12S Servo Module
Status: Active
Current Owner: @Jerry Tian
Previous Owner: @Stanley Hao
- 1 Needs Assessment
- 2 Market Research
- 3 Requirements
- 4 Block Diagram
- 5 Possible Implementation Methods
- 6 Component Selection
- 6.1 Buck ICs
- 6.2 MP9928 Buck Controller Specs
- 6.2.1 Typical Application Circuit Diagram
- 6.2.2 IC Pinout
- 6.2.3 Important Equations
- 6.3 Buck Controller Passive Components Selection
- 6.3.1 Output Feedback Voltage Divider
- 6.3.2 Vout Current Limit
- 6.3.3 Vcc Circuit
- 6.3.4 Power Good Pull-up Resistor
- 6.3.5 Enable/Sync Pin Pull-up Resistor
- 6.3.6 AAM/CCM Resistor
- 6.3.7 Soft-Start Capacitor
- 6.3.8 Switching Frequency
- 6.3.9 Inductor
- 6.3.10 Input Capacitor
- 6.3.11 Output Capacitor Selection
- 6.3.12 Compensation Components
- 6.3.13 RC Snubber
- 6.4 Power FET Selection
- 6.4.1 Requirements
- 6.4.2 Decision Matrix
- 6.4.3 Bootstrap Circuit
- 6.5 Common Mode Choke
- 6.5.1 Needs Assessment
- 6.5.2 Requirements
- 6.5.3 Options
- 6.6 Efuse Component Selection
- 6.6.1 Efuse IC Selection
- 6.6.2 Input and Output Capacitor
- 6.6.3 Current Limit Resistor
- 6.6.4 Slew Rate Control Capacitor
- 6.6.5 Input Zener Diode
- 6.6.6 Output Schottky Barrier Diode
- 6.7 PWM Buffer Component Selection
- 6.7.1 Servo Motor Market Research
- 6.7.2 Requirement
- 6.7.3 5V LDO
- 6.7.4 Input Pulldown/Pullup Resistors
- 7 PCB Layout Notes
- 7.1 Layout Topology
- 7.2 Layout Justifications
- 7.2.1 Trace Width Calculations
- 7.2.2 Via Current Calculation
- 7.2.3 Via Size
- 7.2.4 Layer Stackup
- 7.2.5 Efuse IC Pad
- 7.2.6 Input & Output Connectors
- 8 BOM notes
Needs Assessment
The 12S Servo Module is a PDB designed to supply power from the battery directly to the servo motors on the fixed wing plane. The board should step down the voltage from the battery to 5-6V to supply the servo motors and be able to cut one out if it fails, to prevent the dead servo from drawing too much power and depowering other components of the drone.
Market Research
Existing https://www.revrobotics.com/rev-11-1144/, interesting LED feature can be implemented.
see if we can get plane position from gyro?? is a gyro chip too big?
or I need to study how to read fixed wing directions from its wing tilt angle?? and we can get this data from servo?? but how to decode!???
or simply we just measure current at each servo rail input and mux the color ?? uhh so many ideas I am not even sure if they make sense lol
I (Jerry) assumed 1 servo motor draw a maximum of 1A from my previous course project. I saw 1 servo motor would draw around 700mA from power supply. Also saw a bunch of servos in the bay and they look the same size as the one I used in the course project, so I assumed 1A current limit.
Requirements
Supports 18 V to 55 V input from XT60 for 6S and 12S support
Input should have common mode choke to filter out noise from the ESC
3x4 header servo inputs and outputs to be consistent with Pixhawk Electrical Connector Standards
.100 pitch, 100mil
Input: PWM, GND from Pixhawk (No 5V power since we are making our own)
Output: buffed PWM, Power, GND
Each output power is 6 V (possibly adjustable between 5 V and 6 V with passive values) and has a current limit somewhere between 500mA and 1A (possibly adjustable with passive values within this range). If one channel is over currented the other channels remain alive.
