Nokia 9000i Service Manual page 154

RAE/RAK–1N
PDA Hardware
The controller has internal 1.23V bandgap reference and soft start
circuitry for power–up. Overcurrent comparator disconnects the controller
in short circuit conditions. After this soft start cycle is made in power–up.
Output ripple voltage is determined by output capacitor ESR value which
is minimized to reduce EMI. Shutdown mode is used when battery
voltage decreases down to 5V to prevent battery from overdischarge. In
shutdown all circuits are in lowest power state.
LCDVEE regulator
LCD bias voltage is made by step–up dc–dc controller. Control scheme is
current limited pulse frequency modulation (PFM). External switch
transistor and current sense resistor are required. Maximum inductor
current goes through current sense resistor. Because required regulator
output current is small (3mA), resistor value is selected so that current
through inductor is low to reduce EMI. Also switch transistor
ON–resistance need not be lowest possible. Pulse frequency control is
made without oscillator, maximum on–time and minimum off–time are
used in pulse control. When output voltage is out of regulation the switch
turns on and it stays on until maximum on–time turns it off or inductor
current reaches its maximum value set by current sense resistor. When
the switch turns off, minimum off–time is waited.
After this the switch stays off until output voltage drops out of regulation.
This control results current pulses which are delivered on load demand
and 'skipped' when in regulation, 'pulse skipping' regulation. Benefits are
very low supply current at light loads because of no oscillator and high
energy conversion efficiency. Because pulses come at unspecified time
intervals switching noise is in broad frequency range. However maximum
inductor current is set to lowest practical value and output capacitor ESR
is selected for low value to reduce EMI.
Output voltage is adjusted by LCDPWM signal from PROCU. This pulse
width modulated signal is first converted to analog dc voltage by RC–filter.
This signal is fed to regulator feedback pin through a resistor which
determines the scale at which the LCD display bias voltage can be
adjusted. Temperature compensation for bias voltage is made by resistor
divider connected to regulator feedback pin. Bias voltage is controlled by
two NTC resistors so that optimum LCD contrast bias voltage is followed
accurately by the regulator in temperature range between –20 C and
+65 C. Because feedback pin comparator voltage is 1.5V and regulator
output voltage is over 20V high accuracy feedback resistors are needed
for fairly accurate output voltage.
Shutdown is controlled by PROCU. Because this is boost regulator there
is current path from input to output which must be cut separately in
shutdown. Control switch is placed between regulator output and load.
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After Sales
Technical Documentation
Original, 08/96