Setting Via Annular Ring Width

0:24

In Flux, a via’s annular ring is set indirectly by the via/pad copper size versus its drill hole size:

Annular ring width = (via outer diameter − drill diameter) / 2

So for a 0.3 mm drill with a 0.15 mm annular ring, set the via/pad copper size to:

0.3 mm + 2 × 0.15 mm = 0.6 mm outer diameter

Use layout rules on the via/pad node:

Set holeSize to the drill size.
Set size, or sizeX/sizeY, to the desired finished copper diameter.
Use connectedLayers to control which copper layers the via connects to.
Use forceAnnularLayers if you need annular rings on all layers or only top/bottom regardless of connected layers. Flux defines connectedLayers as the rule controlling which layers have annular rings, while forceAnnularLayers can force annular rings on all layers, top/bottom only, or only connected layers .

Example target:

Text
Drill: 0.3 mm
Annular ring: 0.15 mm
Via copper diameter: 0.6 mm

Typical values from the PCB guidance are:

Table

Use case	Drill	Annular ring
Signal via	0.2 mm	0.15 mm
0.5–1 A power via	0.3 mm	0.2 mm
1–3 A power via	0.4 mm+	0.25 mm

Set via dimensions

Find via rules

Check fab limits

Force all rings

Run via DRC

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Ask about this doc

CN3085 Design Notes — Dual AA NiMH Charger

Scope

This file records the key design rationale and calculations discussed for the USB-C powered 2×AA NiMH charger using the CN3085. It is intended as a reference for future schematic/layout decisions. No schematic or PCB layout changes are implied by this document.

User Preferences / Workflow Constraints

Do not modify the schematic unless explicitly instructed.
Do not modify the PCB layout unless explicitly instructed.
Calculations and design rationale should be saved to project files when they affect future design choices.

Charger Architecture Decision

The design uses a standalone CN3085 linear NiMH charger rather than a switching charger or MCU-controlled solution.

Reasons:

The target battery format is 2×AA NiMH, which is common in consumer devices.
A 3-cell pack would reduce the voltage drop from USB 5V and improve thermal efficiency, but is less practical because most devices use two cells.
A 4-cell pack would be close to the available USB 5V input headroom near full charge and is not a good fit.
Standalone switching NiMH charger ICs are scarce, often old/EOL, expensive, unavailable, or require significant external circuitry.
MCU-based charging can be flexible but adds firmware/labor cost.
The CN3085 is simple, low-cost, and standalone, but thermally inefficient because it is a linear charger.

Key CN3085 Behavior Used in Calculations

USB input assumed: 5.0V.
Fast-charge current is set by RISET.
Precharge current is approximately 10% of fast-charge current.
Maintenance charge current is approximately 60% of fast-charge current.
For the 1.45V/cell maximum-voltage design procedure, the datasheet indicates roughly 40% of capacity is delivered before maintenance mode and the remaining 60% is delivered during maintenance charge.
The maintenance timer therefore simplifies to:

Text
T = C_BAT / I_CH

because:

Text
T = (0.6 × C_BAT) / (0.6 × I_CH)

Two-Cell Voltage Divider

The two-cell divider was selected around the CN3085 recommended ratio for 1.45V/cell operation.

Original ideal values:

R2 = 100kΩ
R1 ideal ≈ 140.7kΩ
Divider ratio ≈ 1 + R1/R2 ≈ 2.407

Practical sourcing note:

140.7kΩ is not a common stocked value.
A practical stocked substitute is 140kΩ, 1%, 0603.
The resulting error is small relative to typical resistor tolerance and acceptable for this design.

Updated lower-impedance divider choice selected for the schematic:

R1 = 51kΩ, 1%, 0603
R2 = 36kΩ, 1%, 0603

Verification of this ratio:

Text
R1/R2 = 51kΩ / 36kΩ ≈ 1.4167
Divider multiplier = 1 + R1/R2 ≈ 2.4167

Compared with the original ideal multiplier of approximately 2.407, this is about 0.40% high:

Text
Error = (2.4167 - 2.407) / 2.407 ≈ 0.40%

Using the same CN3085 FB reference assumptions as the earlier calculation, this would move the approximate two-cell maximum-voltage point from about 2.90V pack to about 3.07V pack:

Text
V_BAT,max ≈ 1.205V × 2.4167 ≈ 2.91V
V_CELL,max ≈ 2.91V / 2 ≈ 1.46V/cell

This note records the selected lower-impedance divider values. The ratio is only slightly above the original ideal target and is acceptable for this design, subject to normal tolerance review.

Precharge Threshold and Worst-Case Fast-Charge Entry Voltage

Using the CN3085 precharge FB threshold discussed:

Text
V_FB ≈ 0.843V

With the selected 51kΩ / 36kΩ divider ratio:

Text
1 + R1/R2 ≈ 2.4167

Pack voltage at precharge exit:

Text
V_BAT = 0.843V × 2.4167 ≈ 2.04V

Per-cell voltage:

Text
V_CELL = 2.04V / 2 ≈ 1.02V/cell

This ~2.04V pack voltage is the key worst-case thermal point, because the charger transitions from low-current precharge into full fast charge while the battery voltage is still low.