5V/6V output can be changed by swapping out resistors
30mm x 30mm PCB size to be compliant with Mounting Hole & Pattern Specifications
Block Diagram
Possible Implementation Methods
Single Buck Converter Option
A single buck converter with external FETs can be used and then four current limited channels
Will need to have the current limit max at 4A
Each servo line will have some form of overcurrent protection, which can shut off that line to the servo if the current exceeds a certain limit
4 Buck Converters Option
Four separate buck converters with internal FETs can be used, each with their own current limit
Adding fuses to each servo line that will trip if the current exceeds allowed parameters?
Methods Pros and Cons
Attribute | Single Buck | 4 Bucks |
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Component Selection |
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Cost |
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Board Layout / form factor |
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Decision: We selected single buck option to save board space.
Component Selection
Buck ICs
Internal FETs
LM76005 3.5-V to 60-V, 5-A Synchronous Step-Down Voltage Regulator datasheet (Rev. A) (ti.com) (Thanks Steven)
For single buck option
External FETs (Buck Controllers for single buck option)
Readily available on Digikey, lower efficiency than the Ti ones (high 80’s at nominal)
Reference Design: rd00002_48v_mpq2908_reference_design_rev1.0.pdf (monolithicpower.com)
MP9928 Buck Controller Specs
Typical Application Circuit Diagram
IC Pinout
This IC is available in TSSOP-20 and QFN-20 packages.
Important Equations
Output voltage divider equation:
Buck Controller Passive Components Selection
Output Feedback Voltage Divider
For 6V output, use 130k & 20k 1%, currently using this
R8 = 130k → RC0603FR-07130KL YAGEO | Resistors | DigiKey
R9 = 20k → RC0603FR-0720KL YAGEO | Resistors | DigiKey
For 5V output, use 43k & 8.2k.
R8 = 43k → RC0603JR-0743KL YAGEO | Resistors | DigiKey
R9 = 8.2k → RC0603FR-078K2L YAGEO | Resistors | DigiKey
Vout Current Limit
Since we have overcurrent protection downstream already, we don’t want to implement a limit on the buck controller itself. Unfortunately, the current sensing is used in the compensation control for the buck, so it needs to be included for the IC to function normally.
Rearranging the inductor equation from Inductor Selection below, Ipp-ripple = 1.149A, then Ipk = 4.5745A, therefore, a current limit of 5A should suffice.
Assuming ILIM pin is GND, V_ILIM = 25mV
Rsense = 25mV/5A = 5E-3Ohms = 5mOhms
Expected power dissipation: P=I^2R, P=5^2 * 5mOhms = 0.125W
5mOhms current sense resistor: https://www.digikey.ca/en/products/detail/rohm-semiconductor/PMR18EZPFU5L00/2094203
Vcc Circuit
The MP9926 has VCC1 and VCC2 to power internal components and the MOSFETS. VCC1 can take input from Vin into an internal LDO. To improve efficiency, can connect VCC2 to output and leave VCC1 pin floating.
Choose a 4.7 uF capacitor for VCC2
4.7uF is chosen to be compliant with datasheet’s typical application example. This cap decouples noise at VCC2 pin. Other values such as 1uF or 10uF would work as well.
Choose 1uF capacitor for VCC1 according to the datasheet
VCC1 cap can’t be drained when charging up the bootstrap capacitor, the 1uF cap is chosen since it is 10x bigger than Cboot
Power Good Pull-up Resistor
PG pin not required in implementation, DNP resistor provided as shown. 100k resistor used in earlier design shown below.
Enable/Sync Pin Pull-up Resistor
selected a resistor divider here to regulate EN pin voltage in high Vin situations
1.28V < V_EN < 50V
Vin = 18V, R1 = 100kohm, R2 = 100kohm, Vout = 9V
Vin = 55V, R1 = 100kohm, R2 = 100kohm, Vout = 27.5V
not using a zener diode because the diode footprint takes too much space
AAM/CCM Resistor
AAM mode improves the efficiency of the the Buck at light or no load, and since we are powered by a battery, it would make sense to improve the efficiency of the board where possible.
