Datasheet

Chapter 2 — Theory of Operation
PB42 Portable Receipt Printer Service Manual 33
The DC-DC converter circuitry is protected by Q2701 and Q2702. The
comparators of U27 turn on Q2701 and Q2702 when the input voltage is
between 8V and 18V. If a voltage spike occurs on the DC input, the
comparator will immediately turn off the transistors until the voltage
returns to an acceptable level. If the printer does not have batteries
installed, power to the printer will momentarily be interrupted.
L1 suppresses high frequency noise. D8 combines the two DC inputs, and
protects the printer from reverse voltage inputs. D1 and D2 protect Q2701
and Q2702 from transients greater than 100V (and less than -100V).
The zener diode D3 is used to detect when the printer is connected to the
contact (hinge) charger.
Conducted Emissions
Conducted emissions are suppressed by the common mode choke, L1 and
the input capacitors of U29. (C2901, C2917, and C2918)
Radiated Emissions
Potential sources of radiated emissions are clocks, high frequency bus lines,
the Bluetooth radio, and the switching power supplies. This section
describes what was done to suppress noise from each of these sources.
Clocks
There are three high frequency clock sources, 18.4 MHz from the print
controller crystal (Y2), 48 MHz from the USB clock oscillator (Y3), and 66
MHz from the U1 PLL to the SDRAM.
Y2 is not buffered outside the chip, so it is not a concern other than being
sure to place it as close to U2 as possible.
Y3 is buffered and goes into U1. A 20 Ω resistor on the output is used to
filter out any high frequency harmonics.
The 66 MHz CLKOUT signal is also filtered with a 22 Ω resistor along
with the rest of the SDRAM control lines. The signal runs on an internal
plane that is between two ground planes.
Bus Lines
The U1 address and data buses are filtered with 22 W series resistors.
Bluetooth Radio
The Bluetooth radio is mounted as far from the power supply circuitry as
possible. The entire module is shielded.