LG CU920 Manual page 32

3. TECHNICAL BRIEF
3.5.4 VCTCXO
The Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO) provides the
reference frequency for all RFIC synthesizers as well as clock generation functions within the
MSM6281 IC. The oscillator frequency is controlled by the MSM6281 IC.s TRK_LO_ADJ pulse density
modulated signal in the same manner as the transmit gain control TX_AGC_ADJ. A two-pole RC
lowpass filter is recommended on this control line.
The PM6650 IC controls the handset power-up sequence, including a special VCTCXO warm-up
interval before other circuits are turned on. This warm-up interval (as well as other TCXO controller
functions) is enabled by the MSM TCXO_EN line. The PM6650 IC VREG_TCXO regulated output
voltage is used to power the VCTCXO and is enabled before most other regulated outputs.
Any GSM mode power control circuits within the MSM6281 IC require a reference voltage for proper
operation and sufficient accuracy. Connecting the PM6650 IC REF_OUT directly to the MSM6275 IC
GSM_PA_PWR_CTL_REF provides this reference. This sensitive analog signal needs a 0.1 µF low
frequency filter near to MSM side, and isolate from digital logic and clock traces with ground on both
sides, plus ground above and below if routed on internal layers
3.5.5 PMIC Functional Block Diagram
■ Input power management
- Valid external supply attachment and removal detection
- Supports unregulated (closed-loop) external charger supplies and USB supplies as input power
sources
- Supports lithium-ion main batteries
- Trickle, constant current, constant voltage, and pulsed charging of the main battery
- Supports coin cell backup battery (including charging)
- Battery voltage detectors with programmable thresholds
- VDD collapse protection
- Charger current regulation and real-time monitoring for over-current protection
- Charger transistor protection by power limit control
- Control drivers for two external pass transistors and one external battery MOSFET - MOSFET is
optional
- Voltage, current, and power control loops
- Automated recovery from sudden momentary power loss
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