Choose 196K resistor, this keeps the V_AAM at 600mV which is above the 480mV (R_FREQ is also 200K so the resistors cancel out which leaves V_AAM at 600mV).
Soft-Start Capacitor
Soft-start capacitor is used to determine the soft-start time, prevents converter output voltage from overshooting rated voltages
If we want a t_ss of ~100ms, I_ss = 4 uA, V_REF = 0.8V, C_ss = 500nF
Easier to find caps with 0.68uF capacity, resulting inh t_ss=136ms, which should still be fine for our purposes
https://www.digikey.ca/en/products/detail/yageo/CC0603KRX7R8BB684/16797741
CAP CER 0.68UF 25V X7R 0603
Justification: We don’t need soft start time to be very fast, having slightly longer gets us less inrush current.
Switching Frequency
There is a trade-off when selecting the switching frequency of the buck controller.
High Fsw → Small input and output ripples, smaller inductor and less capacitors but will increase switching losses
Low Fsw → Big inductor but small switching losses
Limits: 100kHz - 1000kHz
Try to use lowest available frequency (100kHz)
Justification (Old): Even with the lowest available frequency available to us, the corresponding recommended inductor size that this corresponds to is just 47uH which isn’t too physically large anyway, and we aren’t very space constrained so larger a couple larger components is okay, thus to maximize efficiency, we can go for the lowest possible switching frequency.
Justification: 100kHz switching frequency provides the highest efficiency. It leads to a 47uH inductor which is fitting our 30mm x 30mm board so this switching frequency is acceptable.
199kOhms → selected R_freq = 196kOhm → Fsw = 101.5 kHz
Comment:
If we ever go for a 2nd revision, start with calculating total losses on the FET and go for the highest Fsw. We want the board size to be small so a smaller inductor and less caps can help with layout. But still watch out for efficiency drops
Inductor
SRP1265A-470M Bourns Inc. | Inductors, Coils, Chokes | DigiKey → 47uH, 6.5A, 90mOhms
Rated DC current is recommended to be 25% higher than typical for safety.
Inductor size equation from datasheet:
V_out = 6V, V_in = 55V, f_s = 101.5kHz, i_ripple = 0.3 * 4A = 1.2A: L = 43.9uH
V_out = 6V, V_in = 18V, f_s = 101.5kHz, i_ripple = 1.2A: L = 32.8uH
At 4A, the inductance drops a little from rated 47uH but is still close to 45uH, also the expected temperature remains low, thus this inductor should be sufficient
Input Capacitor
When the switching FETs turn on and off, current suddenly start and stop flowing and this causes ripple on the input side. Due to the parasitic inductance, we see the ripple voltage on Vin and we need to filter that out.
Must be capable of handling input ripple current, some power will be dissipated by the capacitor ESR. Typically quoted values of ESR for ceramic capacitors are between 0.01 and 0.1 Ω. The ripple current can be defined as follows:
1.25A@V_in=55V, 1.89A@V_in=18V
Voltage rating ideally greater than 100V to compensate capacitor DC bias effect
Below: Input capacitance equations from TI appnote:
See efficiency chart below for duty cycle calculation
@V_in = 55V: D = 6/(55*0.85) = 12.8%
@V_in = 44.4V: D = 6/(44.4*0.85) = 15.9%
@V_in = 22.2V: D = 6/(22.2*0.92) = 29.4%
@V_in = 18V: D = 6/(18*0.92) = 36.2%
f_sw = 101.5kHz, I_o = 4A
To save board space, we decided to go with ceramic capacitor only. Chosen 4 10uF 100V X7S ceramic capacitors as our input capacitors. See the screenshot below for DC bias and Temp Rise due to I_rms.
Let Vpp be 2% of Vin, this would yield a Vin_max of 55.55V. There is still 4.5V margin till the Vds_max for our switching FET.