RISET Calculation Examples

CN3085 fast-charge current equation used:

Text
I_CH = 1218 / R_ISET

For 450mA:

Text
R_ISET = 1218 / 0.45 ≈ 2706.7Ω

Practical value:

Text
R_ISET = 2.7kΩ, 1%

This gives:

Text
I_CH ≈ 1218 / 2700 ≈ 451mA

Maintenance Timer Calculation

CN3085 timer equation used:

Text
T = 2654 × R5 × C1 + 4980 × C1 × 10^3

Solving for R5:

Text
R5 = (T/C1 - 4,980,000) / 2654

Case A — 2400mAh Battery, 450mA Charge Current

Timer target:

Text
T = 2400mAh / 450mA = 5.33h = 19,200s

With C1 = 10µF effective:

Text
R5 = (19200 / 10e-6 - 4,980,000) / 2654
R5 ≈ 721.6kΩ

Practical value:

Text
R5 = 720kΩ
C1 = 10µF effective

Verification:

Text
T = 2654 × 720000 × 10e-6 + 4980 × 10e-6 × 1000
T ≈ 19,158.6s ≈ 5.32h

Important capacitor note:

C1 must be the effective capacitance at the RC pin operating bias, not just nominal capacitance.
Very small high-value MLCCs may derate heavily under DC bias.

Case B — 2400mAh Battery, 1000mA Charge Current

Maintenance current is 60% of fast-charge current:

Text
I_MAINT = 0.6 × 1000mA = 600mA

Remaining capacity after fast-charge phase:

Text
C_REMAINING = 0.6 × 2400mAh = 1440mAh

Timer target:

Text
T = 1440mAh / 600mA = 2.4h = 8640s

Equivalently:

Text
T = 2400mAh / 1000mA = 2.4h

With C1 = 10µF effective:

Text
R5 = (8640 / 10e-6 - 4,980,000) / 2654
R5 ≈ 323.7kΩ

Practical value:

Text
R5 = 324kΩ
C1 = 10µF effective

Smaller Timing Capacitor Option for 1000mA Case

To reduce DC-bias derating and cost, a smaller nominal timing capacitor in a larger package can be preferable.

For C1 = 4.7µF effective and T = 8640s:

Text
R5 = (8640 / 4.7e-6 - 4,980,000) / 2654
R5 ≈ 688kΩ

Practical values:

Text
R5 = 690kΩ, 1%
C1 = 4.7µF effective, X7R, larger package preferred

Verification:

Text
T = 2654 × 690000 × 4.7e-6 + 4980 × 4.7e-6 × 1000
T ≈ 8680s ≈ 2.41h

This is a good practical timing choice for a 2400mAh battery when I_CH = 1A, provided C1's effective capacitance is close to 4.7µF at the RC pin bias.

Thermal Limitation of the Linear Charger

The major weakness of the 2×AA-from-USB design is the voltage difference between USB 5V and the low two-cell pack voltage.

Worst-case fast-charge entry voltage:

Text
V_BAT ≈ 2.03V

Without added series resistance, charger IC power at 1A:

Text
P_IC = (5.0V - 2.03V) × 1A ≈ 2.97W

This is likely too high unless the PCB thermal path is excellent. The CN3085 will likely enter thermal regulation at high current if heat is not spread effectively.

Thermal Resistance Assumption Examples

Assuming:

Text
T_A = 25°C
T_J_LIMIT ≈ 135°C
ΔT_ALLOWED = 110°C

If effective thermal resistance is 100°C/W:

Text
P_MAX = 110°C / 100°C/W = 1.1W
I_MAX = 1.1W / (5.0V - 2.03V) ≈ 370mA

If effective thermal resistance is 50°C/W:

Text
P_MAX = 110°C / 50°C/W = 2.2W
I_MAX = 2.2W / (5.0V - 2.03V) ≈ 740mA

Heat-Spreading Series Resistor Concept

A series resistor can shift some dissipation away from the CN3085 into external resistor(s), similar to the approach shown in TP4056-style linear charger guidance.

