Hi,
I just replaced SMPS boards in my QMX with a linear regulator board, and
removed the power switch (I always found it annoying). Some tips, if
someone wants to do the same:
1. 3V3 regulator (I used the 78M33 which was normally used to "bootstrap"
the CPU before SMPSes took over) will get bloody hot. QMX draws around 100
mA on this line, which isn't much, but at 12 V it translates to 870 mW of
power dissipated in the regulator. Make sure you provide as much ground
plane for heatsinking as possible. I did, and my regulator is still almost
90°C. I'm not sure how it will behave long term.
2. CPU needs two fake signals: ADC_5V (2.5 V) and ADC_3V3 (1.65 V). The
reason is that it still tries to drive nonexistent SMPSes, expecting
feedback on these two lines. It won't boot without them. Just provide
resistive dividers, like in the original design (resistor values are not
critical, I used 18k).
3. Lines PWM_5V, PWM_3V3 and LIN_REG_EN can be ignored (unconnected).
4. 100 uF capacitors (instead of 470 uF ones used in original boards) seem
to be sufficient. Maybe even smaller ones would be OK.
When it comes to the power switch removal:
1. V_IN can be hooked directly to +12V (but you lose reverse polarity
protection this way).
2. PWR_HOLD signal can be ignored.
3. PWR_ON signal can be ignored, but the original SMPS board has a 100k
pullup (to V_IN) on this line. Make sure your board contains this pullup
as well (although I used a much lower value, 4k7). If you keep this line
floating, then the (very sensitive) gate of Q202 will get triggered by
literally anything.
4. QMX will power on automatically. When you want to turn it off by long
pressing the knob, it will show "Shutdown!" and hang. At this stage, the
MCU releases PWR_HOLD line, but without the power switch this does
nothing. Only power cycle seems to be able to recover from this state
(understandable).
5. CPU seems to save current state (mode, frequency) to EEPROM only upon
shutdown, so if you just remove the power without shutting QMX down with
the knob press, current state won't be retained. This can be a problem or
an advantage.
Hope it helps.