Cin @ 55V >= 0.128 * (1 - 0.128) * 4 / (1.1* 0.1015) = 3.86 uF, effective capacitance = 2.22*4 = 8.88uF > 3.86 uF
Cin @ 44.4V >= 0.159 * (1 - 0.159) * 4 / (0.888* 0.1015) = 5.74 uF, effective capacitance = 2.9*4= 11.6uF > 5.74 uF
Cin @ 22.2V >= 0.294 * (1 - 0.294) * 4 / (0.444* 0.1015) = 17.81 uF, effective capacitance = 5.84*4= 23.36uF > 17.81uF
Cin @ 18V >= 0.362 * (1 - 0.362) * 4 / (0.36* 0.1015) = 24.44 uF, effective capacitance = 6.69*4 = 26.76uF > 24.44 uF
Added one 0603 0.1uF cap at the buck controller input to match the datasheet guide.
Conclusion: We can go with ceramic capacitors only for our design. Vpp_max is 1.1V, which can cause problems at the SW node if the noise frequency matches the converter’s resonance frequency. However, we also have boot resistor, snubber circuit and gate resistor to suppress the SW node ringing.
Comparing Different Capacitor Options
| Ceramic 250V | Ceramic 250V (Stacked) | Al Polymer 125V | Al Polymer 160V | Al Polymer 250V | Electrolytic 400V | Film 450V |
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Description | |||||||
Effective Capacitance @ 18V to 60V (Estimate) | 2uF → 1.9uF | 3.3uF rated | 12uF (no DC bias) | 10uF (no DC bias) | 8.2uF (no DC bias) | 10uF (no DC bias) | 10uF (no DC bias) |
ESR (mOhms) | -- | -- | 69 @100kHz | 110 @100kHz | 458 @100kHz | Not in Datasheet | 10.9 @10kHz |
Price per piece | $4.35 | $5.33 | $6.57 | $2.32 | $3.77 | $1.17 | $8.30 |
Package | 2220 | SMD WxHxL (mm): 5x5x6 | SMD (10mm diameter, 10mm height) | Thru-hole cylindrical (8mm diameter, 12mm height) | Thru-hole cylindrical (10mm diameter, 12mm height) | Thru-hole cylindrical (10mm diameter, 20mm height) | Thru-hole rectangular WxHxL (mm): 15x24.5x31.5 |
Availability (Digikey) | 14k+ in stock | 128k+ in stock | 5.9k+ in stock | 9.4k+ in stock | 10k+ in stock | 21k+ in stock | 18k+ in stock |
Quantity Required | 4 | 3 | 1 | 1 | 1 | 1 | 1 |
Total Cost (Per Board) | $17.4 | $15.99 | $6.57 | $2.32 | $3.77 | $1.17 | $8.30 |
Other Notes | Expensive | Janky | Relatively low stock, rated voltage may be low | Relatively low stock | Relatively high ESR, relatively low stock | Tall, also no ESR rating in DS | Big
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Selection: Al Polymer 160V CAP ALUM POLY 10UF 20% 160V T/H
Justification:
Rated for 160V which should provide ample safety room to the rated 60V design voltage for the board
Quite affordable compared to using just ceramic capacitors
Space efficient, requires only one to provide adequate capacitance
Low ESR, at 110mOhms tested at 100kHz compared to regular electrolytic capacitors
Through-hole makes it easier to solder(?)
Small input ceramic capacitor to handle input spikes and phase-node ringing
Output Capacitor Selection
Cap impedance should be low at switching frequency (101.5kHz) as the inductor current oscillates at this frequency.
Datasheet uses 22uF and 220uF capacitors for 5V application
22uF: GRM21BR61E226ME44K
Keep output ripple voltage to a minimum
Vout = 6V
Vin = 55V
f_s = 101.5kHz
L = 47uH
R_ESR = 0.004R / 3
Co = 8.192uF * 3
Delta V_out = 0.0562V if put three 22uF capacitors, very quiet Vout, very nice
Compensation Components
Assuming we did have a R_sense resistor of 5 mOhms:
Gm = 500uA/V
Gcs = 1/(12*Rsense) = 16.67
Co = 8uF * 3
fc = 0.1 * fsw = 10.15kHz
Vfb = 0.8V
R5 = 1357 Ohms → 1300 Ohms
C6 > 49nF → C6 = 100nF
C7 = DNP for tuning.