General resistor equation used:

Text
R_S = (V_USB - V_BAT - P_IC,max / I_CHG) / I_CHG

For 1A charge current, 50°C/W IC thermal assumption, and P_IC,max = 2.2W:

Text
R_S = (5.0V - 2.03V - 2.2W/1A) / 1A
R_S ≈ 0.77Ω

A practical design option considered:

Text
Two 2Ω resistors in parallel → R_S = 1Ω total

At 1A:

Text
V_R = 1A × 1Ω = 1V
P_R,total = 1^2 × 1Ω = 1W

Each 2Ω resistor carries 0.5A:

Text
P_each = 0.5^2 × 2Ω = 0.5W

With R_S = 1Ω, IC dissipation at the worst-case fast-charge entry point:

Text
P_IC = (5.0V - 2.03V - 1.0V) × 1A
P_IC ≈ 1.97W

At 50°C/W:

Text
ΔT ≈ 1.97W × 50°C/W = 98.5°C
T_J ≈ 25°C + 98.5°C = 123.5°C

This leaves roughly 11.5°C margin below a 135°C thermal regulation point under the stated assumptions.

Resistor Power Rating Discussion

For two 2Ω resistors in parallel at 1A:

Each resistor dissipates approximately 0.5W.
2W parts are thermally comfortable but may be much more expensive.
1W parts are a likely cost/performance compromise: each runs at about 50% of rating.
0.5W parts are not preferred because they operate at their full rating during the hottest operating point.

PCB Thermal Assumptions

Planned PCB:

45mm × 80mm board
2 layers
Copper pours on both sides
Vias connecting pours between sides

Board area:

Text
45mm × 80mm = 3600mm² = 36cm²

Both sides together:

Text
≈72cm² copper/board surface area, if effectively used

Rough natural-convection estimate using h ≈ 10W/m²K:

Text
A = 0.0072m²
G = h × A = 10 × 0.0072 = 0.072W/K
θ_BA ≈ 1 / 0.072 ≈ 13.9°C/W

This is optimistic because not all board area is equally hot and heat must spread from the IC into copper and through vias.

Practical working estimate for the charger IC with good exposed-pad soldering, large copper pours, and via stitching:

Text
θ_JA,effective ≈ 35–60°C/W

A conservative planning value used:

Text
θ_JA,effective ≈ 50°C/W

If thermal pad connection is poor or copper/via stitching is sparse, the effective thermal resistance could be much worse, possibly 70–100°C/W.

Layout Guidance for Thermal Success

No layout changes are authorized by this document, but future layout should consider:

Solder CN3085 exposed pad well to GND copper.
Use large continuous copper pours connected to the exposed pad.
Stitch top and bottom copper with many vias near the IC thermal pad.
Keep heat-sharing resistors physically away from the CN3085 so resistor heat does not directly heat-soak the IC.
Give the series resistors their own copper area for heat spreading.
Avoid thermally isolating the IC pad with narrow spokes if maximum heat spreading is needed.
Use test pads or accessible measurement points to validate charge current and thermal behavior during bring-up.

Overall Design Assessment

The CN3085 + external heat-sharing resistor approach is a defensible low-cost compromise for a standalone 2×AA NiMH USB charger.

Advantages:

Low component/labor cost.
No firmware or MCU programming.
Uses the common 2×AA battery format.
Large PCB area can help with heat spreading.
External resistors can move a meaningful portion of heat out of the charger IC.

Risks / limitations:

Still thermally marginal at 1A unless PCB copper and via stitching are good.
Linear topology is inefficient when charging low-voltage 2-cell packs from 5V USB.
Practical maximum current may be determined by thermal regulation rather than RISET alone.
High-capacity modern AA NiMH cells make low-current charging slow.

Most realistic current range:

Around 350–700mA without aggressive heat spreading, depending on actual thermal resistance.
1A may be plausible with ~1Ω series heat-sharing resistance, good PCB copper, and careful thermal layout, but should be validated empirically.

Open Validation Items

Before finalizing production values:

Confirm selected R5/C1 values against the exact final charge-current setting.
Confirm C1 effective capacitance at RC pin bias using the capacitor datasheet.
Confirm RISET value for the desired charge current.
Thermal-test the assembled PCB at worst-case battery voltage near the precharge-to-fast-charge transition.
Measure whether the CN3085 enters thermal regulation at the chosen charge current.
Decide final series resistor value and power rating based on measured temperature rise.

Scope

User Preferences / Workflow Constraints

Charger Architecture Decision

Key CN3085 Behavior Used in Calculations

Two-Cell Voltage Divider

Precharge Threshold and Worst-Case Fast-Charge Entry Voltage

RISET Calculation Examples

Maintenance Timer Calculation

Case A — 2400mAh Battery, 450mA Charge Current

Case B — 2400mAh Battery, 1000mA Charge Current

Smaller Timing Capacitor Option for 1000mA Case

Thermal Limitation of the Linear Charger

Thermal Resistance Assumption Examples

Heat-Spreading Series Resistor Concept

Resistor Power Rating Discussion

PCB Thermal Assumptions

Layout Guidance for Thermal Success

Overall Design Assessment

Open Validation Items

Reviews

Documents

CN3085 Design Notes — Dual AA NiMH Charger

Final Component Choices Summary — USB-C Dual AA NiMH Charger

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Project Name

USB-C powered Dual AAA NiMH charger

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Properties

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