Power Amplifier Module (Sky77318) - LG 410G Service Manual

3. H/W Circuit Description

3.2 Power Amplifier Module (SKY77318)

Figure. 3-2 SKY77318 FUNCTIONAL BLOCK DIAGRAM
The SKY77318 Power Amplifier Module (PAM) is designed in a low profile (1.2 mm), compact form
factor for quad-band cellular handsets comprising GSM850/900, DCS1800, and PCS1900 operation.
The PAM also supports Class 12 General Packet Radio Service (GPRS) multi-slot operation.
The module consists of separate GSM PA and DCS1800/PCS1900 PA blocks, impedance-matching
circuitry for 50 Ω input and output impedances and a Power Amplifier Control (PAC) block with an
internal current-sense resistor. The custom BiCMOS integrated circuit provides the internal PAC function
and interface circuitry. Fabricated onto a single Gallium Arsenide (GaAs) die, one Heterojunction Bipolar
Transistor (HBT) PA block supports the GSM bands and the other supports the DCS1800 and PCS1900
bands. Both PA blocks share common power supply pins to distribute current.
The GaAs die, the Silicon (Si) die, and the passive components are mounted on a multi-layer laminate
substrate. The assembly is encapsulated with plastic overmold.
RF input and output ports of the SKY77318 are internally matched to a 50 Ω load to reduce the number
of external components for a quad-band design. Extremely low leakage current (2.5 μA, typical) of the
dual PA module maximizes handset standby time. The SKY77318 also contains band select switching
circuitry to select GSM (logic 0) or DCS/PCS (logic 1) as determined from the Band Select (BS) signal.
In Figure 1 below, the BS pin selects the PA output (DCS/PCS_OUT or GSM_OUT) and the Analog
Power Control (VAPC) controls the level of output power.
The VBATT pin connects to an internal current-sense resistor and interfaces to an integrated power
amplifier control (iPAC™) function, which is insensitive to variations in temperature, power supply,
process, and input power.
The ENABLE input allows initial turn-on of PAM circuitry to minimize battery drain.
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