RC Snubber
Purpose: reduces the voltage ringing spike at SW node
How it works:
The PCB trace parasitic inductance and LS FET output capacitance (Coss) forms an LC circuit. The SW node is right in the middle of L and C so it will see a lot of voltage ringing. The snubber circuit serves as a low impedance path for the high frequency voltage oscillations similar to a 1st order RC low pass filter. Half of the energy will be dissipated through Rsnb while HS FET is conducting and the other half will be stored in Csnb. While LS FET is conducting, current will flow out of Csnb and dissipates energy through Rsnb again.
Do we need it?
We don't want a transient to damage the switching FET
DNP the snubber the circuit for the 1st revision, we will find out if we need them during testing
Research Links:
Power FET Selection
Requirements
Vds > 55V max?
Ids > 5A (Id_min = 4.5745A)
lower Rds the better
Small size → son-8 package
Decision Matrix
MOSFET — Power, Single, N-Channel 60 V, 6.1 mΩ, 71 A (onsemi.com)
Overkill, too big
DMN10H170SK3-13 Diodes Incorporated | Discrete Semiconductor Products | DigiKey
Vds = 100V
Ids = 12A
Rds = ~150mOhms
Total Switching Loss = Gate Charge switching loss + Output Capacitance switching loss = 1/2 * Qg * Vgs * fsw + 1/2 * Coss* Vds^2 * fsw
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Cost | $2.83 | $0.83 | $1.66 | $0.70 | $1.48 | $1.32 |
Vds | 60 V | 100 V | 100 V | 60 V | 60 V | 60 V |
Ids | 71 A | 12 A | 35 A | 10.8 A | 13 A | 12 A |
Gate Charge | 9.0 nC | 4.9 nC | 59 nC | 8.4 nC | 11.1 nC @ 4.5 V | 7.3 nC @ 4.5 V |
Rdson | 8.8 mohm | 150 mohm | 28.5 mohm | 24 mohm | 12.4 mohm | 15.6 mohm |
Switching Loss | 0.5 * 9.0 [nC] * 5 [V] * 100 [kHz] + 0.5 * 640 [pF] * 55 [V] ^2 * 100 [kHz] = 0.09905 [W] | 0.5 * 4.9 [nC] * 5 [V] * 100 [kHz] + 0.5 * 36 [pF] * 55 [V] ^2 * 100 [kHz] = 0.00667 [W] | 0.5 * 59 [nC] * 5 [V] * 100 [kHz] + 0.5 * 180 [pF] * 55 [V] ^2 * 100 [kHz] = 0.041975 [W} | 0.5 * 8.4 [nC] * 5 [V] * 100 [kHz] + 0.5 * 282 [pF] * 55 [V] ^ 2 * 100 [kHz] = 0.0447525 [W] | 0.5 * 11.1 [nC] * 4.5 [V] * 100 [kHz] + 0.5 * 19 [nC] * 55 [V] * 100 [kHz] = 0.05475 [W] | 0.5 * 7.3 [nC] * 4.5 [V] * 100 [kHz] + 0.5 * 24 [nC] * 55 [V] * 100 [kHz] = 0.06764 [W] |
Conducting Loss | 4.5745 [A] ^2 * 0.0088 [ohm] = 0.1841 [W] | 4.5745 [A] ^2 * 0.150 [ohm] = 3.1389 [W] | 4.5745 [A] ^2 * 0.0285 [ohm] = 0.5964 [W] | 4.5745 [A] ^2 * 0.024 [ohm] = 0.5022 [W] | 4.5745 [A] ^ 2 * 0.0124 [ohm] = 0.2595 [W] | 4.5745 [A] ^ 2 * 0.0156 [ohm] = 0.3265 [W] |
Maximum Operating Temperature | 175 C | 150 C | 175 C | 175 C | 150 C | 150 C |
Calculated Operating Temperature (Assume Id = 4.5745 A, T_ambient = 25C) | 25 [C] + (0.1841 + 0.09905) [W] * 41[C/W] = 36.61 [C] | 25 [C] + (3.1389 + 0.00667) [W] * 44 [C/W] = 163.41 [C] | 25 [C] + (0.5964 + 0.041975) [W] * 40 [C/W] = 50.535 [C] | 25 [C] + (0.0447525 + 0.5022) * 47 [C/W] = 50.71 [C] | 25 [C] + (0.05475 + 0.2595) * 50 [C/W] = 40.71 [C] | 25 [C] + (0.06764 + 0.3265) * 55 [C/W] = 46.67 [C] |
