Power Supply - Motorola LX2 plus Service Manual

LX2 plus / advanced and LX4 plus / advanced
3.3 Charging Unit (PCB No. 228.01.xx)
Charging Unit with PCB No. 228.01.xx
3.3.1

Power Supply

The Charging Unit is powered by an external AC/ DC Adapter plugged into the wall power
socket (U
I = 0,4A to supply the Charging Unit.
L
The adapter is connected to the Charger using socket BU1 of the Charger.
The electrolythic capacitor C12 removes residual noise and buffers the supply voltage so
that mechanically caused short connection interrupts cannot lead to malfunction. Addi-
tionally components R22 (Varistor) and D7 (Supressor Diode) can be placed to prevent
over-voltage problems.
To supply the internal microcontroller an integrated voltage regulator (IS2) produces a
stable 3V supply. Capacitors C7, C8 und C9 are necessary for a stable operation of the
regulator. Diode D3 prevents supply via the output whenever the cable of the adapter is
removed.
C13 is a decoupling capacitor placed near to the microcontroller. Resistor R49 discharges
the capacitors quickly if the Charger is turned off by removing the adapter cable. This is to
prevent the microcontroller from working with supply voltages outside the specified range.
A decoupled voltage to supply external devices is available on socket BU2. That supply is
current-limited by R20 and protected by fuse SI1.
3.3.2 Charge Current
The chargeable battery inside the pager gets charged whenever one of the two current
paths is turned on (switching transistor T1 or T10 enabled by the microcontroller). The re-
spective base resistor R5 or R19 is drawn to ground by an open-drain port of the control-
ler. R4 and R18 are used to make sure that the transistors turn off properly.
Charging is done by a more or less constant current. The current is set by the respective
limiting resistance. In case of the high charge current (T1 turned on), only resistor R7 is
effective:
I
L accelerated
during normal charging (T10 turned on) the current is:
I ˜ (U
L normal
Diode D12 prevents the battery from being discharged when no charge current is avail-
able and the inductor L3 guards against RF interference.
B2270901 Content is subject to change without notice!
= 230V, f = 50Hz). The adapter delivers an unstabilized DC voltage U
AC
˜ ( U -U ) / R = (6V-1,4V) / 47Ω = 98mA,
= batt 7
-U ) / (R +R +R
= Akku 7 23
) = (6V-1,4V) / (47Ω + 56Ω + 68Ω ) = 27mA.
24
= 6V at
=
9