photobooth01

Photo Booth Project¶

Feb 17, 2023 Update

Found several discrepancies between the board schematic and the typical application shown in the schematic with the exact required specification; i.e. 12V in at 6A. I know the spec is less than 6A but margin is prudent.

RefDesign

Executive Summary

Both swithcers are now working, producing 5V with no load. For details see below.
I'm still a bit worried about the inductor choice so I haven't yet checked the 4A max.
I have not tested the USB-C driver. I'm waiting for a USB-C tester which should arrive today. The specs of it weren't clear but I'm hoping that it will check all possible voltage / current outputs. If you have a tester let me know.
There are still things to do ..
- Verify max loads (mentioned above)
- Verify thermals for switcher and inductor. What is the environment for the board .. shirt sleeve? ... a mobile trailer in the Arizona desert? This can be important in determining the thermal design.
- Verify USB-C for all voltages / currents (also mentioned above)
- Input protection? ESD, Spikes, etc.
- Input voltage tolerance. I was wrong before about the USB-C chip making 24V problematic. It's powered by the switcher not $V_{in}$. However, the absolute max for the switcher is 26V. So a typical off the shelf 24V supply can be 26.4V (+10%) exceeding this. If you supply the supply (pun intended) then you're fine.

Details

PG (power good) is tied to $V_{in}$ but datasheet says it must not go above 4.5V. This was the 'LED' issue. The switcher is has it's own internal LDO that supplies 3.6 volts to the outside world. This should be used for the pull-up source as shown above but without the LED ..
- The MODE pin is floating in the reference design, indicating PFM, Ultrasonic Mode. On your board it's tied high indicating forced PWM mode .. This should be Ok
- R6 should be left open. There is an error in the datasheet indicating that the internal pull-down resistor is 600K. It's a lot lower. So, stuffing R6 caused the EN (enable) pin to be a logic low, keeping the chip from turning on. The board design would have been Ok if the datasheet was correct. See design verification below.
- LED circuit should be changed to the VCC source in series with a 330 ohm resistor, connected to the switcher's VCC pin. The resistor value can be changed until you get the desired brightness.
- C1, C17 - the previous rework is fine.

Design Verification Feb 15¶

To make sure we didn't have problems below is are some rough design calculations per the sometimes correct datasheet.

In [1]:

import Text.Printf

-- We don't want to blow up the chip
-- so compute vEN to be sure that
--     4.0 > vEN > 1.4
vIn = 12
r6 = 300e3
pullDown = 600e3
rp = (r6 * pullDown) / (r6 + pullDown)
vEN = (vIn * rp) / (1.5e6 + 600)
printf "vEN at 12V -> %5.2f" vEN
printf "vEN at 24V -> %5.2f" (2*vEN)

vEN at 12V ->  1.60

vEN at 24V ->  3.20

r6 at 300k yields a good vEN for 12 and 24v

In [2]:

vRef = 600e-3
r1 = 41.2e3

-- Calculate R2 for output volage of 5V
r2 = (vRef * r1) / (5.0 - 0.6)
printf "r2 = %5.2f" r2

r2 = 5618.18

Board values for FB resistors are correct for Vout is 5v.

In [3]:

vPG = 0.4 -- at I = 4ma
vCC = 3.6 -- nominal

Inductor Choice

In [4]:

fSW = 700e3
deltaI = 2.8 -- 30% of output current
vOut = 5.0
vIn = 12.0
indL = (vOut/(fSW*deltaI)) * (1.0-(vOut/vIn))
printf "L -> %5.2e" indL

L -> 1.49e-6

This means that the ripple current will be 2.8 amps

Peak Inductor Current

In [5]:

iOut = 6.0
iPeak = iOut + (vOut / (2.0 * fSW * indL)) * (1.0 - (vOut / vIn))
printf "Ipeak -> %5.2e" iPeak
putStrLn "This is below the max switch current of 10A"

Ipeak -> 7.40e0

This is below the max switch current of 10A

Power Good Circuit

The VCC output is 3.6 volts and the $V_f$ of the LED is 1.8V so the series resistor should be approximatly,

In [6]:

rLED = (3.6 - 1.8 - 0.4) / 4e-3
printf "LED series resistor -> %5.2f" rLED
putStrLn "So make it 330 for about 4ma"

LED series resistor -> 350.00

So make it 330 for about 4ma

Feb 9, 2023¶

Talked with Joey.

Received Initial spec, schematic, gerber, BOM, and board images.
Downloaded relevant datasheets
Reviewed USBCtllr, MOSFets, Switcher IC datasheets
The USB-C interface appears quite simple which makes sense as there is no data transfer, only power.
Power input protection (12V) is almost non-existent. No surge protection, and no ESD protection.
Circuit isolation from the power input is also missing which may cause FCC conduction approval issues.
Chip choices are appropriate.
PCB trace sizes appear to follow good design practices.
Fab may not have undergone normal electrical test. This increases the possibility of PCB errors.
The LED behavoir mentioned makes no sense and initially implied that U4 was reversed. Subsequent conversation with Ryan show this is not true. This is currently the biggest mystery.
A rough design was done to determine the inductor specification per the MPS website.The inductance indicated was 15 times larger than the one on the PCB (22uhy vs 1.5uhy). The physical size was also larger (roughly 13mm square vs 9mm square). The inductance and physical size could indicate a potential saturation issue (saturation = poof). A deeper dive into the inductor spec will be useful. Unfortunately a PSpice model is not available for this switcher IC.

Datasheets¶