Comments | Losses are low and it doesn’t get hot but slightly expensive | Not selected, it will burn. Rdson too high | Moderate losses and reasonable operating temperature, reasonable price. | low gate loss and switch losses, cheapest price. Large package. Previously selected. | Low losses and small package. selected | Low losses and small package. Not selected because the package is too small. 0.9mm trace width may not be enough to handle 5A |
Decision: we choose Diodes Incorporated DMTH6016LK3-13 as our buck converter’s switching FETs.
Decision: Selected Texas Instrument CSD18534Q5A as the switching FET for low cost, small package and small losses.
Bootstrap Circuit
Purpose: generate bias to drive the gate of the High Side FET
How it works:
When SW node is GND, internal 5V VCC charges up Cboot through forward biased diode
When SW node is Vin, Cboot still maintains that 5-Vf voltage since the charges on the capacitor has nowhere to go and V = Q/C. The schottky diode blocks current from flowing back to the voltage source.
High side switching FET gate capacitance: Cg = Qg / (VCC1 - V_bootDiode)
Cg = 11.1 nC / (5 V - 0.45 V) = 2.440 nF
Cboot > 10 * Cg
0.1 uF >> 0.0244 uF
Conclusion: Using a 0.1 uF capacitor for Cboot will work. DC derating must not lower the capacitance at 5V.
Two resistors can be placed in the bootstrap circuit for tuning.
Rboot: Placed in series between Cboot and the diode to limit the charging inrush current to the bootstrap capacitor.
Iboot = Cboot * Vboot / charging time = Cboot * (VCC - Vf) / (1/fsw*(1-DutyCycleMax)) = 0.1 * (5-0.45) / 1/100k*(1-0.362) = 71.32mA
Conclusion: we don’t need a inrush limiting resistor.
Rbst: Placed in series between Cboot and the BST pin to slow down the rise time of the HS FET. Note that a large value on this resistor will trigger the UVLO on the HS gate driver.
This resistor is also helpful to reduce the ringing effect on the SW node by slowing the rise time. It is the most efficient method as the gate resistor and snubber circuit dissipate energy when the HS FET is turning off while Rbst does not do that!!
Diode selection:
This diode needs to recover fast to block charges on the SW node when HS FET is conducting. And also need to have low Vf to charge Cboot.
Vr > Vin - VCC1 = 50V
Io = Iboot = 71.32mA
selected https://www.digikey.ca/en/products/detail/diodes-incorporated/DFLS160-7/765614
Common Mode Choke
Needs Assessment
Do we need this filter?
Motor magnetic field can get coupled to the buck Vin. If there is noise on the buck Vin when the drone is flying in the sky, it is almost impossible to RCA. The common mode filter keeps us away from that situation. So, we should keep the common mode filter if there is space on the PCB
How does the common mode choke work?
This noise, known as the common mode noise, will affect both power and ground rails. To maintain the voltage at our buck converter’s input, we use a common mode choke chip to attenuate the common mode noise.
When differential currents are coming through, the magnetic field substracts so currents pass through without any impedance. When common mode noise are coming through, the magnetic field adds so the noise current will experience a lot more impedance and most of the noise will be blocked at the choke.
