so it does not weaken the default-off guarantee.

Modulation algorithm

Two layers of filtered noise (approach 1, layered as that page suggests) — a slow "breathing" layer and a faster "flicker" layer, summed around the knob's base brightness. Approach 3 (recorded-flame LUT) is rejected because no such dataset exists for this project; approach 2's bounded random walk is folded in as the slow layer's character rather than kept as a separate mode.

/* ---- fixed constants (tune against the real diffuser, not on the bench) ---- */
#define FRAME_HZ            50
#define SLOW_ALPHA          12    /* one-pole IIR alpha, /256 - lazy breathing   */
#define FAST_ALPHA          64    /* one-pole IIR alpha, /256 - candle flicker   */
#define SLOW_DEPTH          70    /* /256 of full scale                          */
#define FAST_DEPTH          38    /* /256 of full scale                          */
#define GAMMA               2.2f

/* CTRL-window mapping, from the divider + AL8860 thresholds above */
#define DUTY_FLOOR_Q16      10617 /* 16.2% of 65535 -> CTRL 0.41V -> 5% of I_NOM */
#define DUTY_FULL_Q16       65535 /* 100%          -> CTRL 2.53V -> 100%          */

static int16_t slow_state, fast_state;
static uint8_t gamma_lut[256];    /* lut[i] = round(255 * pow(i/255, GAMMA)) */

/* ---- one animation frame, called at FRAME_HZ ---- */
void animation_frame(void)
{
    /* 1. knob -> base brightness, perceptual 0..255 */
    uint8_t base = knob_count_clamped();          /* TIM2 CNT, saturated 0..255 */

    /* 2. two layers of low-pass-filtered noise */
    int16_t slow_raw = (int16_t)random_uniform(-128, 127);
    int16_t fast_raw = (int16_t)random_uniform(-128, 127);
    slow_state += ((slow_raw - slow_state) * SLOW_ALPHA) >> 8;
    fast_state += ((fast_raw - fast_state) * FAST_ALPHA) >> 8;

    int16_t modulated = (int16_t)base
                      + ((slow_state * SLOW_DEPTH) >> 8)
                      + ((fast_state * FAST_DEPTH) >> 8);
    uint8_t perceptual = (uint8_t)clamp_i16(modulated, 0, 255);

    /* 3. thermal derate (NTC on PA4) - see apply_thermal_derate() below */
    perceptual = apply_thermal_derate(perceptual, ntc_temp_c_x10());

    /* 4. hard off, or gamma -> CTRL window */
    if (perceptual == 0) {
        pwm_set_duty_q16(0);                      /* CTRL -> 0V, driver off      */
        return;
    }
    uint8_t g = gamma_lut[perceptual];
    uint32_t duty = DUTY_FLOOR_Q16
                  + ((uint32_t)g * (DUTY_FULL_Q16 - DUTY_FLOOR_Q16)) / 255u;
    pwm_set_duty_q16((uint16_t)duty);
}

The pipeline is modulation -> thermal derate -> gamma LUT -> CTRL duty, exactly the composition control-safety describes. The 256-byte gamma LUT is trivial against 64 KB of flash.

Thermal derate

RT1 is an NCP18XH103F03RB 10 kΩ B=3380 K NTC (C13564) placed inside the LED array, in a 10 kΩ divider to PA4 (ADC_IN4).

The thresholds sit under the LED's own −40…+85 °C operating range with margin. Divider current is 165 µA, so NTC self-heating is 0.27 mW against a 100 mW rating — negligible. The derate function is control-safety's unchanged.

Net-connectivity table

Complete pin coverage for every active device. STM32G031F8P6 pin numbers are from ST DS12992 Rev 3 Table 12 (TSSOP20 column); AL8860 from Diodes DS39014 Rev 5; AP63203 from Diodes DS41326 Rev 3.

Power input and protection

LED driver (U2, AL8860MP-13, MSOP-8EP)

LED array (8 strings x 3 series)

Strings are numbered 1–8; LEDs LED1–LED24 in order, so string n holds LED(3n−2), LED(3n−1), LED(3n).

Logic rail (U4, AP63203WU-7, TSOT-23-6)

MCU (U3, STM32G031F8P6, TSSOP-20), all 20 pins

Control and interface nets

Ground

SW1.MP1 / SW1.MP2 are the encoder's mounting lugs. If the fitted EC11 SKU turns out to carry a push-button (the parts DB lists 7 solder joints, which is consistent with a switch variant — the knob research flagged this as unverified), its two switch terminals are left unconnected; the design does not use them.

Connectors and pad groups

Power budget

Against the 45 W / 3.0 A PD contract cap.

The binding constraint is thermal, not the contract

The 45 W figure is a ceiling, not a target. Scaling this lamp to use it would mean ~7x this dissipation inside a closed 3D-printed enclosure with no forced air — which the thermal budget page's own numbers say plain FR-4 cannot shed. ~6 W is the design's real ceiling, set by board thermals, and the design sits at it deliberately.

Dissipation hot spots

No site exceeds its rating, and the two ICs barely warm. The worst-case imbalanced LED at 94% of its package rating is the number to watch: it is derived from the datasheet's full 2.6–3.2 V Vf spread, which is far wider than a single reel will actually deliver, but it is the only bound the datasheet guarantees. If a built board shows visibly uneven strings, that is the symptom, and the fix is a larger ballast value at the cost of duty-cycle headroom.

References

• LED candidates — the 2835/5730/COB comparison and the current-sharing analysis

• Driver ICs — the PT4115 / AL8860 / TPS92511 comparison

• Thermal budget — the 1 W/site FR-4 guideline

• Logic rail — the LDO-vs-buck dissipation math

• MCU candidates — CH32V003 / STM32 / PY32 and their toolchains

• Knob candidates — why an encoder, not a potentiometer

• Modulation algorithms — gamma, flicker thresholds, the three approaches

• Safe default-OFF — the hardware-not-firmware principle

• Diodes Incorporated DS39014 Rev 5 (AL8860) and DS41326 Rev 3 (AP63200/1/3/5)

• ST DS12992 Rev 3 (STM32G031x4/x6/x8), Table 12 pin assignment

• Honglitronic B-17-A-0596 Rev A/2 (HL-AM-2835H421W-S1-08-HR3)