Nokia 9000i Service Manual page 155

After Sales
Technical Documentation
VCC5 regulator
5V output is used only for data writing to FLASH memory and RBUS
signals. Regulator is taken out of shutdown when it is needed. This
simple linear regulator has pnp control transistor and
overcurrent/overtemperature protection circuitry. Maximum current is
50mA. Peak current is 100mA. At higher loads case heats up and
regulator is shut off for very short time and restarted to check if high load
remains. If so thermal cycling results. Typical turn–on time for the
regulator is 50µs. Only small output capacitor is needed as external
component. However, its ESR value must be within certain limits
depending on capacitance and load current in order to get stable regulator
output.
Reset circuit
Purpose of the reset circuit is to generate proper reset to the CPU and
also disable CPU operation when Vsys is below CPU Vcc range. When
the battery is plugged in reset circuit generates proper reset pulse when
Vsys is risen up to CPU Vcc range. Threshold difference between "power
good" and "power not good" is about 0.02V.
Undervoltage lockout (UVLO)
Battery voltage is compared to accurate reference diode to detect too low
battery voltage. Below this HW limit comparator shuts down VSYS
regulator to prevent battery from overdischarge. There is higher SW limit
for PDA but VSYS regulator current drains the battery when left unused
for long period. After UVLO there is only reference diode and two
comparators taking current from battery. UVLO has hysteresis and is
cancelled when battery voltage is risen to 6.0V. Only way to do this is by
charging or plugging in fresh battery. Hysteresis is made to avoid
unsuccessful power–ups. When lockout voltage level is reached battery
voltage rises because load is removed.
Input filter
LC–lowpass filter is used between battery and regulators. EMI is not big
problem in regulator outputs but EMI conducted from switchmode
regulator inputs to battery line needs filtering. Also attenuation of EMI
from CMT devices in battery line to PDA regulators is welcomed. Good
ground planes and placement of PDA power block to metal cavity and low
output ripple voltages keep radiated EMI at low level.
Backup battery
Real time clock is kept running by backup battery only when main battery
is not connected. At nominal RTC load 130mAh capacity of backup
battery gives over two years of RTC operation when main battery is not
connected. Backup battery is not chargeable.
Original, 08/96
RAE/RAK–1N
PDA Hardware
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