Research Notes:
https://electronics.stackexchange.com/questions/79752/what-is-common-mode-noise
Requirements
Current rating > 4.7A
Voltage rating > 55V
Filter capabilities has little impact because we need fit this big chip on our tiny board
Options
Efuse Component Selection
Efuse IC Selection
The table below shows some overcurrent IC options I have found:
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Choice Summary | Might be overkill, has OV and UV protection features which aren’t required | Jank current-limit resistor equation, needs ratio between nominal output current and current limit | Looks okay? |
Name | MAX17813C
| TPS1H000-Q1 (Other options possible in family) | TCKE800NA (because Auto-retry) |
Input Voltage Range | 4.5-60V | 3.4-40V | 4.4-18V |
Current Limit Range | 0.15-3A | 0.05-1A | 0-5A |
Available Modes |
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Cost | ~$9/chip (Mouser) | ~$1/chip (Mouser) | ~$2.40/chip (Digikey) |
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Adjustable Current Limit of Smart Power Switches (Rev. B) (ti.com)
This series of TI chips can only run a max input voltage of 40V, we can put them after the buck and it be okay
TPS1H000-Q1 40-V, 1-Ω, Single-Channel Smart High-Side Switch datasheet (Rev. C) (ti.com)
Can externally set current limit from 50mA to 1A
Can be set using a resistor at the CL pin to GND, value is set using the following equation:
R_CL is the resistor
Where V_CL(th) is internal band-gap voltage (0.8V for this IC)
K_(CL) is the ratio between output current and the set current limit value
I_OUT is the output current
3 Modes of operation when device is tripped:
Auto-retry probably the one we want to go for as it can retry letting current through at intervals
Could just pullup delay input to Vs
Can be controlled by a MCU and work standalone
Could maybe even add some breakout connections for DIAG_EN and FAULT if in the future we want to had diagnostics with a microcontroller
This might also work…
XC8108.pdf (torexsemi.com) (has max voltage of 5.5V)
Selected Efuse IC: TCKE8xx Series
The auto-retry version was chosen so that each servo will continue to attempt working while we are in the air and so that we don’t lock out a servo mid-flight.
Input and Output Capacitor
https://www.digikey.ca/en/products/detail/samsung-electro-mechanics/CL10B104KA8NNNC/3886664
Also, assuming a 2.6ms rise time, and a very low resistance on the line, two 1uF for the input capacitor should be enough to allow the voltage output to rise in 2.6ms from 0V to 6V.
Current Limit Resistor
RILIM = 117kΩ if IILIM is set to 1A
https://www.digikey.ca/en/products/detail/yageo/RT0603DRE07117KL/7708397
Slew Rate Control Capacitor
Use 1nF capacitor from altium library.
This results in a t = 2.6ms rise time for 6V output.
Input Zener Diode
The zener diode protects the chip from inductive voltage spikes from sudden current increases. Think of V = L * di/dt but di/dt is infinity.
Calculation of input voltage spike from the datasheet:
Assuming perfect conductors on the input rail, this formula is a simple ohm’s law where the R becomes the characteristic impedance of the input rail, which is sqrt(L/C)
Lin = 47 uH (buck inductor)
Cin = buck input caps + buck output caps + 4 current limit ic input caps = 10uF*2 + 22uF*3 + 1uF*8 = 94uF
Vin = 6V (worst case)
Iout = 1A
Vspike = 6.707V which is less than the 18V absolute maximum input voltage.
Conclusion: we don’t need a zener diode at input.
Output Schottky Barrier Diode
This optional SBD on the output of the Overcurrent IC may be needed such that no instant changes of the voltage to the motor/servo or when the current suddenly shuts off to 0A creates a negative voltage spike (although ideally servo has flyback internally to deal with this)
The SBD will clamp the output voltage at -0.28V for chip protection since the datasheet specified the absolute minimum Vout is -0.3V. It will be placed for the 1st revision just in case if things go wrong.
