1054417, V
ER
. 01
P
AGE
20
Alice
®
PDx
TM
Service & Technical Reference Manual
There are separate supplies for both the +3.3V_ANA and +3.3V_DIG busses, consisting of U24 and U25,
respectively.
A third DC to DC converter, U6, supplies power to the +5V bus. In this case, output voltage would always be
higher than input voltage, allowing the use of a boost-only converter topology. The part chosen for this circuit
was the LT3526B, by Linear Technologies.
The original circuit for the +5V supply was an ON Semiconductor NCP1421. While highly efficient, this part
generated a rather noisy +5V supply. Since the +5V supply feeds the pressure sensor, noise on the +5V bus
was appearing on the snore signal, lowering the signal to noise ratio of the snore signal. The +5V supply was
changed to the LT2526B in order to improve the signal to noise ratio of the snore signal.
Amplifier U39 provides an indication of battery voltage to the microcontroller. The output of U39 (Vbatt) follows
the relationship below:
Y = MX + B
Where:
Y = voltage measured directly on the battery terminals
M = 1.922
X = VBATMON
B = 0.4212
The battery monitoring circuits are located on the digital board, rather than on the battery board. As a result,
the battery monitoring circuit must measure battery voltage after it has passed through a considerable amount
of resistance, due to traces in the flex circuit, EMC choke coils, and other devices. Main power for the unit also
passes through these resistances, causing voltage drops to develop between the battery and the battery
monitoring circuit. The equation above accounts for these voltage drops, assuming the device is drawing a
nominal load current of about 78 mA. If load current should deviate from this value, it may be necessary to
recalculate the M and B constants.
The system processor monitors VBATMON and will issue a low battery warning when the battery terminal
voltage falls below approximately 3.1 volts. The low battery warning will cause the system to halt further
accesses to the SD card, as well as flash the battery icon on the LCD.
If the battery continues to drain to a terminal voltage approximately 2.82 volts, battery supervisor U7 will issue
a “battery critically low” warning to the processor via the BATT_GOOD line. This will cause the processor to go
into an immediate system shutdown. Due to the voltage drops mentioned above, it was necessary to set the
trip point of the voltage supervisor to 2.55 volts.
The control circuit for the power supplies is shown on sheet 6 of the Digital Board Schematic.
When the power button is pressed, a logic low appears at the /PR input of D-type flip flop U17, which puts the
flip flop into the preset state. When in the preset state, output /Q is at logic low, which causes MOSFETS Q3A
and Q3B to not conduct, enabling the power supplies. A similar condition can be achieved if the RTC (U5)
pulses the /IRQ/FOUT pin low in response to the passing of a preset time condition.
While any further presses of the power button have no direct effect on the power control circuit, the
microcontroller is notified of button pushes via a port pin. This button push may be detected by the program
running in the microcontroller and cause the microcontroller to initiate a shutdown of the power system. When
the microcontroller places a logic high on the gate of Q2A, flip flop U17 is forced into the CLEARED state,
applying a logic high on MOSFETS Q3A and Q3B, which causes a shutdown of the power supplies.