PWM Buffer Component Selection
Servo Motor Market Research
Most servos can handle 3.3V & 5V logic levels but some high torque servos only take 5V logic level and doesn’t respond to 3.3V PWM signals. Therefore we want to buff the 3.3V PWM signal coming from the flight controller (Pixhawk 6) to 5V.
Google search “step down servo motor PWM signal from 5V to 3.3V” gives guides on the opposite… so we don’t need to worry about 5V PWM not compatible. Plus we have jumpers to get around the buffer if we really want a 3.3V PWM signal.
Requirement
4-channel
5V Vcc
Level shift PWM signal voltage from 3.3V to 5V https://docs.holybro.com/autopilot/pixhawk-baseboards/pixhawk-baseboard-ports
Selected SN74AHCT126PWR for cheap price and small package.
3.3V buffer backup: SN74HC125PWR
5V LDO
Current consumption of the buffer is 25mA max, Vdrop = 6V- 5V = 1V, Ploss = 25mW worst case. No need for a big package to dissipate heat. Selected AP7375-50SA-7 for cheap price, small package and VDROP = 350mV @ IOUT = 100mA (Typ.).
3.3V LDO backup: AP7375-33SA-7
Input Pulldown/Pullup Resistors
I couldn’t find Pixhawk PWM rail leakage current. Assuming 100uA leakage current from Pixhawk and open drain configuration, 4.7k pulldown will create a 0.47V input, which is below the 0.8V logic low threshold. 4.7k pullup will create a 0.47V drop, which would still above the 0.7 * Vcc threshold at 3.3V Vcc.
Selected 4.7k as pulldown/pullup resistor value.
PCB Layout Notes
Layout Topology
XT60 connector comes in at the middle-left of the PCB, following by the common mode choke
HS FET on the right then LS FET next to it, inductor is placed under the FETs
All buck controller components stay on top layer, except for jumpers, they can be anywhere
4-channel buffer on the top of the bottom layer, then the Efuse ICs
Mounting holes at 4 corners, no extra space beyond them
Layout Justifications
Trace Width Calculations
I assumed the board can handle 20C temperature rise just from air cooling. Source: Saturn PCB
For 1A traces in and out of the efuse IC, I’m using trace width >10mil
For 4.57A traces in and out of the buck, I’m using trace width >50mil
Via Current Calculation
Via Size
Via hole: 0.15mm
Minimum Annular ring: 0.13mm
Via diameter: 0.15 + 0.13*2 = 0.41mm
This setting is chosen so that the board uses the smallest via size possible while not paying JLCPCB extra. 0.15/0.41 size vias also makes it easier to terminate ground copper dead ends. Though some cases we can use a larger via (e.g. 0.3/0.6), it increases drill sizes and increase manufacturing cost.
Layer Stackup
Copied JLCPCB 4-layer stackup since I want internal ground layers to be as close to the top and bottom layers as possible. Reason being I want the ground plane to absorb as much noise as possible on the top and bottom layers, especially the magnetic field from the inductor.
Efuse IC Pad
Removed vias on the ground pad of efuse IC, so everything can fit on a 30x30 mounting hole pattern. Justification:
Maximum power dissipation would be 1A*1A*38mohm = 38mW.
Assuming a 50 C/W thermal resistance, temperature rise is only 1.9 C.
Input & Output Connectors
Input connector and output connector both follow the pixhawk flight controller PWM connector 3x4 pattern. Top row is ground, mid row is power and bottom row is signal.
Online 3D body for the input & output header in the PCB file is wrong so it is removed from the layout. The correct mating length is 5.84 mm which matches PWM cables.
As of 2023-11-19, 12-position-3-row female sockets are out of stock, link to its datasheet: https://suddendocs.samtec.com/catalog_english/ssq_th.pdf
Alternative is to use two 6-position-3-row sockets together if the above remains out of stock
BOM notes
Add 2.2 ohm resistor for bootstrap tuning
Order 4.7k resistor for buffer input pullup/pulldown